AlkanesCambridge IGCSE Chemistry: Revision notes

Alkanes are a homologous series of hydrocarbons with the general formula $C_nH_{2n+2}$.

The bonding in alkanes is entirely single covalent bonds, meaning alkanes are saturated hydrocarbons — every carbon-carbon bond is a single bond. You should be able to name and draw the structural and displayed formulae of unbranched alkanes containing up to four carbon atoms (methane, ethane, propane, butane).

Alkanes are described as generally unreactive, except in two specific types of reaction:

1. Combustion — alkanes burn in oxygen, releasing energy (this is why they are used as fuels)
2. Substitution by chlorine — under the right conditions, chlorine atoms can replace hydrogen atoms in an alkane

Outside of these two reaction types, alkanes do not readily react — this fits with them being saturated, with no reactive double bond to attack.

The substitution reaction of alkanes with chlorine is a photochemical reaction — this means ultraviolet (UV) light provides the activation energy needed for the reaction to start.

During the reaction, one hydrogen atom on the alkane is replaced by one chlorine atom (limited to monosubstitution at this level), producing a chloroalkane and hydrogen chloride gas.

You should be able to draw the structural or displayed formulae of the monosubstituted product.

Large alkane molecules can be broken down into smaller, more useful molecules — including alkenes — by cracking, using a high temperature and a catalyst. This links alkanes directly to the production of alkenes and hydrogen, and is done because shorter-chain hydrocarbons (like those used as fuels) are in higher demand than the longer chains obtained directly from crude oil.