Recap - Alkenes

The second homologous series is the alkenes. Their names all end in –ene, for example ethene.

Alkenes all contain a carbon to carbon double bond which makes them more reactive than the alkanes. The alkenes have the general formula {C_n}{H_{2n}}.

The first four alkenes. Ethene (C2H4) has two carbons double-bonded to each other. The remaining two bonds on each carbon connect to a single hydrogen. Propene (C3H6) has a carbon with two hydrogens double-bonded to a carbon with one hydrogen, joined to a carbon with three hydrogens (CH2CHCH3). Butene (C4H8) has a carbon with two hydrogens double-bonded to a carbon with one hydrogen, joined to a carbon with two hydrogens, joined to a carbon with three hydrogens (CH2CHCH2CH3). Pentene (C5H10) has a carbon with two hydrogens double-bonded to a carbon with one hydrogen, joined to a carbon with two hydrogens, joined to a carbon with two hydrogens, joined to a carbon with three hydrogens (CH2CHCH2CH2CH3).

The alkenes are unsaturated. This means that they have a carbon to carbon double bond. The alkanes are saturated because they only contain single bonds. An unsaturated hydrocarbon decolourises bromine water quickly. Alkanes do this slowly. Alkenes can be used as fuels, solvents and starting materials for everyday products such as plastics and alcohol.

Naming alkenes

Naming alkenes has the same rules as alkanes.

However, the position of the carbon to carbon double bond must be identified.

Example

A carbon atom with two hydrogens double-bonded to a carbon with one hydrogen, joined to a carbon with one hydrogen and a methyl group, joined to a carbon with three hydrogens.3-methylbut-1-ene

Note that the functional group (in the case of alkenes, the double bond) is given the lowest possible number first, before any branches are numbered.

General formula, functional group and properties of alkenes.