Looking for a Tutor Near You?

Post Learning Requirement »
x

Choose Country Code

x

Direction

x

Ask a Question

x

Hire a Tutor
Dubai

Grade 12 Organic Chemistry

Loading...

Published in: Chemistry
5,342 Views

Organic Chemistry Notes along with Worksheets and Answers

Nathan P / Al Ain

15 years of teaching experience

Qualification: Higher Diploma in Education. High School. Majoring in Physics, Mathematics and Chemistry.

Teaches: Maths, IGCSE/AS/AL, Chemistry, Physics

Contact this Tutor
  1. -c-c-c-c- 12 KA : Matter and Materials : Organic Chemistry TOPIC Organic ( or carbon ) molecules Organic molecules are molecules containing carbon atoms. Carbon has amazing abilities to form bonds. Carbon atoms have four valence electrons in a tetrahedral arrangement. c - cc 1 C has a valency of four. — C — represents a carbon atom Because these bonds are symmetrical, stretching out evenly in 3-dimensions, carbon atoms are able to bond to each other without producing an imbalance of charge. Organic molecules are molecules containing carbon atoms with the exception of CO , CO, diamond, graphite, carbonates, carbides and cyanides. Organic compounds form when carbon atoms bond covalently to other carbon atoms in short or long chains. longer carbon chain -c-c-c-c-c-c-c-c- All living things are made of organic compounds. Carbon is the basic building block of organic compounds that recycles through the Earth's air, water, soil and living organisms including human beings. There are also millions of synthetic ( man-made ) organic compounds eg. about 700 different plastics. Carbon can form single, double or triple covalent bonds. with itself and many other elements eg. C N- cyanide ion -c-c-c- -c=c- -CEC- Carbon can form straight chains, branched chains or ring structures by covalently bonding to other carbon atoms. H —c— H H H H H —c—c H H H I-I-C H H and H H aliphatic : straight chains Carbon can form bonds with non-metals like H, branched chain. O, S and N and halogens. o=c=o co 2 s=c=s cs 2 H -CEN HCN CH 4 CH3Br bromomethane aromatic : ring CH CC chloromethane CH3F fluoromethane carbon dioxide carbon disulfide hydrogen cyanide methane Alkanes Normal alkanes or n-alkanes are the normal or unbranched or straight chain alkanes. Hydrocarbons are organic compounds that consist of hydrogen and carbon only. H-C—C propane butane —C—C-H 4 10 methane C H ethane C H Alkanes are hydrocarbons. 4 CH CH 3 CH CH CH 3 structural formula H H 1-1 H-C—C—C—C—C-H pentane C H 5 12 condensed structural formula 3
  2. 12 KA : Matter and Materials : Organic Chemistry Alkanes have only C —H and C —C single bonds —c—c —c—c c=c—c—c —c 2 Structural formula of a compound shows which atoms are attached to which within the molecule. Atoms are represented by their chemical symbols and lines are used to represent all the bonds that hold the atoms together. H H-C H H H H H —c— H H-C H H H H H H H H H H hexane heptane H octane Condensed structural formula shows the way in which atoms are bonded together in a molecule but does not show all bond lines. 3 C H 3 (C H 2)5C H 3 Molecular formula is a chemical formula that indicates the type of atoms and the correct number of each in a molecule. C6H 14 C7H16 C8H 18 Saturated compounds are compounds in which there are no multiple bonds between C atoms in their hydrocarbon chains. Every carbon atom is attached to two hydrogen atoms, except those at the ends of the chain, which bear three hydrogen atoms. Each C atom is surrounded by the maximum number of H atoms. A saturated compound has no double or triple bonds or ring and contain only covalent single bonds. Alkanes are generally not very reactive but they are combustible and burn well without much soot Thus they are our most important source of fuel and energy ( so called "fossil fuels" ). Homologous series are a series of organic compounds that can be described by the same general formula. OR a series in which one member differs from the next with a CH2 group. All members have the same functional group. The functional group for the alkanes is a single bond in a hydrocarbon. We can represent the alkane homologous series by a general formula : C H All the members of a homologous series have similar chemical behaviour ( alkanes react very slowly ) The physical properties of members change slightly and regularly as their molar masses and boiling points increase steadily as the molecules get bigger. Butane is a gas. molecular model Microscopic representation of atoms and bonds. Alkenes have carbon-carbon double bond structural formula H H molecular formula 4 10 Symbolic representation. F CH2 z CH3 Semi condensed structural formula CH 3 CH 2 An alkene ( or olefin ) is an unsaturated chemical compound containing at least one carbon—carbon covalent double bond. The bond angles in ethene C H General formula for alkenes : C H n Unsaturated compounds are compounds with one or more multiple bonds between C atoms in their hydrocarbon chains. Count the carbons from the left or from the right to the start of the double bond. Use the smaller number. ethene 1200 ( From the left hand side ) ( From the right hand side ) ( From the left hand side ) H H H-C—C—C pent-2-ene H H H H pent-I -ene ( infixed number ) propene pent-2-ene H c H H H H " diene " means two double bonds. c=c=c propdiene H H H propene pent-I ,3-diene Turning a molecule around does not change it. If the double bond is attached to carbon-I from the left and also to carbon-I from the right, then do not use an infixed number as there is no ambiguity ( no chance of muddling up the number ).
  3. 12 KA : Matter and Materials : Organic Chemistry Propene is a gas. • each time a covalent double bond forms, two H atoms are removed But-2-ene • they are more reactive than alkanes. and being combustible they serve as a source of energy • alkenes are unsaturated as they contain one or more covalent double bonds between adjacent C-atoms so each C-atom is not surrounded by the maximum number of H atoms Alkenes decolourise yellow brown bromine water rapidly. To make bromine water : add a few drops of JIK to some dilute HCe to make chlorine water. The yellow green colour of CC2 in suspension can be seen. Pour a little of this chlorine water into a KBr solution in a test tube. The yellow brown ( amber ) colour of bromine water is visible. Br + 2 e- —i 2 cc cc + 2 e- 2 ce- CC + 2 Br- -i Add a little of this Br water to colourless cyclohexene. Shake the mixture. The colour disappears quickly. Jik I-ICC chlorine water KBr 3 CC2 water bromine Br2 water cc Br water If you use cyclohexane you would have to leave the mixture overnight to get rid of the colour as there are no double bonds to quickly snap. The concentration of the I-ICC affects the colour of the bromine water which can range from yellow-brown to red. Instead of bromine water, one could use a purple potassium permanganate solution. The purple colour quickly disappears when added to cyclohexene. cyclohexene We distinguish between saturated and unsaturated hydrocarbons using bromine water. Alkynes have carbon-carbon triple bond are unsaturated hydrocarbons which have at least one carbon—carbon triple covalent bond between two carbon atoms. Unsaturated compounds are compounds in which there are some double and / or triple bonds between carbon atoms and thus they do not have the maximum number of hydrogen atoms attached to the carbon atoms. Alkynes react quickly. 1800 Number from the end closest to the double bond. ethyne or acetylene C2H2 propyne C3 H 4 hex-2-yne C6 HI o • alkynes are unsaturated with a general formula C H n 2n-2 • alkynes are combustible and serve as a source of energy eg. oxy-acetylene blowtorches used for welding and cutting of metals use ethyne ( acetylene ) H Number from the end closest to the triple bond. H H H H H oct-l ,5-diyne Preparation of ethyne . C8 H 10 Cac + 2 H hept-l ,3-diyne Ca (OH) calcium carbide Propyne is a gas. CH C CH 3 ethyne
  4. 12 KA : Matter and Materials : Organic Chemistry 4 Halo-alkanes ( or Alkyl halides ) have halogen atom bonded to a saturated C atom. Most are poisonous. • substitute one or more H-atoms with halogen atoms naming the halogens in alphabetical order • a halogen which replaces a H-atom is called a substituent • ignore prefixes di-, tri- etc when working out alphabetical order • write the whole name as one word putting the halogens first • number from left and right and then choose the smaller numbers • number the longest chain beginning at the end nearest the first substituent whether it is an alkyl group or a halogen CH3 CH2 CH2 CHFCHBr CHCC CH2 CH CH CHBr CH2 CHBr CH2 CH 3 3 4-bromo-3-chloro-5-fluorooctane 2,4-dibromohexane If Br and F are both on the first carbon, from either end, give Br preference as it comes first in the name Use smaller numbers. I-bromo-2-fluoroethane ( alphabetical preference ) H Br H-C—C— F 1 1 1 -bromo-l -fluoro-l ,2-diodoethane The right hand C has more substituents, so it becomes carbon number one. cc -c —c —c -cc CC H 1 , 1 , 1 ,3-tetrachloropropane not 1 ,3,3,3-tetrachloropropane mono di tri tetra cc (2) (3) (4) F — C — CC cc dichlorodifluoromethane CFC's ( ozone killer ) Chlorofluorocarbons. cc cc cc cc dichloromethane less toxic laboratory solvent cc cc tetrachloroethene drycleaning solvent 1 1 1 Primary halo-alkane. Halogen attached to end carbon atom that is only attached to one other C atom. cc cc • adding more halogen atoms make alkyl halides more poisonous, less combustible and less water-soluble • used in fire extinguishers but the poisons they emit are causing a return to CO fire extinguishers cc cc cc alkyl halides are used as solvents and anaesthetics trichloromethane or chloroform ) • also used as refrigerants and as a domestic cleaning agent ( oven window cleaners, toilet bowl cleaners ) • CCC is non-polar, does not dissolve in water and does not burn but is very toxic. Banned as an ozone killer by the Montreal Protocol of 1986. All CFCs were also banned. tetrachloromethane / carbon tetrachloride cc cc cc cc cc cc
  5. 12 KA : Matter and Materials : Organic Chemistry Substituents ( Side chains and atoms ) 5 A substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon. A substituent is a piece that is stuck onto a carbon chain, that is longer than the substituent, and gives rise to a branched chain if the substituent has one or more carbon atoms ( an alkyl group ). For simplicity we can • the longest possible chain of C atoms gives the name of the compound • straight chains and branched chains are called aliphatics CH 4 CH — 3 2-fluorobutane F is a substituent. Secondary halo-alkane. Halogen is attached to a C atom inside a chain that is joined to two other C atoms. CH CHF CH CH 3 pretend that all the bonds are at right angles. methylbutane Butane is the main chain. The methyl group is a substituent and a side chain ( an alkyl group ). CH3 CH(CH3) CH2 CH 3 C5 HI 2 Alcohols ( alkanols ) have hydroxyl group bonded to a saturated C atom. Alcohols are organic compounds containing the hydroxyl group. The hydroxyl group is Primary Alcohols (C atom at the end of a chain ) A primary alcohol is one in which the carbon having the - OH ( hydroxyl ) group is bonded to only one other carbon atom. H H propan-l-ol (C H OH ) Number from the end closest to the OH group. Secondary Alcohols (C atom inside a chain ) In a secondary alcohol the carbon to which the hydroxyl group is attached is also bound to two other carbon atoms. H H—C—C H H o O O —OH is the functional group. O O O propan-2-ol (C H OH ) Tertiary Alcohols need a side chain In a tertiary alcohol, the carbon which holds the hydroxyl group is bonded to three other carbon groups. methylpropan-2-ol (C H OH ) Identical molecules O O methanol CH OH or CH O This formula describes all four molecules. Rotate the methanol molecule anti-clockwise. This does not change the molecule. parent group propan-l -01 functional group R- OH can represent any alcohol. R represents any C chain or alkyl group.
