1. AQA Organic Chemistry Unit 2 AS Summary

2. Formulae of organic molecules Empirical formulae – the simplest whole number ratio of atoms of each element found in the compound. Often deduced form combustion analysis data Molecular formulae – the actual number of atoms of each element found in a molecule

3. Formulae of organic molecules Structural formulae – the minimal detail that shows the arrangement of atoms, e.g. CH 3 CH 3 for ethane Displayed formulae – shows the relative position of all atoms and bonds between them, e.g. ethene Skeleton formulae – the simplest representation of organic molecules, removing all hydrogen atoms, leaving only the outline of the carbon skeleton and adding only any associated functional groups, e.g.1-chlorobutane

4. Nomenclature and Isomerism Homologous series – compounds that differ by a CH 2 unit, sharing the same functional group and exhibiting trends in their properties Know the first ten alkanes – methane to decane Functional group – (group of) atoms that are chemically reactive in molecules

5. Nomenclature Locate longest chain Identify functional groups and positions Identify side groups and their positions CH 3 CH 2 CH 3 propane CH 3 CH 2 CH 2 OHpropan-1-ol CH 3 CHCH 3 I CH 3 2-methyl propan-1-ol

6. Structural Isomerism Structural isomers are molecules with the same molecular formulae but with different structural formulae. 1-chlorobutane 2-chlorobutane Both these molecules have the same molecular formula (C 4 H 9 Cl) but one cannot be converted to the other without breaking and making bonds. These are examples of positional isomers

7. More structural isomerism CH 3 CH 2 CH 2 CHO and CH 3 CH 2 COCH 3 CH 3 CH 2 CH 2 CH 3 and (CH 3 ) 2 CHCH 3 butanal and butanone butane and 2-methyl propane Functional group isomers of C 4 H 8 O chain isomers of C 4 H 10

8. Stereoisomerism Same structural arrangement, but different spatial arrangement E/Z isomers e.g. of alkenes Z -isomer (groups on one side of C=C bond) E -isomer (groups on opposite sides of C=C bond) Where the groups on both sides of the double bond are the same – the terms cis and trans apply

9. Petroleum and Alkanes Petroleum = Crude oil A mixture consisting mainly of alkanes Fractional distillation used to separate into component fractions with different boiling ranges Temperature gradient in fractionating tower enables this to happen

10. Cracking Alkanes Involves breaking c-c bonds in alkane molecules Converts heavy fractions of oil into higher value products, increasing profits for oil companies Produces alkenes for the petrochemical industry e.g. to make poly(ethene)

11. Combustion of Alkanes Complete Combustion Only CO 2 and H 2 O produced CO 2 is a greenhouse gas causing global warming Incomplete combustion CO, NO x and unburnt hydrocarbons produced in car engines Can be removed by catalytic converters Toxic fumes Global dimming Photochemical smog Acid rain Crude Oil fractions contain sulphur as an impurity. If not removed, this burns to form SO 2 – causes acid rain

12. Chlorination of Alkanes Free radical substitution i.e. homolytic breaking of covalent bonds CH 4 + Cl 2 CH 3 Cl + HCl Overall reaction equation Conditions ultra violet light excess methaneto reduce further substitution

13. CH 4 + ClCH 3 + HCl Cl 2 Cl + Cl CH 3 + Cl 2 CH 3 Cl + Cl CH 3 ClCH 3 + Cl initiation step two propagation steps termination step ultra-violet CH 3 CH 3 + CH 3 minor termination step Free radical substitution mechanism

14. CH 3 Cl + Cl 2 CH 2 Cl 2 + HCl Further reaction equations Conditionsultra-violet light CH 2 Cl 2 + Cl 2 CHCl 3 + HCl CHCl 3 + Cl 2 CCl 4 + HCl excess chlorine Further free radical substitutions

15. H H H H C C         bonds are exposed and are therefore more vulnerable to attack by electrophiles Bonds are regions of high electron density Alkenes

16. Planar alkene molecules With only three areas of bonding electrons around the double bonded C atoms, the shape of atoms around each carbon atom is trigonal planar The π electrons repel the C-H bonding electrons effectively preventing the C-H bonds from rotating about the C=C double bonds This causes cis-trans isomerism

17. Hydrogenation of Alkenes Ethene (example) reacts with hydrogen in the presence of a finely divided nickel catalyst at a temperature of about 150°C. Ethane is produced. Margarine is made by hydrogenating C=C double bonds in animal or vegetable fats and oils. Temperatures of only 60°C are needed. Vegetable oils often contain high proportions of polyunsaturated and mono-unsaturated fats, and as a result are oils at room temperature. That makes them messy to spread on your bread or toast, so many producers use hydrogenation to raise the melting point of these fats.

