Esterification

Esterification is the reaction between a carboxylic acid and an alcohol, catalysed by concentrated sulfuric acid, that produces an ester and water. It is the named reaction that builds the ester linkage, –COO–. Malay: pengesteran · Chinese: 酯化.

In the reaction the –OH of the alcohol and the –OH part of the acid’s –COOH group come together, an ester bond forms between the two molecules, and a molecule of water is released. The concentrated sulfuric acid speeds the reaction up and also absorbs the water, and the mixture is warmed gently rather than boiled hard. The product is recognised by its sweet, fruity smell. The reaction can be summarised as carboxylic acid + alcohol → ester + water, with the sulfuric acid written above the arrow because it is a catalyst rather than a reactant. Naming follows the two parents: the alkyl part from the alcohol is written first and the part from the acid, ending in -oate, second.

Example. Ethanoic acid + ethanol, warmed with concentrated sulfuric acid, gives ethyl ethanoate + water. In the laboratory the ester is smelled directly, or poured into water where it forms a fragrant oily layer, confirming that esterification has taken place.

Confusion to avoid. Do not mix esterification up with neutralisation. Both involve an acid, but neutralisation needs a base and gives a salt and water quickly, while esterification needs an alcohol and a sulfuric acid catalyst and gives an ester and water. Remember that here the sulfuric acid is a catalyst, not a reactant, so it is not used up.

Esterification is the reaction that joins the alcohols and the carboxylic acids, the two oxygen-bearing homologous series, so it sits at the heart of the reaction map of Carbon Compounds and appears often in the essay questions of SPM Chemistry.