Alkenes

The alkenes form a homologous series. Like all homologous series, the alkenes:

General formula

The general formula for the alkenes is CnH2n, where n is the number of carbon atoms in the molecule.

Worked example

Decene is an alkene. Its molecules contain 10 carbon atoms. Predict the molecular formula of decene and explain your answer.

The formula will be C10H20. This is because n = 10. So, 2n = (2 × 10) = 20.

Question

Hexene is an alkene. Its molecules contain 6 carbon atoms. Predict the molecular formula of hexene.

The formula will be C6H12.

Structures

The table shows four alkenes, their molecular formulae and their structures.

Alkenes table

The ball and stick models on the right show that the covalent bonds are not really at angles of 90°, as shown in the structures.

The alkenes are unsaturated hydrocarbons:

  • hydrocarbons, because they are compounds containing hydrogen and carbon only
  • unsaturated, because they contain a C=C double bond, which means that they have two fewer hydrogen atoms than the corresponding alkane

The C=C bond is the functional group in the alkenes. It is responsible for the typical reactions of alkenes.