HookThe plastic that was an accident — and then won a war
On 27 March 1933, two chemists at ICI's Winnington laboratory in Cheshire — Eric Fawcett and Reginald Gibson — squeezed ethene gas to nearly 2,000 times atmospheric pressure and left it overnight. In the morning they found the reaction vessel coated with a white, waxy solid nobody had ordered. It was poly(ethene), the first modern plastic, and it was so hard to reproduce that it took another two years, and a trace of leaked oxygen, to work out how it had formed. Within a decade that accident was insulating the cables of airborne radar in the Second World War — light, waterproof and a superb electrical insulator — and today the world makes over 100 million tonnes of it a year.
Every idea in C7 sits on the road between a barrel of crude oil and that sheet of plastic. Carbon is the only everyday element that chains to itself almost without limit, and this chapter is the study of what those chains do: how crude oil is a mixture of them, how we sort it by boiling point, how we break the long ones to make the short reactive ones, and how those reactive molecules are bolted together into fuels, alcohols, acids and the polymers — natural and synthetic — that build both a carrier bag and a strand of your DNA.
ModelCarbon's chaining habit — hydrocarbons and the homologous series
A hydrocarbon is a compound of hydrogen and carbon only — nothing else. Crude oil is a finite resource formed over millions of years from the buried remains of ancient plankton, and it is a thick mixture of hundreds of different hydrocarbons, most of them alkanes. Alkanes are saturated: every carbon–carbon bond is a single bond, so the molecule holds as many hydrogen atoms as it possibly can. They share one general formula, \(C_nH_{2n+2}\), and the first four you must know cold are methane \(\mathrm{CH_4}\), ethane \(\mathrm{C_2H_6}\), propane \(\mathrm{C_3H_8}\) and butane \(\mathrm{C_4H_{10}}\).
These four open a homologous series — a family whose members share the same general formula, differ from their neighbour by one \(\mathrm{CH_2}\) unit, react in the same way, and show a smooth gradient in physical properties as the chain lengthens. That gradient is the examiner's favourite handle, because once you know the trend for the first four you can predict the hundredth. The reason the whole thing works is a single fact about carbon: it forms exactly four covalent bonds, which lets it build chains, branches and rings no other everyday element can match.
Predict the formula of the alkane with five carbons (pentane). Substitute \(n=5\) into \(C_nH_{2n+2}\): the hydrogen count is \(2\times5+2=12\), so pentane is \(\mathrm{C_5H_{12}}\). The same rule sanity-checks a formula. Is \(\mathrm{C_6H_{14}}\) a possible alkane? \(2\times6+2=14\) — yes. Is \(\mathrm{C_6H_{12}}\)? That is two hydrogens short, so it cannot be a saturated alkane; it fits the general formula \(C_nH_{2n}\) of an alkene instead.
MechanismFractional distillation and the property gradient
Crude oil is nearly useless as it comes out of the ground; its value is unlocked by fractional distillation, which separates the mixture into fractions — groups of hydrocarbons with similar chain lengths and therefore similar boiling points. The oil is heated until most of it vaporises and fed into the base of a tall fractionating column that is hot at the bottom (around 350°C) and cooler towards the top. As the vapours rise they cool, and each fraction condenses when it reaches its own boiling point: bitumen and heavy fuel oil stay near the hot bottom, then diesel and kerosene, then petrol, while the refinery gases escape from the cool top.
The split works because of a clean property gradient tied to chain length. The longer the molecule, the stronger its intermolecular forces, and that single cause drives four linked trends: longer chains have higher boiling points, are more viscous (thicker, flow less easily), are less volatile, and are less flammable and harder to ignite. Short chains are the opposite — runny, volatile, easy to light — which is exactly why petrol powers a car and bitumen surfaces the road. The fractions are not only fuels; they are the feedstock of the petrochemical industry, the raw material for solvents, detergents, lubricants and, through cracking, the polymers.
Burn any hydrocarbon in plenty of oxygen and you get complete combustion: the carbon is oxidised fully to carbon dioxide and the hydrogen to water, releasing energy. Both elements in the fuel are oxidised, which is the formal meaning of burning.
