Phenols in ester formation

This reaction between an acid and an alcohol relies on the nucleophilic properties of the lone pair of electrons on the oxygen atom in the alcohol group. However, in the phenol, this oxygen lone pair is partly delocalised in the adjacent benzene ring. This reduces the electron density on the oxygen atom, and makes it less nucleophilic. Consequently, phenols are reluctant to react with acids to form esters in the same way that alcohols did. However, they do react in a similar way to alcohols when mixed with the more reactive acid anhydrides or acyl chlorides to form esters. This is the reaction for phenol and ethanoyl chloride:

The reaction between methyl 4-hydroxybenzoate and ethanoic anhydride produces a similar ester: