An ester is a derivative of a carboxylic acid in which the hydrogen of the carboxyl group is replaced by an alkyl group (Section 1.3E). Draw a structural formula of methyl acetate, which is derived from acetic acid by replacement of the \(\mathrm{H}\) of its \(-\mathrm{OH}\) group by a methyl group. Determine if proton transfer to this compound from \(\mathrm{HCl}\) occurs preferentially on the oxygen of the \(\mathrm{C}=\mathrm{O}\) group or the oxygen of the \(\mathrm{OCH}_{3}\) group.

Acetic acid has the chemical formula \(\mathrm{CH}_{3}\mathrm{COOH}\). The structure consists of a methyl group \((\mathrm{CH_{3}})\), a carbonyl group \((\mathrm{C=O})\), and a hydroxyl group \((-\mathrm{OH})\). The structural formula for acetic acid is: \(\mathrm{CH_{3}-C(=O)-OH}\)

To form methyl acetate, we replace the hydrogen atom of the -OH group in acetic acid with a methyl group \((\mathrm{CH}_{3})\). The structural formula of methyl acetate is: \(\mathrm{CH_{3}-C(=O)-OCH_{3}}\)

When HCl donates a proton to methyl acetate, either the oxygen of the C=O group or the oxygen of the OCH3 group can accept the proton. If the oxygen of the C=O group accepts the proton, the intermediate is: \(\mathrm{CH_{3}-C(+)=O-H}\) If the oxygen of the OCH3 group accepts the proton, the intermediate is: \(\mathrm{CH_{3}-C(=O)-OCH_{3}^{+}}(\mathrm{H})\) The protonated form with a positive charge on the carbonyl oxygen is less stable as the positive charge is placed on a highly electronegative atom. On the other hand, the protonated form with a positive charge on the OCH3 oxygen forms a more stable intermediate, as the positive charge can be better stabilized through resonance with the carbonyl group. Therefore, proton transfer to methyl acetate from HCl occurs preferentially on the oxygen of the \(\mathrm{OCH_{3}}\) group.