IGCSE Chemistry: organic chemistry from crude oil to polymers

The homologous series you need

Cambridge lists the first four general formulae as Core. Ester naming and formulae are Cambridge Supplement and Edexcel Paper 2.

Key ideas board by board

Cambridge 0620 Core: displayed formulae, general formulae, functional groups, homologous series, saturated and unsaturated; naming methane, ethane, ethene, ethanol and ethanoic acid; fossil fuels and fractional distillation (fractions get shorter chains, more volatile, lower boiling point and less viscous towards the top, with named uses from refinery gas to bitumen); alkanes; cracking and the bromine water test; making ethanol by fermentation (25 to 35 °C, yeast, no oxygen) or from ethene and steam (300 °C, 6000 kPa / 60 atm, acid catalyst); ethanoic acid with metals, bases and carbonates; addition polymerisation to poly(ethene) and the problems plastics cause. Supplement adds structural formulae, structural isomers (C4H10 and C4H8), naming unbranched compounds up to four carbons, esters, photochemical substitution, addition reactions with drawn products, oxidation of ethanol with acidified potassium manganate(VII), condensation polymers (nylon, PET) and proteins.

Edexcel 4CH1 for all students: formula types, naming, isomerism, classifying reactions as substitution, addition or combustion, crude oil fractions, complete and incomplete combustion, why carbon monoxide is poisonous, nitrogen oxides and sulfur dioxide and acid rain, cracking, alkanes with halogens in UV light, alkenes with bromine, and addition polymers including poly(propene), poly(chloroethene) and PTFE. Alcohols (oxidation by acidified potassium dichromate(VI), manufacture with a phosphoric acid catalyst at about 300 °C and 60 to 70 atm, or by fermentation at about 30 °C), carboxylic acids, esters including the ethyl ethanoate practical, and polyesters are Paper 2 only.

Worked example 1: equations you must be able to write

Complete combustion of propane: C3H8 + 5O2 -> 3CO2 + 4H2O. Balance carbon first (3 CO2), then hydrogen (4 H2O), then count oxygen atoms on the right (6 + 4 = 10, so 5 O2).

Substitution: CH4 + Cl2 -> CH3Cl + HCl, in ultraviolet light. Addition: CH2=CH2 + Br2 -> CH2BrCH2Br (1,2-dibromoethane), which is why bromine water turns from orange to colourless. Hydration: C2H4 + H2O -> C2H5OH. Fermentation: C6H12O6 -> 2C2H5OH + 2CO2.

Cracking: C10H22 -> C8H18 + C2H4. Check the atoms balance: 10 carbons and 22 hydrogens on each side. Cracking is done because there is more demand for short-chain fuels such as petrol and for alkenes to make polymers than crude oil supplies.

Worked example 2: naming, esters and polymers

Name and formula: an alkene with four carbons has formula C4H8 (n = 4, 2n = 8). With the double bond at the end it is but-1-ene, CH2=CHCH2CH3; in the middle it is but-2-ene, CH3CH=CHCH3. These are structural isomers: same molecular formula, different structural formula.

Ester: ethanoic acid + ethanol -> ethyl ethanoate + water, CH3COOH + C2H5OH -> CH3COOC2H5 + H2O, with a concentrated sulfuric acid catalyst. The alcohol gives the first part of the name (ethyl), the acid the second (ethanoate). So methanol with propanoic acid gives methyl propanoate.

Addition polymer: propene, CH2=CH(CH3), gives poly(propene). Open the double bond and draw the repeat unit as -CH2-CH(CH3)- inside brackets, with bonds extending through the brackets and an n outside.

Common mistakes

• Writing that bromine water is "decolourised to clear". Clear is not a colour change term; write orange to colourless.
• Drawing displayed formulae with missing hydrogens, or carbon atoms with five bonds.
• Saying alkanes "have no double bonds so are unreactive" without naming combustion and substitution as exceptions.
• Mixing up the ester name order: ethanol plus methanoic acid gives ethyl methanoate, not methyl ethanoate.
• Saying cracking happens because long alkanes are "useless". The reason is supply and demand, and making alkenes.
• Leaving the bonds off the ends of a polymer repeat unit, or leaving the double bond in it.

Exam technique

Organic chemistry is very predictable. Make one summary sheet of every reaction with conditions (temperature, pressure, catalyst, UV light) and practise writing each equation and drawing each product until it takes seconds. Conditions are frequently a separate mark.

For comparisons of the two ways of making ethanol (Cambridge Supplement, Edexcel 4.32C and 4.33C), use a balanced table in your head: fermentation uses renewable sugar, low temperature, but is slow, batch and gives impure ethanol; hydration of ethene is fast, continuous and gives pure ethanol, but uses non-renewable crude oil and high temperature and pressure.

How one-to-one lessons help

Students often lose organic marks on drawing and naming rather than understanding. In lessons a tutor asks the student to draw displayed and structural formulae on the shared whiteboard, corrects bond counts as they happen, and drills naming and equations across all the series. The tutor also checks which content is Supplement or Paper 2 for the student's board so revision time goes where the marks are.

Self-check

1. Give the general formula and functional group of alkanes, alkenes, alcohols and carboxylic acids.
2. Describe fractional distillation and the trend in properties up the column.
3. Write equations for combustion, substitution, addition and cracking.
4. Describe the bromine water test and its result.
5. Compare fermentation and hydration of ethene for making ethanol.
6. Name and draw an ester from a given alcohol and acid.
7. Draw the repeat unit of an addition polymer from its monomer, and back.