4.4 AlkenesEdexcel IGCSE Chemistry: Revision notes

Alkenes are a homologous series of hydrocarbons containing the functional group >C=C< — a carbon-carbon double bond. Their general formula is CnH2n.

Because they contain a double bond, alkenes are described as unsaturated hydrocarbons: the double bond can open up so that more atoms can be added across it, which alkanes (with only single bonds) cannot do.

The double bond must be shown in structural and displayed formulae. The unbranched-chain alkenes with up to four carbons are named using the stem for chain length plus the suffix '-ene':

Note there is no one-carbon alkene, since a double bond needs at least two carbon atoms. From butene onwards, the position of the double bond must be numbered (e.g. but-1-ene has the double bond between carbons 1 and 2).

Alkenes undergo addition reactions because the C=C double bond can open up. When an alkene reacts with bromine, the bromine molecule adds across the double bond to form a dibromoalkane, with no other product formed:

CH2=CH2 + Br2 → CH2BrCH2Br (ethene + bromine → 1,2-dibromoethane)

This is an addition reaction, not substitution, because the whole bromine molecule is added and nothing is lost.

Bromine water is an orange solution. When shaken with an alkene, the bromine adds across the double bond and the solution rapidly turns from orange to colourless, because the coloured bromine is used up in the addition reaction.

Alkanes have no double bond to react with, so when shaken with bromine water, the solution stays orange — no reaction occurs (alkanes cannot undergo addition).

This simple test is used to tell an unknown hydrocarbon sample is saturated or unsaturated.