  6. Counting from the left, the OH is attached to the third C atom. Counting from the right, the OH is attached to the first C atom. Rule : always count from the side that is closest to the functional group. So the right hand C-atom is number 1. A functional group is a reactivity centre. A functional group is a bond or an atom or a group of atoms that determine(s) the physical and chemical properties of a group of organic compounds. The functional group is the cause of the similar chemical behaviour of all the members of a homologous series. Homologous series are a series of similar compounds that have the same functional group and whose consecutive 12 KA : Matter and Materials : Organic Chemistry Alcohols are formed when one hydrogen from an alkane is substituted for, by a hydroxyl or — OH group. Alcohols are combustible and serve as a source of energy. H-C—C-O methane CH 4 methanol CH OH ethanol (wine ) Methane becomes methanol. H H-C— H Notice that the alcohols are named by dropping the final "e" of the alkane name and adding "ol": H H H—C—C butan-2-ol The 2 is an infixed number. The OH is attached to the second C-atom. O molecular model butan-2-ol 6 C2H50H c—c—o H H ethan-l ,2-diol members differ by — CH 2 Making ethanol ( the natural product when sugar or starch ferments ) : CH O 2 C H OH (ethanol) +2 CO 6 12 6 2 CO is a greenhouse gas that increases global warming. In fermentation, bacteria, yeasts, or moulds act on sugar and break it down into ethanol and carbon dioxide: The enzyme zymase does the breaking down of the sugar. Carboxylic acids (weak acids K < 1 ) ) Oxidation occurs when adding oxygen to a molecule. Wine can easily go sour if exposed to the dioxygen (O ) in the air, especially if warm. ethanol Two thirds of the Earth is covered in water, so water loves to form in a reaction. H and OH are lost by the same C-atom which can now form a double bond. ethanoic acid The sour vinegar lasts indefinitely. Vinegar was the staple ration for the Roman wine H vinegar army. carbonyl group One O atom from O 2 in the air joins this double bond on C H -c O aldehyde H O —C— H H H air The fruity aroma of wine is due to the aldehyde ethanal that forms as wine oxidises But aldehydes are a toxin ( poison ) to the liver. The other O is added. An O-atom just snips a single bond and uses its two bonds to re-connect. ( oxidation ) H —C —C—H +0+ H H H ethanoic acid / vinegar Fermentation of starch or sugar by enzymes causes alcohol ( ethanol ). Over fermentation leads to acid formation.
  7. An acid catalysed condensation reaction between an alcohol and a carboxylic acid produces an ester and water. 12 KA : Matter and Materials : Organic Chemistry Esterification Reactions H-C—C—O H—C—C—O 7 Carboxylic acids end in —COOH ( called an oic acid" ) The functional group for carboxylic acids is — COOH ( a carboxyl group ) This O draws the electron density 4from the C, further polarising the O—H bond and making the H more positive. O methanoic acid HCOOH Esters H O vinegar ethanoic acid CH COOH —c—o/ \ The molecule donates this H + when acting as an acid. butanoic acid C H COOH ( responsible for bad odour of rancid butter ) Add an alcohol surplus to a carboxylic acid in the presence of a little cold concentrated sulphuric acid. An ester is the product of an acid catalysed condensation between an alcohol and a carboxylic acid. 5 me ethanol 4 me methanoic acid in a tall test tube 2 me H2S0 4 paper add slowly towel Methanoic acid plus ethanol 0 mixture. Heat in a water bath at 600C for 10 - 15 minutes hot water ester cold water Marble chips keep the temperature uniforn by absorbing hot spots so that it stays at 600C heat Water bath keeps alcohol fumes away from flame. High temperatures can degrade the ester. The sulphuric acid, a dehydrating agent, acts as a catalyst ( H + ) which splits out the water. Heat for about 10 - 15 minutes in a water bath with a piece of paper towel soaked in cold water wrapped around the upper part of the test tube with a rubber band which helps condense the escaping ester fumes. • if vapour bubbles form in the test tube, add some inert porcelain chips to dampen things down • cool the test tube in a beaker of cold water and smell the ester. ( adding about 10 drops of the base sodium carbonate, to neutralise the excess acids, may help ) • pour the liquid into cold water to absorb any smell of the sulfuric and ethanoic acids and alcohol fumes • the oily ester floats to the top. You can smell it. Esters cause identifiable aromas ( like the fragrant smell and odour of fruit ) and are used in perfumes. When the alcohol was made from radioactive oxygen-18 then no traces of radioactivity were found in the water that formed as a product. Thus the oxygen in the water does not come from the alcohol. It must thus come from the acid. The concentrated sulfuric acid extracts an H and an OH to form water. alcohol donates H acid donates OH ( to form water ) H H H H H o double bond ( acid ) o C—O—C H ester H H H SO 4 ethanol + methanoic acid ethyl methanoate. Boiling point 540C H Ethyl ethanoate boils at 770C. It is very volatile. Esterification is an acid catalysed condensation between an alcohol and a carboxylic acid. The reverse reaction is hydrolysis. Split a water molecule and add the two parts to two subunits of an ester molecule to form an acid and an alcohol. Naming esters Alcohol first , ending in - yl ( ethyl ) acid comes second and ends in - oate. Eg. Hexanol and propanoic acid Functional group for esters . ethyl butanoate ( pineapple aroma ) methyl butanoate ( apple aroma ) H H O H H ester group H H —c— H H H H O alcohol R—O—C—R' acid R and R' represent alkyl groups Eg. methyl. ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl hexyl propanoate
  8. • the ketone functional group is . 12 KA : Matter and Materials : Organic Chemistry Ketones have carbonyl group bonded to two C atoms. — C = O attached to two alkyl groups ( also called the keto group ) are formed by the carbonyl group • ketones usually have aromas that resemble food —c—c I-I-C —c—c I-I-C —c-—c 8 O c (R and R ' represent any alkyl chain eg. methyl, ethyl etc. ) Ketones have 2 alkyl groups ( R and R' ). Ketones are never on an end C. H c H H O H propanone pronounced " propa known ( simplest ketone ) H H H H pentan-2-one O Condensed structural formula does not show all the bonds • the ketone functional group is on the second carbon ( or any other inner carbon atom ) • the ketone double bond is never on an end carbon • number the longest chain beginning at the end nearest the carbonyl group ( or keto group ) CH3CCH2CH2CH 3 O Aldehydes have formyl group are formed by the carbonyl group and a H. ( also called the aldo group ) O To form an aldehyde, shift the double bond holding the Thus an aldehyde only has one alkyl group ( R ). The C is always an end C. O atom ( the carbonyl group ) to an O R END H —c atom. O O H CHO methanal H H-C CH3 CHO ethanal CH3 CH CHO propanal • aldehydes usually have aromas that resemble food • number the longest chain beginning at the end nearest the aldo group I-I—C H H H H H O hexanal general formula for both : C H O n 2n ketones and aldehydes Structural isomers are organic molecules with the same molecular formula, but different structural formulae. They are different compounds with different properties ( eg. different boiling points ) Positional isomers have the same molecular formula but different positions of the side chain, substituents or functional groups on the parent chain. Use numbers to differentiate the C atoms Count the C-atoms from the side closest to the double bond Separate letters and numbers with a hyphen. Same functional group appears in a different position. c=c—c H H but-I-ene C H 4 8 H H H H F H H H H H butan-2-ol CHO 4 10 2-fluorobutane O C H F H H H H H H H H—C but-2-ene C H butan-l -01 1 -fluorobutane
  9. The ketone and aldehyde can also be regarded as functional isomers as they involve different functional groups. Chain isomers have the same molecular formula but different types of chains. The atoms are arranged in different chains. 12 KA : Matter and Materials : Organic Chemistry Functional isomers have the same molecular formula, but different functional groups. Isomers are formed by different functional groups c— —c —c—c—c—c—c—o 9 H H H H H propyl ethanoate pentanoic acid Esters and carboxylic acids form isomers H H-C H general formula for both molecular formula for both —C — C —C — H butan-2-one H 5 10 2 H H—C—C—C H H H H butanal CHO H H H H H H c H H pentane H H methylbutane dimethylpropane Ethyl and Methyl Substituents ( Side chains ) 2-chloromethylpropane Tertiary halo-alkanes need a side chain. In a tertiary halo-alkane, the carbon which holds the halogen atom is bonded to three other carbon groups. alkyl halides H CC — C —C H H ( halo-alkanes ) H H H —c—c H c Number from the left H H H H H 1 -bromo-l , 1-dichloro-4-ethyl-3, 7-dimethyloctane Number the longest chain from the end closest to the first substituent regardless of whether it is an alkyl or a halogen. • halogens in alphabetical order • ethyl before methyl ( alphabetical order ) * halogens come first in the name, then alkyl groups 2,7-dibromo-2-chloro-4-ethyl-5-methyloctane Number from toe right 2,7-dibromo-7-éhlprb-5-ethyl-4-methyloctane Longest chain can be numbered from either side. Give alphabetical preference to substituents Choose the lower number where the first number difference occurs i.e. 2 rather than 7 for chloro. 1 , 1-difluoro-2,3-dimethylbut-2-ene H H 1-bromo-4-ethyl-3-methylhexan-1-01 H H LIMITS per compound . 1 functional group, 2 halogens 3 ethyl, methyl side chains H H H H H H Number from the end nearest to the OH functional group.