18. Halogenation of Alkenes Alkenes react easily with the halogens in an addition reaction. This has led to bromine water being used as a test for alkenes. Alkenes decolourise bromine water. CH 2 =CH 2 + Br 2  CH 2 BrCH 2 Br Brown  colourless

19. Br Electrophilic addition mechanism CH 3 H H H C C ++ -- H H H C C Br + - carbocation CH 3 H H H C C Br 1,2-dibromopropane bromine with propene Bromine molecule is spontaneously polarised as it approaches the electron rich π bond

20. Alkenes with hydrogen Halides Alkenes react with gaseous hydrogen halides at room temperature. If the alkene is also a gas, you can simply mix the gases. If the alkene is a liquid, you can bubble the hydrogen halide through the liquid. This is not done in aqueous solution due to the polar nature of the water molecules, which would ionise the H-X molecule to H + and X - and side reactions could occur.

21. Electrophilic addition mechanism CH 3 H H C C H H C C H + carbocation CH 3 H H C C BrH 2-bromobutane hydrogen bromide with trans but-2-ene -- ++ Br H -

22. Summary of Electrophilic addition CH 3 CH=CH 2 + Br 2 CH 3 CHBrCH 2 Br bromine with propene hydrogen bromide with but-2-ene CH 3 CH=CHCH 3 + HBrCH 3 CH 2 CHBrCH 3 2-bromobutane 1,2-dibromopropane

23. Polymerisation of Alkenes Addition polymerisation – very many monomer molecules add together, forming chemical bonds making a very long chain polymer molecule TETRAFLUORO -ETHANE PROPENE CHLOROETHENE ETHENE POLY(ETHENE) POLY(PROPENE) PVC PTFE

24. Processing of waste polymers Collection, sorting and separating into different types and then recycling Combustion for energy production –Need to remove toxic waste gases i.e. HCl during combustion of PVC and other halogenated plastics Feedstock for cracking Also aiming to develop more biodegradable polymers e.g. from maize, starch

25. Halogenoalkanes Polar molecules with δ+ve carbon centres Susceptible to attack from nucleophiles – species able to donate lone pairs able to make covalent bonds Reaction with aqueous alkali – hydrolysis via nucleophilic substitution reaction

26. Haloalkanes with potassium hydroxide CH 3 CH 2 Br + KOH CH 3 CH 2 OH + KBr Conditions: boil under reflux with aqueous potassium hydroxide The bromoethane has been hydrolysed to make ethanol This is a nucleophilic substitution mechanism

27. ++ -- CH 3 H Br C H - OH CH 3 H OH C H Br - Aqueous hydroxide ion with bromoethane Nucleophilic substitution mechanism ethanol

28. Identification of the halide group 1.Warm the substance with aqueous sodium hydroxide 2.Add dilute nitric acid 3.Add silver nitrate solution (AgNO 3 ) Ag + (aq) + X - (aq) AgX (s) Results: A silver halide precipitate forms (AgX), White precipitate indicates chloride ions Cream precipitate indicates bromide ions Yellow precipate indicates iodide ions The halide groups can also be identified by adding ammonia solution to the silver halide precipitates: Chloride ions dissolve in dilute NH 3 (aq) Bromide ions dissolve in concentrated NH 3 (aq) Iodide ions are insoluble in concentrated NH 3 (aq)

29. Alcohols An alcohol is a compound where an OH functional group has replaced one or more H atoms on an alkane Alcohols can be separated into 3 groups –Primary (1°) –Secondary (2°) –Tertiary (3°)

30. Differences between Alcohols In a primary alcohol, the carbon bonded to the OH group is only attached to one alkyl group. In a secondary alcohol, the carbon is attached to two alkyl groups In a Tertiary alcohol the carbon is attached to three alkyl groups

