Nucleophilic addition of one equivalent of alcohol to an aldehyde or a ketone forms a hemiacetal comprising an OH and an OR group.
The higher energy of hemiacetal than the corresponding carbonyl compound disfavors its formation.
Since alcohols are weak nucleophiles, the formation of hemiacetal is slow under neutral conditions; however, the rate can be enhanced using an acid or a base.
In acid catalysis, a strong acid first protonates the alcohol.
The generated acid catalyst then transfers a proton to the carbonyl oxygen. The resulting strong electrophile is then attacked by alcohol, giving an oxonium intermediate.
Subsequent deprotonation by another molecule of alcohol gives hemiacetal and the regenerated catalyst.
In base catalysis, a base first deprotonates the alcohol, forming a strongly nucleophilic base catalyst — the alkoxide anion. Following this, a nucleophilic attack by the alkoxide forms a carbonyl addition intermediate.
Finally, proton transfer from another molecule of alcohol gives hemiacetal and the regenerated catalyst.