  10. 12 KA : Matter and Materials : Organic Chemistry Electronegativity is the ability of an atom in a covalent bond to attract electrons to itself. 10 Polar Covalent bonds form a continuum of electron pulling abilities ( ionic ) on shared ( covalent ) electron pairs. Thus all bonds have covalent and ionic character. Pure covalent bond between identical atoms. Electrons shared evenly between two atoms. 1 1 The two atoms form a neutral molecule. 11 Non-polar molecule. For identical atoms in a diatomic molecule eg.l-l or 1 2 AEN= O Non-polar bond Any two different atoms will have an uneven sharing of electrons and thus form a polar covalent bond. Difference in electronegativity scale, AEN : This illustrates what the bonds look like between the atoms. Polar covalent uneven sharing of electrons AEN = C very weakly polar covalent bond C has a slightly stronger pull on the shared electron pair than H. 1 Bonds between atoms . weak bonds Type of molecule formed . Non-polar molecule. The charge distribution in C - H bonds cannot be completely symmetrical, but the resultant bond polarity is generally regarded as trivially small. Ignore these slight positive and negative charges at the poles. Regard the molecule as if it has no polarity. Hence in hydrocarbons the description non-polar is applied. O polar covalent bond O has a much stronger pull on the shared electron pair than H. strong bonds A dipole. Polar molecule. Slight positive and negative charges at the poles are noticeable. Very polar covalent Very uneven sharing of electrons. "Transfer" of electrons. ( called "ionic" bond ) very polar covalent bond very strong bonds Ion-pair in contact. Ionic network An ion is a particle that has a single charge, either positive or negative A dipole is a polarised molecule that has partial ( slight ) positive and negative charges. Dipole-dipole forces form between two polar molecules. C-H bond : AEN = - - This produces very weakly polar covalent bonds between the atoms. Thus the hydrocarbon molecules are considered to be non-polar as their bond polarity is trivial. Intermolecular forces All molecules form very weak London dispersion bonds or induced dipole forces between molecules that are also called Van der Waals forces. Larger non-polar molecules form stronger Van der Waals forces between molecules. Weak Van der Waals forces between non- polar molecules. Not water soluble. stronger Van der Waals forces The weak Van der Waals dispersion forces depend on the surface area of the non-polar molecule. Increasing the length of the chain increases the surface area of the molecule. Then individual molecules can attract each other better. The strength of the Van der Waals forces is proportional to the surface area of the molecules.
  11. 12 KA : Matter and Materials : Organic Chemistry Van der Waals intermolecular forces As the non-polar hydrocarbon chains get even longer, there will be many regions where they can line up alongside each other and the weak Van der Waals forces accumulate their strength to exert a noticeable attractive effect. As the chain length increases even further these longer molecules become wrapped around each other and get tangled up like strands of spaghetti. Much more energy is then needed to separate them compared to the short chain molecules that only had weak attractive forces between them. Thus their boiling points increase with chain length. tangled long chain molecules TOPIC Physical properties of saturated hydrocarbons depend on the structure of the molecule Eg. the chain length Boiling points indicate the strength of the forces between molecules. The stronger the molecules stick together, the more energy it will take to rip them apart and blast them off into the atmosphere as separated gas molecules. Boiling points of the saturated hydrocarbons, or Alkanes Name methane ethane propane butane Molecular Formula CH 4 10 Melting Point CC) -183 -183 -190 -138 Boiling Point CC) -164 -89 —42 State at 250C gas Boiling points below 250C Boiling points above 250C means they have already boiled and are thus gases. means they are still liquids as they have not yet begun to boil and move apart to form gases pentane hexane heptane octane 5 12 C6H14 7 16 8 18 -130 -95 -57 Boiling point 36 69 98 125 Boiling points increase more uniformly with increasing chain length than melting points. In melting points, only some of the bonds break to enable molecules to slide past each other. The bonds that do not break can cause complications. In boiling points, all the bonds have to break to form gas molecules so the situation is much simpler. Example : Solid ethane forms rectangular plates in layers that are closely packed. The atoms are close and strongly bonded. Solid butane forms hexagonal plates with bigger atoms that are also closely packed in layers. solid ethane solid propane solid butane In between is solid propane which forms five-sided plates that cannot pack closely together. Thus the solid molecules are further apart and more weakly held together which explains its lower melting point. 120 100 80 60 40 20 -20 -60 -80 -120 -140 -160 liquid number of C-atoms in chain But in general, melting points increase with longer chain length. Short chain molecules have low boiling points which mean that they readily evaporate to form gas molecules. Lots of gas molecules give them a high vapour pressure as all the gas molecules collide with the sides of the container. Vapour pressure increases with shorter chain lengths. Smell or aroma is due to vapour molecules entering the nose. Liquids with low boiling points evaporate readily and are called volatile. These gas molecules convey aroma. Aroma increases with shorter chain lengths. So does flammability as it is the vapour that burns.
  12. 12 KA : Matter and Materials : Organic Chemistry Larger non-polar molecules have higher boiling points than similar smaller molecules. Boiling points of alkanes 12 boiling po- t ( oc ) butane propane — 45 ethane -89 ' methane - 161 30 44 16 58 relative molar mass M Effect of intermolecular forces The strength of the induced dipole forces increases with molecular size. A temporary dipole induces a temporary dipole in a molecule that is close to it. Increasing the molecular mass means adding more carbon atoms into the chain. This increases the number of electrons surrounding the molecule. This increased electron density thus increases the strength of the intermolecular van der Waals forces. Bigger molecules have more electrons, so more distortion of the electron cloud occurs forming more dipoles and thus more points of electrostatic contact form, requiring more energy to snap these bonds. Thus they have a higher boiling point. London dispersion forces are forces of attraction between fluctuating dipoles in all types of atoms and molecules that are close together. These are also called weak Van der Waals forces. Stronger intermolecular forces require more energy ( thermal energy ) to break them. Larger molecules thus tend to have higher melting points and boiling points and increased viscosity ( less runny liquids ) Boiling point is the temperature at which the vapour pressure of a substance equals the atmospheric pressure. Volatile liquids evaporate fastest • and have high vapour pressure • and low boiling points Consider three liquids in closed containers that are all at 250C. The liquid with the weakest intermolecular forces has the lowest boiling point and evaporates most to produce the most vapour molecules. The more vapour molecules there are, the more they collide with the sides of the container to produce a higher vapour pressure. A liquid in a closed container will evaporate until the rate of evaporation of the liquid is equal to the rate of condensation of the vapour. This is called dynamic equilibrium. Put equal volumes of three liquids into 3 identical closed containers. At equilibrium, the visible level of liquid in each container remains constant. least volatile lowest vapour pressure little vapour evaporate condense faster evaporation and condensation heptane at 250C high boiling point 980c 7 16 hexane at 250C boiling point 690c C6H14 most volatile highest vapour pressure most vapouro,o pentane at 250C C H 5 12 low boiling point 360c Stronger intermolecular forces cause stronger bonds and less evaporation, thus a lower vapour pressure. Weaker intermolecular forces cause weaker bonds and more evaporation occurs resulting in a higher vapour pressure. Vapour pressure is caused by the gas particles colliding against the sides of the container. Straight chains have higher boiling points. Branched chains can have a lower boiling point due to a smaller area of contact. The molecules are more compact and cannot get too close together, resulting in weaker Van der Waals Pentane The molecules have a larger surface area of contact, and thus more delocalised electrons, close to each other. Stronger Van der Waals forces develop. ( Represented by the dotted lines ) Boiling point - - 360C Methyl butane forces. Boiling point = 280C
  13. 12 KA : Matter and Materials : Organic Chemistry Van der Waals forces are surface-area dependent. Branched chains have a lower boiling point due to a smaller area of contact. 13 As the molecules get more compact only smaller portions can get close to each other resulting in weaker Van der Waals forces. Increased branching decreases the boiling point. The closer a molecule is to a sphere the smaller its surface area is and the fewer Van der Waals forces operate between the molecules. The more rod-like the molecules are eg. pentane, the better they are able to line up and bond together. As an analogy, macaroni is more spherical and does not stick together as strongly as the more rod-like spaghetti does. Dimethyl propane Boiling point = 90C Thus physical properties are influenced by the degree of branching of the chains. Hydrogen bonding is a strong intermolecular force. A special case of dipole-dipole forces. 3+ 3+ H This polar molecule is slightly positive at the H-atoms and has a slightly negative pole at the O-atom. The water molecule has a characteristic bent shape. The H-atom with its partial charge 8+ must line up in a straight line between the Ione pairs on two O-atoms that are on two separate water molecules. This minimises the repulsion due to the electron pairs that are on either side of the H-atom as those two electron pairs would then be furthest apart from all the other electron pairs. dotted line is a hyrogen bond water molecule The electron pair on the right hand water molecule is repelled by two electron pairs on the left hand molecule when the dotted line hydrogen bond is at an angle to an electron pair. This extra repulsion weakens the hydrogen bond. The H-atom has a small volume and thus the small partial positive charge on the H-atom nevertheless has a large charge density which creates a strong bond between molecules. Each H-atom, due to its high charge density, can form a hydrogen bond and the O-atom having two lone pairs, can form two hydrogen bonds with two other hydrogen atoms. Each water molecule can form FOUR hydrogen bonds. In water these hydrogen bonds break and reform trillions of times every second so about 2,4 to 3,6 hydrogen bonds form at any one time, on average, between a water molecule and its neighbours. H Thus the hydroxyl group OH is a source of hydrogen bonds. Hydrogen bonding is due to forces between molecules in which hydrogen is covalently bonded to nitrogen, oxygen or fluorine. Hydrogen bonding is a special case of dipole-dipole bonding. Dipole-dipole forces exist between two polar molecules. Induced dipole forces or London forces exist between dotted line represents a hydrogen bond water molecule non-polar molecules. intermolecular forces H interatomic forces or chemical bonds The strongest dipoles involve the H atom bonding to N, O or F. These atoms have a high electronegativity value and by strongly pulling the bonding pair of electrons closer to themselves they become slightly negative. But these atoms are in the second row of the periodic table which means that their valence electrons only occupy the second electron orbit. This means that these atoms are very small. So this slight or partial negative charge due to a Ione pair is confined to a very small volume which results in a high charge density. Water's unique features are due to this strong hydrogen bonding in solid, liquid and gaseous water. As water runs over underground rocks, the stickiness of water slowly wears down the rock surfaces and thus water is able to extract minerals from the rocks. Over a length of time water becomes almost a universal solvent and healthy mineral water bubbles up from underground springs.
