Alkanes

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

General formula

The general formula for the alkanes is CnH2n+2, where n is the number of carbon atoms in the molecule.

Worked example

Decane is an alkane. Its molecules contain 10 carbon atoms. Predict the molecular formula of decane and explain your answer.

The formula will be C10H22. This is because n = 10. So, 2n + 2 = (2 × 10) + 2 = 20 + 2 = 22.

Question

Hexane is an alkane. Its molecules contain six carbon atoms. Predict the molecular formula of hexane.

The formula will be C6H14.

Structures

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

Table showing four alkanes, their molecule formulae and their structuresTable showing four alkanes, their molecule formulae and their structures

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 alkanes are saturated hydrocarbons:

  • Hydrocarbons, because they are compounds containing hydrogen and carbon only.
  • Saturated, because they only contain single bonds. Each atom has formed its maximum number of bonds.