Hemiacetals are formed by reacting one equivalent of alcohol with the carbonyl group of an aldehyde or ketone. This reaction is acid catalyzed and is equivalent to the formation of hydrates.

Reaction Mechanism:

Stage 1. Protonation of carbonyl oxygen.

Stage 2. Nucleophilic attack of methanol on the protonated carbonyl.

Stage 3. Deprotonation of the hemiacetal