Balance the complete combustion of propane, \(\mathrm{C_3H_8}\). Write the skeleton \(\mathrm{C_3H_8 + O_2 \rightarrow CO_2 + H_2O}\). Balance carbon first: three carbons need \(\mathrm{3CO_2}\). Balance hydrogen next: eight hydrogens need \(\mathrm{4H_2O}\). Now count the oxygen on the right: \(3\times2=6\) in the carbon dioxide plus \(4\times1=4\) in the water gives ten atoms, so you need \(\mathrm{5O_2}\) on the left. The balanced equation is \[\mathrm{C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O}\] Always balance oxygen last: it sits in two products and is easiest to adjust once carbon and hydrogen are fixed.
MechanismCracking, and the reactive world of alkenes
Fractional distillation creates a business problem: the Earth hands us more long-chain heavy fractions than anyone wants, too little of the short-chain petrol drivers demand, and none of the reactive alkenes industry needs to make plastics. Cracking fixes the mismatch by breaking long, saturated alkane molecules into shorter ones. Every crack splits one long alkane into a shorter alkane (a useful fuel) plus at least one alkene. There are two routes: thermal cracking uses high temperature and pressure, while catalytic cracking passes the vapour over a hot catalyst at around 550°C, which is faster and cheaper.
Alkenes are the point of the exercise. They are unsaturated hydrocarbons, meaning they contain a carbon–carbon double bond, \(\mathrm{C=C}\), and they follow the general formula \(C_nH_{2n}\) — two hydrogens short of the alkane with the same number of carbons. That double bond is a reservoir of reactivity, which is why alkenes do addition reactions that alkanes cannot: the double bond opens and a new atom bolts on to each of the two carbons. Add hydrogen over a nickel catalyst and the alkene becomes an alkane (hydrogenation); add steam with a catalyst and it becomes an alcohol (hydration); add a halogen such as bromine and it becomes a dihalogenoalkane.
That last reaction is also the standard test that tells an alkene from an alkane. Shake the unknown with orange bromine water: an alkene adds the bromine across its double bond and the orange colour disappears, leaving the solution colourless; an alkane has no double bond to react, so the bromine water stays orange.
A molecule of decane, \(\mathrm{C_{10}H_{22}}\), is cracked into octane plus ethene. Write and check the equation. The products are octane \(\mathrm{C_8H_{18}}\) and ethene \(\mathrm{C_2H_4}\): \[\mathrm{C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4}\] Check the atoms balance — carbon: \(10 = 8+2\); hydrogen: \(22 = 18+4\). Notice the alkane product \(\mathrm{C_8H_{18}}\) obeys \(C_nH_{2n+2}\) while the alkene \(\mathrm{C_2H_4}\) obeys \(C_nH_{2n}\); a cracking equation that leaves you with two saturated products has a mistake in it, because cracking must generate at least one double bond.
ModelAlcohols and carboxylic acids — meet the functional group
From here the chapter is about functional groups — the specific cluster of atoms that gives a family its characteristic reactions, so that once you know the group you can predict the chemistry of every member. The alcohols carry the \(\mathrm{-OH}\) (hydroxyl) group; the first four are methanol, ethanol, propanol and butanol. They dissolve in water to give neutral solutions, burn as fuels, react with sodium to fizz off hydrogen, and can be oxidised — by chemical oxidising agents or slowly by air and microbes — into carboxylic acids.
Ethanol is made in two very different ways, and the contrast is a favourite exam comparison. Hydration of ethene passes ethene and steam over a catalyst, \(\mathrm{C_2H_4 + H_2O \rightarrow C_2H_5OH}\); it is fast, continuous and high-purity, but its feedstock is finite crude oil. Fermentation instead lets yeast convert sugars from crops into ethanol at about 30–37°C in the absence of air, using enzymes as biological catalysts; the feedstock is renewable, but it is slow, works in batches and yields an impure, dilute product that must be distilled.
