Alkanes

Alkanes are saturated hydrocarbons of general formula CₙH₂ₙ₊₂, while cycloalkanes have the formula CₙH₂ₙ because closing the chain into a ring removes two hydrogen atoms.

Both contain only single C–C and C–H bonds, which are strong and non-polar, making them relatively unreactive at room temperature.

Key Definition Structural isomers are compounds that share the same molecular formula but differ in the arrangement of their atoms.

In alkanes only chain isomerism is possible, because there is no functional group whose position could be varied.

The number of possible isomers grows rapidly as the carbon count increases, and IUPAC nomenclature names each one from the longest carbon chain plus locants that show the positions of any branches.

Crude oil is separated by fractional distillation: the column is hot at the bottom and cool at the top, so longer-chain hydrocarbons, which have higher boiling points, condense low down at the hot bottom, while shorter chains rise and condense higher up.

Cracking breaks long-chain alkanes into shorter, more useful alkanes and alkenes, while reforming converts straight-chain alkanes into branched, cyclic or aromatic compounds.

Combustion of alkanes produces carbon dioxide and water, but it also generates several pollutants, particularly when combustion is incomplete.

Carbon monoxide, formed by incomplete combustion, is toxic because it binds to haemoglobin (forming carboxyhaemoglobin), reducing the blood's capacity to carry oxygen.

Oxides of nitrogen (NOₓ), formed at the high temperatures inside engines, and sulfur dioxide (SO₂), from sulfur impurities in the fuel, are acidic oxides that dissolve in rainwater to form acid rain, while CO₂ is a greenhouse gas linked to climate change.

Sustainability and emissions concerns motivate the search for alternative fuels.

Bioethanol is in theory carbon-neutral, because the CO₂ released when it burns was previously absorbed from the atmosphere by the crop during photosynthesis, but in practice farming and distillation typically rely on fossil fuels, so it is not truly neutral.

Hydrogen produces only water on combustion and so releases no carbon dioxide at the point of use, but its overall carbon neutrality depends on the source of the electricity used to generate it by electrolysis.

Alkanes react with halogens (such as chlorine or bromine) in the presence of UV light by free radical substitution, which proceeds in three stages:

1. Initiation: UV light breaks the halogen–halogen bond homolytically to form two halogen radicals.
2. Propagation: the chain is sustained through alternating radical–molecule reactions, with a new radical regenerated at each step.
3. Termination: two radicals combine (pair up), removing radicals from the system and stopping the chain.

Initiation: Cl₂ → 2Cl• (UV light) Propagation: Cl• + CH₄ → CH₃• + HCl Propagation: CH₃• + Cl₂ → CH₃Cl + Cl• Termination: Cl• + Cl• → Cl₂ Termination: CH₃• + Cl• → CH₃Cl Termination: CH₃• + CH₃• → C₂H₆

Half-arrows (fish-hook arrows) show the movement of single electrons, distinguishing radical mechanisms from the full (double-headed) curly arrows used for polar mechanisms.

Examiners require correct dot notation on every radical species at every stage of the mechanism.

The reaction has limited use in synthesis because further substitution and reaction at different C–H positions produce mixtures of products and isomers, so isolating a single pure halogenoalkane is difficult and inefficient.