  14. Matter and Materials : The strong electronegativity of the O-atom in water turns water into a polar molecule that can produce hydrogen bonds. The very high boiling point of water, compared to the other similar molecules in the same group ( or column ) of the periodic table, indicate how strong the hydrogen bond is as an intermolecular force, compared to the other intermolecular forces such as dipole-dipole, dipole-induced dipole and induced dipole-induced dipole and London dispersion forces. All these other intermolecular forces are called Van der Waals forces and are much weaker than the hydrogen bonds. BUT hydrogen bonds are much weaker than covalent bonds. 12 KA: I-I-c H-C —c —cc Organic Chemistry 14 boiling point ( oc ) of group 16 hydrides 100 50 -50 H2Te molar mass ( g ) 20 40 80 130 Hydrogen bonds can form between alcohol molecules as they have an OH functional group. These strong intermolecular forces raise the boiling point of ethanol to 780C. H H ethane H c H H H Ethane can form no hydrogen bonds. Very weakly polar covalent bonds form between the C and H atoms as LEN = -2,1 Thus weak Van der Waals forces exist between the ethane molecules. Boiling point of ethane is —890C. Chloroethane can form no hydrogen bonds. Very weakly polar covalent bonds form between the C and C! atoms as AEN= 3 -2,5 = < 1. Thus weak Van der Waals forces exist between the ethane molecules. ethanol Ethanoic acid has the highest boiling point 1180C. But C! has far more mass than H. Molecules with more mass have stronger Van der Waals forces due to their bigger surface area. CC has a stronger electronegativity which makes this molecule a stronger dipole which causes stronger dipole-dipole intermolecular forces. Boiling point of chloroethane is 120C. This O-atom draws the electron density from the C. In a sense, it also helps the other O-atom to pull on the shared pair of electrons between the O-atom and the H-atom on the right hand side. This further polarises the O—H bond, making the H-atom more positive as there are ffectively two O-atoms pulling on electron pair between O and H. This stronger polarity creates stronger hydrogen bonds on the O-H site as well as a stronger dipole on the O-atom with the double bond. The molecule is more polar due to the double bond which causes additional dipole - dipole intermolecular forces The carboxyl group ( COOH ) of a carboxylic acid is more polar than the hydroxyl group ( OH ) of an alcohol. Thus physical properties of substances depend on the intermolecular forces and the number and type of the functional groups.
  15. 12 KA : Matter and Materials : Organic Chemistry Ability to form stronger hydrogen bonds affects the solubility of substances in water 15 methanol ethanol C H OH CH3CH OH propan-l -01 CH CH2CH OH butan-l -01 CH CH CH CH OH These 4 alcohols are water-soluble ( hydrophilic i.e. love water ) because the hydroxyl group forms hydrogen bonds with water. • the melting points and the boiling points increase proportionally when molar mass increases with the chain length the molecule has a non-polar end, where there is no -OH group, that forms weak Van der Waals forces and thus mixes well with other non-polar substances eg. pentan-l-ol CH CH CH2CH CH OH polar head non-polar tail The same happens with ketones As a ketone gets longer, it gets less soluble as the non-polar alkyl group gets larger and more oily which cancels out the solubility of the polar carbonyl group and makes the molecule non-polar rather than polar. c O O water CH propanone acetone ( highly water soluble ) Hydrogen bond can form between a water molecule and the oxygen atom in a ketone butan-2-one is 25,6 % water-soluble. pentan-2-one is 5,5 % water-soluble octan-2-one is not water-soluble at all. CH CH CH CH CH CH 2 non-polar alkyl group CH 3 c polar carbonyl group O Aliphatic ( straight chain ) alcohols with 5 or more carbon atoms are not water-soluble as the non-polar alkyl group is large enough to make the molecule overall more non-polar than polar. The longer alcohols look more like alkanes. Longer alkane chains get more oily. More of the hydrogen bonds in water must be broken to make room for the longer hydrocarbon chain. The number of hydrogen bonds between water molecules that are lost or broken when the longer alcohol molecule mixes into the water are not completely compensated for by the hydrogen bonding of the water molecules to the alcohol's -OH functional group. Thus the solubility of the alcohol decreases as its carbon chain gets larger. But the shape of the hydrocarbon portion is important. Side chains make the molecule more compact and thus they require less room and break fewer of the water hydrogen bonds in their aqueous solutions. So alcohols with more side branches are more miscible ( soluble ). methylpropan-l -01 11% water-soluble. CH 3 CHG-CH- CH2-OH methylpropan-2-ol Infinitely water-soluble. CH 3 CH -C- CH 3 OH The more hydroxyl groups an alcohol has, the more polar and the more water-soluble ( miscible ) it becomes. anti-freeze or ethan-l ,2-diol glycerine or glycerol (a moisturiser) or propan-l ,2,3-triol o H o H o —C—H Highly soluble glucose ( a source of quick acting energy for athletes ) has 5 hydroxyl functional groups on a molecule. High viscosity means a fluid has little flow because the particles are stuck together by stronger bonds. Thus glycerine and honey do not flow well. Stronger intermolecular forces produce liquids that have a greater viscosity. They are more sticky. Melting point is the temperature at which the solid and liquid phases of a substance are in equilibrium. Boiling point is the temperature at which solid and liquid phases of a substance are in equilibrium. Stronger intermolecular forces produce higher melting points and boiling points. Vapour pressure is the pressure exerted by a vapour at equilibrium with its liquid in a closed system. Stronger intermolecular forces produce lower vapour pressure.
  16. CH3CH2CH2CH2CH20H I-pentanol. Boiling point - - 1370C 3-pentanol. Boiling point - - 1150C Physical properties Carboxylic acids 12 KA : Matter and Materials : Organic Chemistry —c 16 A hydrogen bond-forming functional group inside a chain is less soluble than an "exposed" function- al group on the end of a chain weaker bonding ability stronger bonding ability If the ketone functional group is in the middle of a long chain it is even less soluble as the non-polar sections can fold around the polar carbonyl group on both sides and stop water molecules from bonding with it. When the functional group for an alcohol is in the middle of a chain, it forms fewer hydrogen bonds with other identical alcohol molecules. CH CH 3 CH CH 2 pentan-2-one is 3 2 pentan-3-one is 4,8 % water-soluble O 3 5,5 % water-soluble OH CH CH2CH CH CH 2 The -COOH end has stronger hydrogen bonds between molecules and it also mixes with water for members with 1 to 4 C-atoms. The other non-polar alkyl end forms Van der Waals forces which overpower the effect of the hydrogen bonds as the acid chain grows longer. So acid chains with six C-atoms, or longer, don't mix with water. methanoic acid HCOOH ethanoic acid CH3COOH butanoic acid CH3CH2CH2COOH very soluble pentanoic acid hexanoic acid CH3CH CH2CH2COOH CH3CH CH2CH CH2COOH very soluble very soluble hydrophilic ( loves water ) Esters slightly soluble insoluble hydrophobic alkyl group ( hates water ) Produce "-oate" salts, with formula written back-to-front eg. sodium ethanoate CH COONa. Esters have London forces and dipole-dipole forces so the melting and boiling points are lower than for carboxylic acids. There is no OH functional group to form hydrogen bonds between ester molecules. Dipole-dipole forces are uppermost for short chain esters, making them soluble. Van der Waals forces are uppermost for long chain esters so they do not mix with water. Physical properties and intermolecular forces are influenced by 1 ) number and type of functional groups 2) chain length Cycloalkanes C H n 2n 3) branched chains. The methyl substituent is carbon-I . C6 HI 2 c H cyclohexane c c H c c c C c H H H H c c H c Cycloalkenes c c c ethylcyclobutane ( double bond is always C on carbon-I ) c X...SH methylcyclohexane c c I-fluorocyclopropene cyclohex-l ,3-diene 1 ,2-dibromocyclopentene
  17. 12 Alkane Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane KA : Matter and Materials : Organic Chemistry Density ( g•cm-3 at 200C ) gas gas gas gas 0,626 (liquid) 0,659 (liquid) 0,684 (liquid) o, 703 (liquid) 0,718 (liquid) o, 730 (liquid) 17 Formula CH 4 4 10 5 12 C6H14 7 16 8 18 9 20 10 22 Boiling point -162 -89 —42 36 69 98 126 151 174 Melting point -182 -183 -188 -138 -130 -95 -91 -57 -54 -30 Longer carbon chains have greater CONTACT surface area, thus more places to set up temporary dipoles, thus stronger London forces. temperature ( C ) 400 300 200 100 -100 -200 -300 Wikipedia. boiling point melting point 16 14 12 Number Of carbon atoms For the melting point, there are still many bonds between the liquid particles. These can complicate things. For the boiling point, all the bonds have to break to produce freely-moving gas atoms. The increase in bond strength with the lengthening of the carbon chain is thus far more uniform. Combustion Reactions Fossil fuels are very important in terms of their ability to transfer chemical potential energy into thermal energy which has a heating effect. Combustion reactions are exothermic. When the combustion of alkanes, alkenes and alcohols takes place in the presence of excess oxygen ( 02 ) the products that are produced are water and carbon dioxide.
  18. If there is not an excess of air then CO ( very poisonous ) and finally C ( black soot ) are also products. Alkanes are our most important fuels. The combustion of alkanes is highly exothermic and carbon dioxide and water are produced Ethanol is the natural product that forms when sugar or starch ferments. Alcohols, like hydrocarbons, are combustible forming H20 and CO Balance C H OH + 02 CO + HO 12 KA : Matter and Materials : Organic Chemistry TOPIC Applications of organic chemistry Burning hydrocarbons ( in excess dioxygen) very exothermic AH < 0 • products are always CO and H O • first balance C, then H, then O by inspection (juggling the numbers till they are equal on both sides ) Balance C H + 02 CO + HO —c—c— 18 5 balance C . balance H : balance O . Balance C 5 balance C . balance H : balance O . 12 C H + 02 5 CO + H O 5 12 5 12 10 5 10 5 10 02 5 CO + 6 CO + H O 02 5 CO + 5 c 5 Combustion reaction H +802 —>5C0 +6 HO 12 AH < O. Double everything CH +7,50 —>5C0 +5 HO 5 10 or 2CH +15 02 —HO co +10 HO 5 10 AH < 0. balance C then H then O C HOH +302 2 CO + 3 HO AH < 0. ( do not forget the O and the H on the alcohol ). We classify reactions according to the structural change that occurs 1. Addition reactions If a double bond between two C-atoms snaps, we can add two monovalent atoms to this molecule. Addition reactions are reactions where atoms are added to a molecule by the breaking of a double or triple bond. between the carbon atoms without the removal of any atoms from the molecule. Thus unsaturated compounds undergo addition reactions to form saturated compounds. • double bond snaps to make 2 single bonds unsaturated compounds become saturated ( a ) Hydrogenation is the addition of hydrogen to an alkene. (The adding pt c=c of H atoms to saturate a compound ) 6 ethene ethane The non-polar ethene is dissolved in a non-polar solvent with a catalyst ( platinum, palladium, or nickel ) in an hydrogen atmosphere. pt PCI The gaseous ethene becomes far more concentrated, increasing reaction rate,when dissolved i.e. when in a liquid form. Making margarine. ( an industrial process using hydrogenation ) • bubble hydrogen gas through sunflower oil • the temperature is 3000C • the catalyst used is nickel Hydrogenation produces saturated hydrocarbons with larger molar masses and higher melting points, thus tending to be solids rather than liquids. O nickel sieve H gas 000 2 sunflower oil heat 3000C 2 As the double bonds snap the margarine becomes saturated with H-atoms, and gets harder. Polyunsaturated margarine has more double bonds and is softer.
  19. 12 KA: Halogenation. Matter and Materials : Organic Chemistry is the reaction of a halogen ( Br , ) with a compound. 19 (b) H-C-C-O + H SO Br-C—C-Br C2 H 4 Br2 1 ,2-dibromoethane Any of the diatomic halogens can be represented as X 2 where X = F or CC or Br or I. ( c) Hydrohalogenation. Reaction conditions : The addition of a hydrogen halide to to any alkene. 2-bromopropane H H About 70% of this major product forms HX is added to the alkene. The acid must be water-free or concentrated. No water may be present. No water must be present or else an —OH will add on instead of the —Br Br only has an electronegativity of 2,8 compared to O with EN = 3,5. Thus the O in —OH is more reactive than Br. Markovnikov's rule • the H atom looks for the most "friends" and attaches to the C atom that already has the greater number of H-atoms ( in this case it will be the end carbon ) • the halogen atom (X) attaches to the C atom that has the more substituted atoms i.e. the least number of H-atoms H H Br H-C—C—C—H 1 -bromopropane Only about 30% of this minor product usually forms. ( d ) Hydration is the addition of H O to a compound.. Hydration is the splitting the water molecule and adding the two parts across a double bond to form a single product, an alcohol. A small amount of concentrated sulfuric acid or phosphoric acid as a catalyst at 3300C and 60 atmospheres. Pass gaseous ethene and steam over a H SO 4 or H PO 4 catalyst at 3000C. and 6 MPa. High pressure is needed to compress the gases together. H H conc H2SO 4 c=c H ethene H H H H The water is in excess. A small amount of HBr or HCC can also be used. ethanol The sulfuric acid is a catalyst that takes part in the reaction and then is recovered unchanged at the end. The H SO splits into H— and — HSO which adds across the double bond, by snapping the double bond. The —HSO then grabs a H from the water to re-form H SO which is thus recovered unchanged at the end, making it a catalyst. H H c = c + H-HSO H H H -C -C- I-ISO H H H H H H H-HSO 4 Cyclohexene will rapidly decolourise bromine water. Add a little yellow-brown Br2 water to colourless cyclohexene. Shake the mixture. disappears quickly as a reliable test for double bonds. The colour H H H Br water cyclohexene H yellow-brown colourless cyclohexene H colourless H 1,2-dibromocyclohexene
  20. 12 KA : Matter and Materials : Organic Chemistry 20 Organic compounds made by people at home include alcohol which is caused by enzymes like zymase that ferment starch found in malt or rice or sorgum beer or that ferment sugar found in grapes, plums, peaches etc. Over fermentation leads to acid formation as the alcohol is oxidised to form a carboxylic acid. H-C-C-O O 2 H-C-C-O O Gluten is a sticky protein found in wheat, barley and rye. ( Millet, brown rice and buckwheat are gluten free ). Gluten is an elastic protein that the body finds difficult to break down. Soaking wheat, rye or barley in excess water in a sealed container for 2 days makes them ferment which helps to partly break down the gluten. Then the intestines can extract more nutrients. Sour porridge is made by soaking mealie meal ( crushed corn ) in excess water, stirring and leaving it in a sealed container for 5 days until it has fermented. This helps to break down the cellulose in the mealie pip outer covering which our bodies cannot digest. Pour off the excess water and stir the fermented mealie meal into boiling water with a little salt. Turn down the heat and cook slowly for 15 to 20 minutes while stirring. The body can gain far more nutrients from the fermented maize which is why sour porridge has been a staple diet for centuries. It is very good for our health. 2. Elimination reactions This reaction turns single bonds between carbon atoms into double bonds. Saturated compounds undergo elimination reactions to form unsaturated compounds. Eliminate atoms or groups of atoms from two adjacent carbon atoms in a saturated single bond compound ( alkanes, haloalkanes and alcohols ) in order to form unsaturated ( multiple bond ) compounds. Elimination or cracking of alkanes, haloalkanes and alcohols Dehydrogenation reaction is the elimination of hydrogen from an alkane. Saturated ethane is cracked to unsaturated ethene and H2 (g) H —C —C—H pt A 8000C c=c Single bonds are strong. At a very high temperature the C atoms vibrate so strongly that each C atom can shake offa H atom. This is a gas phase elimination reaction driven by high temperature. CH3CH3 (g) CH2 = CH2 (g) + 1-12 (g) Platinum is used as a catalyst. Large straight-chain alkanes can also be cracked to make shorter chained alkenes and branched alkanes. Cracking is the chemical process in which longer chain hydrocarbon molecules are broken down to shorter more useful molecules. • catalytic cracking requires lower temperatures and pressures in the presence of a catalyst • thermal cracking requires high pressures and temperatures without a catalyst Dehalogenation reaction of haloalkanes is the elimination of hydrogen and a halogen from a haloalkane. H H c=c + ZnBr 2 heat Reaction with zinc dust. Zinc is very reactive as it is high on the Ee table which tells us that it forms an ion Zn2+ with two bonds. Thus Zn can attract the two Br atoms. The empty bonds left by the Br atoms snap together to form the double bond. Acid catalysed dehydration of alcohols is the elimination of water from an alcohol. Heating of ethanol with an excess of concentrated sulfuric or phosphoric acid. Concentrated sulfuric acid ( or phosphoric acid ) is a dehydrating agent that absorbs water by removing an H + and an OH- from a molecule and then combining them to make water. Acid catalysed dehydration of ethanol by heating the alcohol with an excess of concentrated H SO (or H PO ). conc H SO c=c O dehydration reaction A We can get the same result by passing ethanol vapour over a heated catalyst such as AC203 powder. CH3CH20H (g) CH2 = CH2 (g) + 1420 (g) This is a gas phase elimination reaction as the liquids are all gases at a hot enough temperature.
  21. 12 KA: Zaitsev's rule I-I—c— Matter and Materials : Organic H c H H H I-I—c H H H H O H O H H conc H SO A conc H SO A H H H —c- H H H c H H Chemistry H H H H 21 If more than one elimination product is possible, the major product ( 70 % ) is the one where the H-atom is removed from the C-atom with the least number of H-atoms. ( Zaitsev's rule ) The minor product, never more than 30%, removes the H-atom from the carbon atom with the most number of H-atoms. Ethanol can dissolve water-soluble ( polar ) and non-water soluble ( non-polar ) substances Ethanol OH C-H bond AEN = -2,1 O-H bond AEN = -2,1 Model of ethanol - < I (a "tail" ) = > 1 (a "head") non-polar tail H H non-water soluble tail organic solvent soluble tail H H polar head of ethanol H water soluble head Very weakly polar covalent bonds form the "tail" which is soluble in non-polar substances. C2 H5— Polar covalent bonds form the "head" which is water-soluble. —OH If a non-polar powder like bromothymol blue indicator will not dissolve in water, dissolve it first in ethanol. non-polar bromothymol blue particle ethanol molecules Only the non-polar tails of ethanol dissolve into the non-polar bromothymol blue particles. Then dissolve this solution of bromothymol blue and ethanol in some water. The polar heads heads of water that stick out of the non-polar bromothymol blue particles are able to form hydrogen bonds with the water molecules that surround the bromothymol blue particles. The ability to form hydrogen bonds enables a substance to dissolve in water. The dotted lines represent hydrogen bonds. water molecule Dehydrohalogenation reaction Elimination of HX from a haloalkane ( X is any halogen ). CH2ce- CH2ce CH2 = CHCC I-I-C cc A heat cc 1 ,2-dichloroethane cc chloroethene Reaction condition : hot concentrated solution of NaOH or KOH in an ethanol solvent i.e. absence of water. Haloalkanes tend to be liquids and must be heated to turn them into gases and then further heated to drive the gas phase elimination reaction. Many haloalkanes are gases at high temperatures. The only haloalkanes that are gases at room temperature are CH CC, CH Br and CH CH CR. Gas phase elimination reactions are well known in industry. But bromo compounds are avoided in industry because bromine is expensive.
  22. Reflux : Stops vapour escaping. colder Cooling column The vapour rises and condenses on the glass plates or trays inside the column and runs back down into the liquid below, refluxing the upflowing vapour. hotter Round-bottomed flask with the liqui4 feed mixture with a few anti-bumping granules that stop big bubbles forming in the boiling liquid. Lots of small bubbles form on the granules. 12 KA : Matter and Materials : Organic Chemistry Zaitsev's rule : if more than one elimination product is possible the major product is the one where the double bond will appear between the carbon atoms where the H is removed from the carbon that already has the least number of H-atoms. ( eg. C number 3 from the left has 2 H) Using a strong base as a reactant causes elimination at a lower temperature. but-I -ene H 22 H H-C—C-C- 2-bromobutane Reaction conditions : concentrated strong base + Na OH heat under reflux Hot enthanolic NaOH solvent H This is the major product that forms ( but-2-ene ). Normally three products form. A concentrated solution of ionic NaOH in pure ethanol is used to dissolve the non-polar haloalkane. Heat the mixture strongly under reflux. This energy helps H and Br to be both eliminated. Reflux means the vapours condense and return to the reaction vessel during heating. The NaOH is less soluble in alcohol than in water thus forming fewer OH- ions which prevents the OH- substitution reaction with the Br-. This reaction needs a strong base. Group I or Group Il metal hydroxides. [ Weak will undergo hydrolysis as it has too few hydroxide ions for elimination l. doing a Heat a solution of these reactants under reflux. Instead of the vapours escaping into the air, the vapours go up a long glass tube or cooling column and condense on the sides of the tube and the plates in the tube, and this condensate returns to the reaction vessel during heating. A concentrated solution of NaOH or KOH in pure ethanol is the solvent i.e. hot ethanolic NaOH or KOH. The NaOH does not fully ionise in the ethanol so there are fewer OH- to substitute with the Br- . Thus Na+ can join with the Br-, and an H+ next to the Br- can join with the OH- thermometer Safety : Do not use open flames with volatile and flammable liquids. Use a fume cupboard, especially if fumes are toxic. water out Liebig ondenser to condense the most volatile vapours that stay in gaseous form all the way to the top of the cooling column. water in The reaction flask will not boil dry for a long time as all the vapours re-condense and flow back into the flask. • ••••••oil bath (water evaporates too readily over a long time period ) electric hot plate ( no open flame to ignite flammable solvents ) In the round-bottomed flask the reaction is thermally accelerated by conducting it at an elevated temperature ( the solvent's boiling point This temperature is constant. The following minor product also forms but is usually never more than about 30% of the total product. concentrated strong base H-C—C-C- 2-bromobutane + NaOH =c- H H + NaBr + O heat under H reflux
  23. A fractionating column has a series of trays, each at a constant temperature, thus many separate distillations are able to occur. Consequently, a liquid mixture can be separated into its different components. Fractional distillation of crude oil. Crude oil is a mixture of many different components. Crude oil evaporates when heated to 7000C. The different vapours of its components are able to condense at different temperatures in the fractionating column. Each fraction consists of hydrocarbon molecules containing a similar number of carbon atoms. crude oil from furnace ( feedstock ) hottest 12 KA : Matter and Materials : Organic Chemistry Cracking Crude Oil hydrocarbons in a Fractionating Column A mixture of gases bubbles though the liquid in a tray that is kept at a certain temperature. Gases of that liquid will condense in the tray, the other gases rise to condense at a lower temperature in the higher trays. -c 23 1200C gases : gases o 00 o o 1700C gases coolest paraffin oil 700C 1200C 1700C 2700C 6000C 7000C LPG q - c 4 liquid petroleum gas naptha for chemicals 7 petrol % - C 10 paraffin and jet fuel CIO - C 16 diesel fuels C 14 wax C - C 20 50 20 fuel oil for ships and factories cm - C 70 bitumen for tar roads 70 The gases cool as they rise through the fractionating column. Cracking means breaking up large hydrocarbon molecules into smaller and more useful bits. Reaction conditions : Thermal cracking at high pressures and temperatures without a catalyst. Or catalytic cracking at lower pressures and temperatures in the presence of a catalyst. Crude Oil is a mixture of different lengths of carbon chains. These long chains are cracked to make them shorter. Those that are split into chains of the right length then become petrol. So we get petrol from oil. SASOL can also produce petrol from coal. Cracking reactions to produce naphtha. Long carbon chains are broken down into shorter carbon chains at about 7000C Molecules in the long chains move so fast that some of the bonds between the carbon atoms are broken. The long chains are thus split apart ( or crack ) to form shorter chains such as light and heavy naphtha. Cracking Naphtha Alkane Chains ( SASOL ) Heavy naptha has 7- 12 carbon atoms in a chain ( boiling points range from 750C to 2000C ) These can be split up or cracked to yield petrol ( gasoline ) and petrochemicals. Light naptha has 5- 7 carbon atoms in a chain (boiling points range from 200C to 750C ) These can be split up or cracked to yield ethene and petrochemicals. The heated chemicals rise up large fractionating columns and the hydrocarbons with the lower boiling points rise up higher before cooling down sufficiently to liquefy and be collected. Modern cracking methods use zeolite catalysts composed mainly of silica and alumina in a micro-porous structure, which break up long alkane chains at lower temperatures and thus act as molecular sieves. Zeolites have a highly regular structure of openings that allows some molecules to pass through, and causes others to be either excluded, or held in these openings for long enough until these molecules are broken down as they flop around vigorously due to the thermal energy. • monomers are small, similar molecules that can join to make a chain • dimers form when two monomers are joined together Zeolite structure A polymer is a large molecule composed of smaller monomer units covalently bonded to each other in a repeating pattern. A macromolecule is a molecule that consists of a large number of atoms. Monomers are small organic molecules that can be covalently bonded to each other in a repeating pattern. Polymerisation is the chemical reaction in which monomer molecules join to form a polymer. • a free radical is a molecular fragment with an unpaired electron that initiates a polymerisation reaction Cracking of hydrocarbons This involves breaking up large hydrocarbon molecules into smaller and more useful molecules. • thermal cracking : high pressures and temperatures without a catalyst. Unbranched products form with low octane rating. • catalytic cracking : lower temperatures and pressures, about 5000C, in the presence of a catalyst form more highly branched alkanes from which petrol is made with a higher octane rating. Fortunately, mixtures of alkanes with similar boiling points are perfectly suitable for use as fuels, solvents and lubricants
  24. 12 KA : Matter and Materials : Organic Chemistry Chemical behaviour depends on the valence electron situation. H20 H —O Electron pairs are a stable state. H —O 24 Ignore lone pairs • if an electron pair is between two non-metal atoms, it forms a bond • if an electron pair is attached to one atom only, it is a Ione pair If an unpaired electron is attached to an atom it forms a free radical and is very reactive as it seeks another electron to form a pair with. Eg. chlorine cc Ignore lone pairs CC A free radical can break the weaker pi component of a double bond and then pair off with one of the electrons. cc-c—c Initiator monomer ( Ignore lone pairs) H H active intermediate snap pi bond Cracking Naphtha Alkane Chains ( SASOL ) to make free radicals and ethene When the carbon chains are just too short to make petrol ( about 5 - 7 carbon atoms in a chain ) then we call this mixture light naptha. These short chains can undergo further cracking to form ethene which is used in many polymers. Cracking enables short chain Naptha to become a source Of ethene. Pyrolysis chemically decomposes organic materials by heating in the absence of oxygen. ( the breaking up of a molecule when heated ) Thermal cracking of alkane chains is still used to produce ethene. With higher alkanes, the cleavage occurs randomly along the chain when the light naphtha alkane is heated. For example, a straight chain ( aliphatic ) pentane molecule can crack into free radical chains Cl and C4 or else into free radical chains C2 and C3 ( three carbon atoms in a chain ). CH CH CH CH CH pentane 3 or CH CH CH CH CH 3 CH . + • CH CH CH CH 3 3 free radicals CH CH 2 3 These free radicals may recombine to form a mixture of different alkanes For example CH CH • + •CH CH CH 3 CH CH CH 3 ethane propane Cracking forms a shorter chain. CH CH • + • CH CH C H CH Or CH CH CH CH CH CH 3 hexane Cracking forms a longer chain. the free radicals can perform another reaction ( called disproportionation ) This is another possible reaction during the thermal cracking and produces alkenes. To make ethene, an ethyl free radical is needed because it has two C atoms. A hydrogen atom ( with its electron ) is transferred from the C atom that is next to the C with the unpaired free radical electron. This H transfer to another free radical produces an alkane and an alkene. H H c=c ethene transfer He This reaction requires a high temperature to enable the fast moving H atom to break its bond and a high pressure to force the gasses together so that they react. Thermal cracking, no catalyst. The two unpaired electrons form a second ( pi ) bond methane CH CH 2
  25. 12 KA : Matter and Materials : Organic Chemistry Pyrolysis often produces ethane as a product but ethene is more useful. Steam cracking ( or thermal cracking ) of ethane to produce ethene A high temperature is required to break the carbon - carbon bonds. 25 To minimise choking or clogging up the pipes, ethene is diluted with steam and passed rapidly through tubes at about 8000C. The cooled gases are passed through a base to remove acidic gases H2S and CO Danger Many of the chemicals involved are corrosive, flammable and toxic. Gases can escape into the air. Hot liquids can spill. Fires and explosions can occur. People must wear protective clothing and be well trained to cope with these dangers as well as how to use the correct safety equipment. Initiation ( homolysis of single carbon - carbon bonds at 8000C ) H H H H Propagation A free radical abstracts a hydrogen atom H —Co + H _ C _ H H methyl free radicals ( proton + electron ) from an ethane molecule 3 ethyl free radical The ethyl free radical decomposes by losing a H H-C- atom ( which has one electron and is thus a free radical ) c=c ethene hydrogen free radical The hydrogen free radical abstracts a H atom from another ethane molecule to make another ethyl free radical 3 The process continues as the ethyl free radicals keep decomposing to make ethene. ethyl free radical SASOL's gas - to - liquid ( GTL ) process Natural gas ( methane CH4 ) from Mozambique is reformed into synthesis gas using steam and O at high tem- peratures ( it is an endothermic reaction ). CH + HO CO + 3142 (synthesis gas or syngas) AH > O The C from CO and the H from 1-12 can be joined together to make many hydrocarbon chains. Sasol uses the Fischer-Tropsch technology to convert synthesis gas ( derived from natural gas or coal ) into petrol and other fuels. The hydrogen and carbon monoxide react under pressure at a moderate temperature. An iron-based catalyst in fluid form is added. This produces a broad spectrum of hydrocarbons, usually in the Cl - C20 range. The C2 rich stream ( those hydrocarbons with two C atoms ) is split into ethene and ethane. The ethane is cracked in a high-temperature furnace, producing ethene. The ethene is then purified. The longer-chain olefins ( alkenes) (C8 - Cll ) are introduced into the fuel pool to make petrol. The double bonds break and enable side chains to form which have better anti-knock properties in petrol. 100 % octane is CH 2,2,4-trimethylpentane CH 3 CH 3 CH 3 — CH CH 3 The more compact molecules compress less and produce less knocking under pressure. Longer fuel molecules compress too much inside the car engine and self-explode at the wrong time ( which is called knocking ) since compression causes heating.
  26. 12 KA : Matter and Materials : Organic Chemistry 3. Substitution reactions ( only HCC and HBr, Br2 and C! ) Two types of saturated structure can be interconverted by substitution : alcohols haloalkanes and haloalkanes alcohols 26 The first type of substitution reaction involves removing an —OH from one molecule and replacing it with a halogen (X) from another molecule. ( Substitution in a sports team involves a reserve going on to the field to replace one of the players in the team who then leaves the field ) Substitution reactions using alcohols and water-free HCC or HBr ( dehydrated hydrohalides i.e. concentrated ) H H H-C—C—C—O alcohol + dehydrated hydrohalide -....................+ haloalkane The mechanism for this reaction is that the HBr acid donates an H + water Concentrated acids produce an increased reaction rate. to join onto a lone electron pair of the O-atom. H H H-C H H H H H H Water easily forms on planet Earth as about 70% of the surface is covered with water. A water molecule can break away if the O-atom takes away the bonding electron pair between it and the C-atom. This leaves a positive charge behind on the C-atom. A positively charged C-atom is called a carbonium ion or carbocation ( carbon cation ) H H The opposite charges then attract. If water were present as a reactant then OH- from the water would be attracted more favourably to the positive carbonium ion, because the O-atom has a greater electronegativity than the Br-atom, and this would simply reform the alcohol. BUT primary and secondary alcohols react slowly and at high temperatures. The carbonium ion has a strong positive charge which attracts the bonding electron pair between it and the O-atom, making it difficult for the water molecule to break away. Carbon 1 and carbon 3 are primary carbons because they are bonded to one other carbon atom. These are the carbons at the ends of the carbon chain. Carbon 2 (the middle carbon ) is a secondary carbon because it is bonded to two other carbon atoms. H H H H H H CH CH CH3 propane H H H H A primary alcohol is a —OH bonded to a primary carbon. CH CH CH OH propan-l-ol A secondary alcohol is a —OH bonded to a secondary carbon. CH3 CH(OH) CH3 propan-2-01 A substitution reaction works best for tertiary alcohols which are converted into haloalkanes using HBr or HCC at room temperature. C (CH ) OH + HBr C (CHO 3 & + 1-120 33 The central carbon in this compound is a tertiary carbon because it is bonded to three other carbons. This is a tertiary alcohol because the —OH is bonded to a tertiary carbon. H HBr H H methylpropan-2-ol O H 2-bromomethylpropane
  27. A proton, H + , from HBr attaches to a lone pair on the O-atom. The positive charge is thus attached to the molecule. C ö+ 12 KA : Matter and Materials : Organic Chemistry Mechanism of a substitution reaction involving a tertiary alcohol : H-C—C H-C—C—C—O + Na-Br 27 c Water loves to form on planet Earth. Over 70% of our surface is covered in water. If the water molecule is to break away, it must take the bonding pair of electrons with it, leaving the positive charge on the carbon atom. This positive charge exerts a strong attraction on the negative electrons, making it difficult for the water molecule to break away. In a tertiary alcohol the carbonium ion is surrounded by three carbon atoms and their electron pairs which are drawn closer to the carbonium ion due to the electron density shifting towards the positive charge. This leaves the three surrounding C-atoms with a slight positive charge which enables them to pull on the electron pairs between the C-atoms and their three H-atoms. As each electron pair is pulled closer to a carbon atom, all the H-atoms become slightly more positive as the hydrogens of the methyl groups assume some of the positive charge. Effectively. the positive charge on the carbonium ion is dispersed or delocalised over all the H-atoms, leaving a much smaller positive charge on the carbonium ion. The water molecule can thus more easily break away from this weaker attraction. The negative bromide ion is then attracted to the positive carbonium ion. The opposite charges cancel and the electron pairs all return to their normal positions in the absence of the positive charge. c H HO Reactions of haloalkanes with bases ( hydrolysis ) to produce alcohols The non-polar haloalkane is dissolved in ethanol so that it can mix with aqueous NaOH and then the mixture is warmed to increase the reaction rate. The non-metal Br is substituted by the non-metals OH H H H H H dilute strong base H H H H Normally only 2 products. H Heat in The same hydrolysis reaction occurs more slowly without the alkali. Water is added to a haloalkane dissolved in ethanol. Water behaves as if it is H— OH. C (CH3)3 Br + HOH C ( CH3 OH + HBr The advantage of this slower reaction is that no elimination occurs. A strong base will cause elimination as an unwanted by product of the substitution reaction. Hydrolysis is the reaction of a compound with water.
  28. 12 KA : Matter and Materials : Organic Chemistry The hydrolysis reaction, without the alkali, takes place very slowly as the concentration of OH- ions is lower. 28 H H Split bromomethane into two parts. Split water into H + and OH- Then exchange functional groups to form two separate molecules, an acid and an alcohol. A hydrolysis reaction is when a compound splits into two parts when reacting with water which also splits into H + and OH-. The functional groups are exchanged to form two products. Substitution reactions are reactions in which one atom or group of atoms is replaced by another These occur VERY SLOWLY in saturated compounds. in the presence of light or heat Halogenation of alkanes ethane + bromine gas ( substitution reaction ) • one Br atom swops places with an H atom to form bromoethane Br2 does not readily attack single bonds in an alkane light energy hf H H — C C —H + Br— Br (g) H —C HBr H H H heating • liquid H —C-Br H C H + Br2 C 2 H 5 Br Alkane to haloalkane. The brown colour of the bromine disappears slowly. ethene + bromine gas ( addition reaction ) C H + Br H c H + Br-Br Br CH Br CH Br The brown colour of the bromine disappears quickly. This is the test for a double bond. Haloalkanes to alcohols CH CC + NaOH (aq) CH30H + Reaction conditions : heat in an alkali solution DANGER Never work in practice with liquid bromine. It causes cancer. It is very ugly. Go back to the third page of these organic chemistry notes and see how to make bromine water. You must only decolourise bromine water when testing for a double bond. Caution C H + HBr C H Br + H 2 So we write C H + HBr has no reaction. • we can't get a high energy product (H 2 ) out of low energy organic reactants • this reaction does not occur ! The reaction of alkenes with an alkaline solution of potassium permanganate ( KMn04 ). Cold, dilute purple Mn04- ions in the presence of OH- ions form MnO The Mn7+ is reduced to Mn4+ by gaining 3 negative electrons from the alkenes. The half-reaction is MnO- + 2 HO + 3 e- -5 MnO + 4 OH- Two of these OH- ions will undergo an addition reaction with ethene. H H + 2 OH- 0-c—c—o A diol forms. This is a vicinal diol as the two — OH are near each other. CH20H CH20H ethan-l ,2-diol or glycol The purple potassium permanganate is thus decolourised. ( It can turn a brownish colour as Mn02 forms ) ( Adding NaOH to the potassium permanganate may turn the solution green ) Acidic MnO - is a strong reaction that attacks organic compounds almost indiscriminately.
  29. A polymer is a large molecule composed of smaller monomer units covalently bonded to each other in a repeating pattern. A macromolecule is a molecule that consists of a large number of atoms. Monomers are small organic molecules that can be covalently bonded to each other in a repeating pattern. Polymerisation is the chemical reaction in which monomer molecules join to form a polymer. A functional group is the particular group of atoms in a molecule that primarily determines how the molecule reacts. Natural polymers are the proteins like silk and wool. The starches in our diet are polymers. The cellulose of cotton and wood are polymers and so is rubber. Being natural molecules, they are bio-degradeable and are decomposed by microorganisms. Plastics are synthetic or man made polymers. There is no natural place for them in Nature so they do not decompose. A chain of monomers can extend to thousands of atoms. How these macromolecules interact with each other enables them to be used as fibres or plastics. If large side chains form there is no close packing and the low melting point means they can be melted and remoulded. They are called thermoplastics. If cross-links form between the polymers then they are hard with high melting points that degrade or decompose if heated. Called thermosetting polymers. Once set, they cannot be remoulded. Making a polymer from ethene molecules The double bonds snap to form two unpaired electrons and in this free radical state new bonds can keep forming with identical neighbouring molecular fragments that are also free radicals as they have one or more unpaired electrons. Addition polymerisation is a reaction in which small molecules join to form very large molecules by adding on double bonds. 12 KA : Matter and Materials : Organic Chemistry TOPIC Plastics and Polymers -c-c-c-c -c-c- 29 H H H H H H H H H H H H H H H H H ethene monomer -c-c-c-c-c-c Step by step the chain grows We represent polythene as H H H H H H H H H n Polymer. This is a chain-growth or addition polymer. An addition polymer is a polymer formed when monomers ( usually containing a double bond ) combine through an addition reaction. The final macromolecule polymer is called polythene or polyethylene. ( poly means many ethenes ) Polythene is only useful if the molecular mass is about 1 million ( tens of thousands of atoms linked ) which is achieved by heating the reaction at 2000C and 1 200 atmospheres of pressure to limit side branches forming. The many side branches that do form prevent close packing of the polymers and this process makes low density polythene ( LDPE ). (40 Polythene is cheap, flexible, durable and chemically resistant. Its recycling number is LDPE Polythene is the most common plastic and makes the familiar supermarket plastic bags, thin plastic films and sheets and squeeze bottles, packaging materials, moulded objects and electrical insulation, cling wrap for protecting food. Using a catalyst at 600C and 1 atmosphere pressure allows few side branches to form as the ethene molecules snap their double bonds and add on to the end of the chain where the ions of the catalyst are. These polymers thus tend to be long straight chains, with few side chains, that can pack together closely to form hard and rigid plastic crates, car fittings and pipes. In this compact form the polymer is called high density polythene. HDPE. We write the polymerization reaction as is the recycling number. (24) HDPE n CH = CH 2 monomers CCH 2 — CH2 n polymer where n is a big number.
  30. 30 represent the acid ) Acid catalysed chain-growth ( addition ) reaction The process involves 3 steps : initiation, propagation and termination 1. Initiation ethene monomer ( a pair of electrons makes a bond — ) ( let H H carbonium ion H or carbocation ( carbo-cation ) Since the proton H + has no electrons to contribute, both electrons from the double bond in ethene are used to form the new bond between hydrogen and carbon. The other carbon is thus short of one electron and hence has a positive charge. Effectively, the positive charge has moved from the H to the end carbon. 2. Propagation carbonium ion H H H H H The two electrons from the double bond form a bond between two carbon atoms, leaving the end carbon with a positive charge as it is one electron short. Thus the unstable carbonium ion grows longer by reacting with ethene molecules. 3. Termination H H polythene polymer CH2 CH2 H H H H C H A H n H c H c-c c- H n written as polythene polymer H n H n Termination occurs when no carbonium ion is produced, usually by the reaction of the carbonium ion and the anion (A - ) part of the acid catalyst HA. If the polythene polymer is long enough then we can ignore the effects of the end atom A that came from the acid. Small amounts of acid catalyst are used. If there is too much acid, too many A- anions are present and termination occurs too soon so that no long polymers form. The repeating monomer that forms polythene is made from the portion in the bracket, by re-forming the double bond. H H H H H H H 1-4 H 1-1 a section of the polythene polymer chain H H becomes H C H repeat unit monomer Similarly, making a polymer from propene, CH melting it and then forcing it through narrow holes to make thin fibres which are then twisted together, enables strong ropes or carpets to be made. Propene is a bigger molecule with stronger van der Waals forces between molecules which makes a ( ethene ) (CH2 CHY n CH 3 stronger material than ethene. More addition polymers PVC Polyvinyl chloride (ethene is called vinyl when something is attached to a carbon atom in the place of H) 12 KA : Matter and Materials : Organic Chemistry :c-c+ .c-c+ H-C- —cuc c-c+ c- -c-c-c-c-c-c -c-c-c-c-c-c- c- c- -c -c-c-c c- c- polypropene or polypropylene The CH = CH— group is called a vinyl group. H H H cc polymer CH2 CH) cc repeat unit I-chloroethene or vinyl chloride H H H monomer CH2 = CH n poly ( I-chloroethene ) H H H H H H H H H cc H H cc H H C H H H H cc H Makes plastic conduit pipes for enclosing electric wires in houses. Floor tiles, gutters a section of the PVC H polymer chain H CC H CC H H CC H H H CC H H cc (30 Recycle number To participate in polymerisation, the vinyl chloride monomer molecule must be able to react at both ends.
  31. Smooth teflon is used to make valves for hearts and walls for veins. Small round discs are placed under the legs of furniture to act as low friction sliders when moving furniture around. 12 Matter and Materials : Teflon c- c- c- -o-c- KA : n CF CF Organic Chemistry F F F F n 2 n polymer F F F F F F F F polymer polytetrafluoroethene PTFE repeat unit monomer tetrafluoroethene 31 F side groups make teflon almost resistant to chemical attacks as F is the most reactive element and cannot be displaced by another atom. Its surface is smoothest and needs no lubrication. Non-stick pans are coated with teflon. Makes a stain repellent for carpets and clothes as it does not react with anything. F F F F F F F F F F c F F - c-c-c-c-c a section of the teflon polymer chain PVA Polyvinyl acetate Acetate ion is the common name for ethanoate ion . F F H F F O F c- F H F F F F F c- F H F F F c- F F F F F F F or CH COO- CH2 CHOCOCH H H H F F or H H OCCH 3 O n OCCH 3 O -OCCH 3 O OCCH 3 O Used for water soluble paints and wood glue. Paper and textile coatings. polymer polyvinylacetate Polystyrene repeat unit monomer : vinyl acetate PVA is considered a scrap plastic that is not recycled. Six carbon atoms in a loop form the benzine ring. If the electrons in the double bonds are delocalised they spread out over a bigger region and repel each other less, making the molecule more stable. We can represent the delocalised electrons as a circle. A simple line formula ignores the H atoms and assumes there is a C atom at each corner of a hexagon. Each line then represents a pair of electrons forming a bond. benzine ring C6 H 6 benzine ring C6 1-16 phenyl C6 145 — Remove one hydrogen atom from benzene to form a bond ( now called the phenyl substituent ) with an ethene molecule. n polystyrene polymer (60 CCH 2 C6H5 recycling number O repeat unit O monomer is styrene or phenylethene Polystyrene is light and rigid and is used for packaging, picnic utensils like disposable cups, insulation in buildings, cooler boxes etc.
  32. 12 KA : Matter and Materials : Organic Chemistry Macromolecules are individual large polymer molecules. 32 A functional group is a chemically reactive atom or group of atoms that imparts characteristic properties to organic compounds containing that group. For example : The carbonyl group C = O is the functional group for ketones. Free radicals are very reactive atoms or molecules which have unpaired electrons. Example Hydrogen peroxide H O 0-0 The bond between the two oxygen atoms breaks first as it is not as stable as the bond between the O —H atoms, Polyiactic acid PLA Condensation ( esterification ) of lactic acid to form lactide. Direct or self-polymerization is not a good option as too many water molecules are liberated, Lactic acid is 2-hydroxypropanoic acid H O O Lactose is the predominant sugar in milk and enzymes turn it into lactic acid. CH 3 we write this more conveniently as O H O When a chain of lactic acid undergoes self-polymerisation, too much water is produced by the esterification ( elimination ) reactions. CH 3 H C O O CH 3 H O CH 3 H C O O CH 3 H O O O It is far more efficient to place two monomers next to each other with one upside down so that the carboxylic and hydroxy functional groups are next to each other. An elimination reaction occurs as water is eliminated to produce lactide. I —CH3 O O O c A self-esterification process. I CH3 Then remove the lactide c + 2 HO ring from the water so that H3C I lactic acid monomer O O c .zcx O lactic acid monomer CH c c acid catalysed H3C I o O lactide is a cyclic di-ester -o- CH 3 H O CH 3 H H3C I o' O cut here O dimer ( repeat unit ) This is ring opening polymerisation lactide CH 3 there is no water present when the lactide ring is cut and opened to produce a water-free dimer of two identical monomers that can be polymerised. Polymerisation of this dimer takes place when identical repeating units of these dimers bond together to form a chain uniformly repetitive polylactic acid ( homopolymer) CH 3 H O CH 3 H o CH 3 H O CH 3 H O H O n No initiator is needed. The reaction is slower because two steps are needed. Lactide must first form and then be polymerised. No double bonds are needed as the acid and alcohol functional groups react by eliminating water.
  33. 12 KA : Matter and Materials : Organic Chemistry Polylactic acid is a bioplastic 33 PLA is produced by fermentation of many natural biological plant materials — corn ( mielies ) in particular. Lactic acid, is the relevant product of this fermentation, and since a molecule of lactic acid has both an OH and a COOH functional group, the one functional group (on one molecule) can form an ester with the other functional group (on another molecule). With suitable control of conditions this polyester formation by polymerisation can continue indefinitely. "Ä (70 recycling number PLA is biodegradable by the enzymes in bacteria. PLA is used for food and cosmetic packaging, agricultural sheeting and household appliances. Plastic polymers cannot be economically recycled like metals, so disposal is a problem. A mixture of polymer plastics has very limited usefulness so waste must be sorted with recycling numbers. But, PLA needs O and microorganisms to degrade. In a compacted land-fill and covered with other material there is not enough O for rapid biodegrading. Thus PLA may not fully solve a city's land-fill problem if PLA takes a long time to decompose. At present it remains our best hope for a biodegradeable plastic. It is mainly used as a packaging material. More plastic is used each year than steel, so plastic waste disposal and litter are major problems. All plastics produce poisonous carbon monoxide CO when burnt. PVC also produces poisonous CC 2 gas and nylon produces deadly hydrogen cyanide HCN when burnt. Often it is better to use paper and glass etc. which can be recycled instead of plastic. One million sea birds die each year when they try to eat plastic and choke to death. 100 000 sea mammals die each year by eating and choking on plastic or getting tangled up in plastic nets and drowning. Types of polymers Polymer chains are regularly oriented and closely packed because there are few branched chains. High density polythene. High tensile strength, high melting point. Thermoplastic as it can be recycled by remelting and reshaping. Polymer chains are irregularly packed. Low density polythene. Because of the many side chains they cannot be closely packed. Molecules are further apart with weaker intermolecular Van der Waals forces producing lower tensile strength and lower melting points. Thermoplastic as it can be recycled by remelting and reshaping. Polymer chains with much cross-linking. Rigid and hard as the cross-links stop the molecules sliding past each other. Fairly brittle as the chains cannot bend and are not flexible. Eg. Bakelite. Chains are firmly held and when heated the plastic decomposes before reaching a melting point. Thus they cannot be recycled. Polyesters are formed by step-growth or condensation polymerisation. Condensation polymerisation occurs when two monomers with different functional groups undergo condensation reactions with the loss of small molecules, usually water. A molecule with two alcohol functional groups reacts with a molecule that has two carboxylic acid functional groups. A water molecule is eliminated, for example, if ethan-l ,2-diol reacts with ethandioic acid O H-O-C-C-04H H-O-C-C-O-C-C O H O dimer The dimer molecules must line up alternately with the acid group next to an alcohol group to undergo elimination H H O O H o-c- H O n H-O-C-C-O O-H O n H;O-C-C-O-C-C-O-H H H + 2nHO polyester A condensation polymer is a polymer formed by two monomers with different functional groups that are linked together in a condensation reaction in which a small molecule, usually water, is lost. Mylar is a polyester that makes thin films like space blankets and Kevlar polyester fibres make bullet proof vests. A polycarbonate polyester makes Lexan which is tough and transparent and acts as shatter-proof glass and riders' helmets.

Need a Tutor or Coaching Class?

Post an enquiry and get instant responses from qualified and experienced tutors.

Post Requirement

Related Study Notes

Upload Study Notes

If you have your own Study Notes which you think can benefit others, please upload on MyPrivateTutor. For each approved Study Note you will get 100 Credit Points and 100 Activity Score which will increase your profile visibility.

Upload Now
Home > Study Notes > Grade 12 Organic Chemistry