Oxidise an alcohol and you reach the carboxylic acids, which carry the \(\mathrm{-COOH}\) group — methanoic, ethanoic, propanoic and butanoic acid, with ethanoic acid the sharp-tasting acid in vinegar. They behave as acids: they dissolve to give acidic solutions and react with carbonates to give a salt, water and carbon dioxide. Here the Higher-tier subtlety matters — carboxylic acids are weak acids. They only partially ionise in water, so at the same concentration a carboxylic acid has a higher pH (is less acidic) than a strong acid such as hydrochloric, whose molecules ionise completely.
Ethanol brewed in a fermenter comes from glucose. The balanced equation is \[\mathrm{C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2}\] — one glucose molecule yields two of ethanol and two of carbon dioxide (the gas that makes bread rise and beer fizz). The Higher-tier weak-acid idea is written as an equilibrium, because the ionisation is incomplete: \[\mathrm{CH_3COOH \rightleftharpoons CH_3COO^- + H^+}\] The double arrow is the whole point — it says only a fraction of the ethanoic acid molecules have given up their hydrogen ion at any moment, which is exactly why a weak acid is weak.
CasePolymers — from a carrier bag to your DNA
Poly(ethene) from the introduction is made by addition polymerisation: many small unsaturated monomers — alkenes — join into one enormous saturated polymer chain, and nothing else is produced. The double bond in each ethene monomer opens up and links to the next, thousands of times over. To draw the repeating unit, take the monomer, change the double bond to a single bond, extend a bond from each carbon out through the brackets, and write \(n\) outside to mean very many. The same trick turns propene into poly(propene) and chloroethene into PVC.
Condensation polymerisation (Higher tier) works differently: its monomers each carry two functional groups, and each time two monomers join they expel a small molecule, usually water. A diol (two \(\mathrm{-OH}\) groups) reacting with a dicarboxylic acid (two \(\mathrm{-COOH}\) groups) builds a polyester while releasing a water molecule at every link — so from two monomers you get two products, the polymer and the water, which is the fastest way to tell condensation from addition in the exam.
Nature was doing condensation polymerisation billions of years before ICI. Amino acids (Higher tier) each carry an amine group and a carboxylic acid group, so they condense together — losing water at each join — into polypeptides and then proteins. The other great natural polymers follow the same logic of small repeating monomers: DNA is two strands of monomers called nucleotides (just four different ones) twisted into a double helix and carrying your genetic code, while starch and cellulose are both long polymers built from sugars such as glucose. The chemistry that produced an accidental lump of plastic in 1933 is, atom for atom, the chemistry that writes every living thing.
Draw the repeating unit of poly(ethene) from its monomer ethene, \(\mathrm{CH_2=CH_2}\). Start from the monomer's double bond; open it to a single bond so each carbon now has a spare bond; extend those two spare bonds out through square brackets, keep the two hydrogens on each carbon, and write \(n\) outside the bracket. In words, the repeating unit is two carbons each bonded to two hydrogens, joined by a single bond, with the chain continuing out of both ends — and the giant molecule is that unit repeated \(n\) times. The marks are for (1) turning the double bond into a single bond, (2) the two continuation bonds passing through the brackets, and (3) the \(n\).
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Naming and classifying earns easy marks if you are precise: an alkane is saturated with the formula \(C_nH_{2n+2}\); an alkene is unsaturated, has a \(\mathrm{C=C}\), and follows \(C_nH_{2n}\). When you describe the bromine-water test, state both the observation and the reason — the orange bromine water is decolourised because the bromine adds across the double bond — because AQA credits the explanation, not just the colour.
Combustion and cracking questions are marked on balanced equations, so always balance carbon, then hydrogen, then oxygen last, and check the atom totals on both sides. For the ethanol comparison, structure your answer as a trade-off: fermentation is renewable but slow, batch and impure; hydration of ethene is fast and pure but uses finite crude oil — a balanced pair of points scores better than a list of facts about one method.
On polymers, the repeating-unit diagram is where marks are won and lost: turn the double bond into a single bond, take the two bonds out through the brackets, and write \(n\). For Higher tier, remember the one-line discriminator — addition polymerisation makes a single product, condensation polymerisation makes the polymer and a small molecule such as water.