Alcohols

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

Functional group

The functional group in the alcohols is the hydroxyl group, –OH. It is responsible for the typical reactions of alcohols. Take care not to confuse the –OH group with the hydroxide ion, OH-.

General formula

The general formula for the alcohols is CnH2n+1OH (where n is the number of carbon atoms in the molecule).

Worked example

Decanol is an alcohol. Its molecules contain ten carbon atoms. Predict the formula of decanol and explain your answer.

The formula is C10H21OH. This is because n = 10. So, 2n+1 = (2 × 10) + 1 = 20 + 1 = 21.

Question

Hexanol is an alcohol. Its molecules contain six carbon atoms. Predict the formula of hexanol.

The formula is C6H13OH.

Structures

The table shows three alcohols, their formulae and their structures.

Covalent structures of methanol, ethanol and propanol in a table.
Question

Draw the structure of butanol, C4H9OH. Show all the covalent bonds.

Covalent structure of butanol.

Reactions of alcohols

The alcohols undergo complete combustion to form carbon dioxide and water. For example, ethanol is used as a fuel:

Ethanol + oxygen → carbon dioxide + water

C2H5OH + 3O2 → 2CO2 + 3H2O

However, the alcohols can also be chemically oxidised, without combustion in oxygen, to produce carboxylic acids. For example, ethanol can be oxidised to ethanoic acid using an oxidising agent. It is easier to understand what happens if ethanol is shown as CH3CH2OH in the balanced equation:

Ethanol + oxidising agent → ethanoic acid + water

CH3CH2OH + 2[O] → CH3COOH + H2O

Each of the two oxygen atoms provided by the oxidising agent are shown as [O]. Notice that the left-hand side of the ethanol molecule is unchanged. The reaction involves the carbon attached to the –OH functional group on the right-hand side. The number of carbon atoms in the carboxylic acid is the same as the number of carbon atoms in the alcohol.

Question

Propanol is oxidised by heating with an oxidising agent. Name the carboxylic acid formed in the reaction.

Propanoic acid.