Alkanes

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

• have the same general formula
• differ by CH₂ in molecular formulae from neighbouring compounds
• show a gradual variation in physical properties, such as their boiling points
• have similar chemical properties

General formula

The general formula for the alkanes is C_nH_2n+2, where n is the number of carbon atoms in the molecule.

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.

Reveal answer

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 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 their carbon atoms are joined by C-C single bonds