Topic E: Fossil Fuels and Carbon Compounds

This part begins organic chemistry by looking at fossil fuels, where we obtain most carbon compounds, and the energy released when they burn. It introduces the homologous series as the organising idea for organic families, then studies the two simplest series, alkanes and alkenes, including their reactions and the idea of isomerism.

Fossil fuels and fractional distillation

Fossil fuels (coal, crude oil and natural gas) formed over millions of years from the remains of dead organisms and are finite, non-renewable resources. Crude oil is a mixture of hydrocarbons separated by fractional distillation in a tall column: the oil is vaporised and rises, and fractions condense at different heights according to their boiling points. Smaller molecules with lower boiling points (refinery gas, petrol) come off near the top, while larger molecules with higher boiling points (diesel, bitumen) come off lower down. The boiling point increases with chain length because longer molecules have stronger intermolecular forces. Each fraction is a useful fuel or raw material, showing how a single resource yields many products.

Combustion of fuels

Hydrocarbons release energy by burning in oxygen. Complete combustion, with plenty of oxygen, produces carbon dioxide and water and releases the most energy, for example CH4 + 2 O2 -> CO2 + 2 H2O. Incomplete combustion, with limited oxygen, produces carbon monoxide and soot (carbon) and releases less energy. Carbon monoxide is a toxic, colourless gas that binds to haemoglobin, while soot causes breathing problems and dirties surfaces. Burning fuels also releases carbon dioxide, a greenhouse gas linked to global warming, and sulfur dioxide and nitrogen oxides that cause acid rain. These environmental costs motivate cleaner fuels and better combustion conditions, an important theme in DSE.

Homologous series and general formulae

A homologous series is a family of organic compounds with the same general formula and the same functional group, where each member differs from the next by a CH2 unit. Members share similar chemical properties because they have the same functional group, while physical properties change gradually: as chain length and relative molecular mass increase, boiling point rises and volatility falls because intermolecular forces grow. This regular gradation lets you predict the properties of one member from its neighbours. The alkanes (general formula CnH2n+2) and the alkenes (CnH2n) are the first two series, and the same idea later organises alcohols, carboxylic acids and other families.

Alkanes and their reactions

Alkanes are saturated hydrocarbons, meaning they contain only single carbon-carbon bonds and the general formula CnH2n+2; methane, ethane and propane are the first members. Because their bonds are strong and non-polar, alkanes are fairly unreactive: they do not react with acids, alkalis or aqueous reagents under normal conditions. Their two important reactions are combustion, which makes them valuable fuels, and substitution with halogens in the presence of ultraviolet light, for example CH4 + Cl2 -> CH3Cl + HCl, where a hydrogen atom is replaced by a halogen atom. The low reactivity of alkanes contrasts sharply with alkenes and explains why alkanes are prized mainly as fuels rather than as chemical feedstocks for synthesis.

Alkenes and addition reactions

Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond, C=C, with the general formula CnH2n; ethene and propene are the first members, often made by cracking larger alkanes. The double bond makes alkenes much more reactive than alkanes and is the site of addition reactions, in which a molecule adds across the double bond to give a single saturated product. Ethene adds hydrogen (with a nickel catalyst) to form ethane, adds bromine to form 1,2-dibromoethane, and adds water (with an acid catalyst) to form ethanol. The reaction with bromine water is the standard test for a C=C double bond: alkenes decolourise orange bromine water rapidly, while saturated alkanes do not.

Structural isomerism

Structural isomers are compounds with the same molecular formula but different arrangements of atoms, and therefore different structures and often different physical properties. For example, C4H10 exists as straight-chain butane and branched methylpropane, which have different boiling points. Isomerism arises because carbon atoms can join in chains or branches and because a functional group can sit in different positions along a chain. As the number of carbon atoms increases, the number of possible isomers grows quickly. Recognising and drawing isomers correctly, and naming them with IUPAC rules, is an examined skill: the molecular formula alone never fixes a structure, so you must consider all the ways the atoms can be connected.