Topic 7: Organic chemistry

Organic chemistry is the study of compounds built around carbon atoms, which can join together to form chains and rings. Most of these compounds come originally from crude oil, a finite resource formed from ancient living organisms. This topic explains how we separate, process and use these molecules, and how the carbon-based building blocks combine into fuels, alcohols, acids and plastics.

Crude oil and hydrocarbons

Crude oil is a thick, dark mixture of a very large number of compounds. Most of these compounds are hydrocarbons, meaning molecules made from hydrogen and carbon atoms only. Crude oil formed over millions of years from the remains of tiny sea creatures and plants buried under layers of rock, so it is described as a finite resource: it is being used far faster than it can ever reform. Because it is a mixture, the different molecules are not chemically joined and can be separated using physical methods based on differences in their boiling points. Crude oil is the main source of fuels and is also a feedstock, supplying raw materials for the petrochemical industry to make solvents, lubricants, polymers and detergents.

Alkanes and the homologous series

The simplest hydrocarbons in crude oil are the alkanes. Alkanes are saturated, which means every carbon-carbon bond is a single bond and the molecule holds as many hydrogen atoms as possible. They form a homologous series: a family of compounds that share the same general formula and similar chemical properties, with each member differing from the next by one CH2 unit. The general formula for the alkanes is CnH2n+2. The first four members are methane CH4, ethane C2H6, propane C3H8 and butane C4H10. Because the bonding is the same all along the series, alkanes react in similar ways, but their physical properties change steadily as the molecules get larger.

Fractional distillation and uses of fractions

Crude oil is separated into more useful mixtures called fractions by fractional distillation. The oil is heated until it evaporates, then the vapours rise up a tall fractionating column that is hottest at the bottom and coolest at the top. As each vapour rises and cools, it condenses back to a liquid when it reaches its own boiling-point level. Hydrocarbons with short chains and low boiling points condense near the top, while long-chain molecules with high boiling points condense near the bottom. Each fraction contains molecules with a similar number of carbon atoms. Useful fractions include petrol and other fuels for cars, kerosene for aircraft, diesel oil, fuel oil for ships and large power stations, and bitumen used to surface roads.

Trends in properties

As the alkane molecules become larger, several properties change in a predictable way. Longer chains have higher boiling points because the forces of attraction between the molecules become stronger and need more energy to overcome. Larger molecules are also more viscous, meaning they flow less easily and feel thicker, so they are harder to pour. Longer-chain hydrocarbons are less flammable, igniting less easily than the short-chain ones. This is why short, light fractions such as petrol are in high demand as fuels, while the heavy fractions are sticky and burn poorly. These trends explain why the smaller, more flammable molecules are the most valuable fuels.

Combustion

Hydrocarbons make good fuels because they release energy when they burn, a reaction called combustion that transfers energy to the surroundings. During combustion the carbon and hydrogen in the fuel are oxidised, meaning they gain oxygen. When there is plenty of oxygen, complete combustion occurs and the only products are carbon dioxide CO2 and water H2O, for example CH4 + 2O2 produces CO2 + 2H2O. If the oxygen supply is limited, incomplete combustion can produce carbon (soot) and the toxic gas carbon monoxide CO, which is dangerous because it is colourless and odourless. Burning fuels also releases carbon dioxide, a greenhouse gas linked to climate change.

Cracking and why it is done

Fractional distillation produces more long-chain hydrocarbons than industry can sell and not enough of the short-chain molecules that customers want for fuels. Cracking solves this mismatch by breaking large, less useful molecules into smaller, more useful ones. There are two main methods: catalytic cracking, which passes the hydrocarbon vapour over a hot catalyst, and steam cracking, which mixes the vapour with steam and heats it to a very high temperature. Cracking always produces some smaller alkanes, which make good fuels such as petrol, and it also produces alkenes. Alkenes are valuable as starting materials for making polymers and many other chemicals, so cracking matches supply to demand and creates important feedstock.

Alkenes and the bromine-water test

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond, written C=C. Because they have a double bond rather than the maximum number of hydrogen atoms, alkenes are described as unsaturated. They form a homologous series with the general formula CnH2n; the first members are ethene C2H4, propene C3H6 and butene C4H8. The reactive double bond makes alkenes more reactive than alkanes, and they react by addition, where atoms add across the C=C bond. This is the basis of the test that distinguishes alkenes from alkanes: when an alkene is shaken with orange bromine water, the bromine adds across the double bond and the solution turns colourless. An alkane leaves the bromine water orange.

Alcohols and carboxylic acids

Alcohols are a homologous series whose molecules all contain the functional group -OH; the first members are methanol, ethanol, propanol and butanol. Ethanol is made industrially by fermenting sugars using yeast, or by reacting ethene with steam. Alcohols dissolve in water to form neutral solutions, react with sodium to give hydrogen, burn in air, and can be oxidised to form carboxylic acids; they are used as fuels and solvents and in alcoholic drinks. Carboxylic acids (Higher) are a series containing the functional group -COOH; examples include methanoic, ethanoic, propanoic and butanoic acid. They are weak acids, only partly ionising in water, so their solutions have a higher pH than strong acids of the same concentration. They react with carbonates to release carbon dioxide and with alcohols, in the presence of an acid catalyst, to make sweet-smelling esters.

Polymers: addition, condensation and natural (Higher)

Polymers are very large molecules made by joining many small molecules called monomers. In addition polymerisation, many unsaturated alkene monomers join together when their double bonds open up; no other product is formed, so poly(ethene) is made from many ethene molecules. Condensation polymerisation (Higher) involves monomers with two functional groups, and each time two monomers join a small molecule such as water is lost; polyesters are made this way from a diol and a dicarboxylic acid. Many natural polymers also exist (Higher): proteins are condensation polymers of amino acids, starch and cellulose are polymers of sugars, and DNA is a natural polymer made of two strands held together to form a double helix and carrying the genetic code.