Carbon forms four covalent bonds and can bond to other carbon atoms, so an enormous variety of carbon compounds exists. Compounds within a family behave alike, so their reactions can be predicted.
Crude oil is a finite resource formed over millions of years from the remains of ancient plankton. It is a mixture of hydrocarbons, compounds of hydrogen and carbon only.
Most are alkanes, a homologous series with the general formula CnH2n+2: methane, ethane, propane, butane. They are saturated, meaning every carbon–carbon bond is a single bond.
Crude oil is separated by fractional distillation. It is heated until it vaporises and fed into a column that is hot at the bottom and cool at the top. Hydrocarbons condense at different heights according to their boiling points, giving fractions, themselves mixtures containing molecules with similar boiling-point ranges.
Short chains condense high up and are used for fuel gases and petrol; long chains condense low down and give diesel, lubricants and bitumen. The fractions are the feedstock for the petrochemical industry, which makes solvents, lubricants, polymers and detergents from them.
The properties change gradually with chain length. Longer chains have higher boiling points, are more viscous and less flammable: stronger intermolecular forces make them less volatile, so they form a flammable vapour less readily.
Complete combustion of a hydrocarbon in plenty of oxygen gives carbon dioxide and water and releases energy. Incomplete combustion, with too little oxygen, may also produce carbon monoxide, a toxic gas that is colourless and odourless, and soot, depending on how little oxygen there is.
There is more demand for short-chain hydrocarbons than fractional distillation supplies, so long chains are broken into shorter ones by cracking.
This is thermal decomposition, done either by heating with steam at high temperature (steam cracking) or by passing the vapour over a hot catalyst (catalytic cracking). The products are a mixture of shorter hydrocarbons: alkanes useful as fuels, and alkenes, which are the starting point for polymers.
Alkenes contain a carbon–carbon double bond, which makes them unsaturated. The open-chain alkenes with one C=C bond have the general formula CnH2n: ethene, propene, butene.
The carbon-carbon double bond is the reactive part, so alkenes undergo addition reactions in which it opens and atoms add across it. Adding hydrogen gives an alkane; adding steam gives an alcohol; adding a halogen gives a dihalo compound.
The test for an alkene is bromine water: it is decolourised from orange to colourless by an alkene, and unchanged by an alkane.
Alcohols contain the functional group –OH: methanol, ethanol, propanol, butanol.
The first four dissolve in water to give a neutral solution, though solubility falls as the chain gets longer. They react with sodium to give hydrogen, burn in air, and the primary alcohols studied here can be oxidised to carboxylic acids. Ethanol is made industrially by hydrating ethene, and biologically by fermentation of sugar with yeast at about 30 °C without air.
Carboxylic acids contain –COOH: methanoic, ethanoic, propanoic, butanoic. Ethanoic acid is what makes vinegar sour.
They dissolve to give acidic solutions, react with carbonates to give carbon dioxide, and react reversibly with alcohols, usually with an acid catalyst, to form an ester and water. They are weak acids: only partly ionised, so a solution has a higher pH than a strong acid of the same concentration.
Many alkene molecules join when their carbon-carbon double bonds become single bonds, forming a long polymer chain. Nothing else is produced, so the repeating unit has the same atoms as the monomer.
Ethene gives poly(ethene); propene gives poly(propene). To draw a repeating unit, change the double bond to a single bond and put a bond out of each end.
In condensation polymerisation each monomer has two functional groups, and a small molecule, usually water, is lost each time a link forms.
A diol reacting with a dicarboxylic acid gives a polyester. That loss of a small molecule is the difference from addition polymerisation, where nothing else is produced.
Amino acids have two functional groups: an amino group and a carboxyl group. They join by condensation polymerisation, losing water each time, to form polypeptides, which make up proteins. The sequence of amino acids determines the protein.
DNA is a natural polymer made from four types of nucleotide, arranged as two strands wound into a double helix. Starch and cellulose are also natural polymers, made from sugars.
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