31. Physical properties Like the alkanes the boiling points of the alcohols increase as the carbon chain length increases. However the boiling points of the alcohols are significantly higher than the alkanes due to the OH group. This is because of Hydrogen bonding between the OH groups! – More energy is required to overcome the bonds between the molecules  higher boiling points

32. Solubility Small chain alcohols dissolve easily into water Longer chain alcohols are less soluble, there is a decrease in solubility as the chain length increases. Whilst smaller alcohols form hydrogen bonds with the water molecules, compensating for the hydrogen bonds broken, larger alcohols break more H-bonds than they replace due to their long hydrocarbon tail which is unable to form H- bonds.

33. Manufacture of Alcohols Alcohols can be manufactured by reacting the corresponding alkene with steam over a catalyst. The catalyst and reaction conditions vary between the alcohols. –Eg –This is carried out at 60-70atm, 300°C Ethanol can also be made by fermentation of sugars with a yeast catalyst. –Eg

34. Comparison of methods FermentationHydration of ethene Type of process Batch processContinuous process Rate of reaction Very slowVery rapid PurityImpure - requires further processing Very pure Reaction conditions Gentle temperatures, atmospheric pressure High Temperatures and pressures Atom economy Low – waste products made BE ABLE TO CALC THIS!! High – only 1 product made BE ABLE TO CALC THIS!! Use of resources Renewable resourcesFinite resources (mainly crude oil)

35. Dehydration of Alcohols Alcohols may be dehydrated to form alkenes This, as the name implies, involves the removal of a water molecule from the alcohol The reaction takes place using conc. Phosphoric acid or conc sulfuric acid as a catalyst Alkene gases are collected over water

36. Oxidation of Alcohols Only primary and secondary alcohols may be oxidised. Primary alcohols are oxidised to aldehydes, and depending upon the conditions may be further oxidised to carboxylic acids. Secondary alcohols may only be oxidised to form ketones. Tertiary alcohols cannot normally be oxidised

37. Formation of Aldehydes A suitable oxidising agent is Cr 2 O 7 2- An acid catalyst is used and the mixture is heated gently. During the reaction orange dichromate(VI) ions are reduced to blue-green chromium(III) ions

38. Formation of Carboxylic acids Aldehydes are further oxidised to Carboxylic acids when heated under reflux To do this you need to use an excess of the oxidising agent

39. Formation of Ketones Secondary alcohols form ketones under an oxidation reaction The ketones do not undergo further oxidation The secondary alcohol is oxidised under the same conditions as the primary alcohol The same colour change from orange to blue- green is displayed

40. Uses Alcohols have three main uses –Drinks –Solvents –Fuel (gasohol in poorer countries) –Methylated spirits Alcohols are also used in other reactions which have not been mentioned –Formation of esters –Triiodomethane (Iodoform) reaction

41. Carbonyls - Aldehydes and Ketones AldehydesKetones R C = O H Where R are alkyl or aryl groups, and may or may not be different The aldehydes are named using the suffix -al E.g. CH 2 O methanal CH 3 CHO ethanal C=O R R1R1 The ketones are named using the suffix -one E.g. CH 3 COCH 3 propanone CH 3 CH 2 COCH 3 butanone CH 3 CH 2 CH 2 COCH 3 pentan-2-one

42. Carboxylic acids and Esters Named as –oic acids, assuming the –COOH carbon to be carbon 1 in the chain e.g. CH 3 CH(CH 3 )CH 2 COOH is named as 3-methylbutanoic acid Named as –yl –oates e.g. CH 3 COOCH 2 CH 2 CH 3 is named as propyl-ethanoate Where R are alkyl or aryl groups, and may or may not be different

43. Esterification Heat a carboxylic acid with an alcohol in the presences of a strong acid catalyst (usually conc. sulphuric acid). This can also be known as condensation reactions as a water molecule is lost.

44. Uses of esters Food flavourings – sweet smells and nice fragrances and flavours (flowers and fruits) Plasticisers – because they make plastic more flexible. Solvents – Esters are polar liquids so dissolve polar compounds, they have a low boiling point (therefore evaporate easily), making them good solvents in glue and printing inks.