A hemiacetal is a functional group or compound containing the function group in the form of:

where R and R' are any carbon backbones.

A hemiacetal can react with an alcohol under very acidic conditions to form an acetal, and can dissociate to form an aldehyde and an alcohol.

Reactions

Synthesis

1. Nucleophilic addition of an alcohol to a carboxyl group
   • aldehyde or ketone + alcohol + acid (catalyst) or base (catalyst) ↔ hemiacetal
     • Nucleophilic addition of an alcohol to a resonance stabilized hemiacetal cation
       • hemiacetal + alcohol + acid (catalyst) ↔ acetal + water

An aldehyde dissolved in water exists in equilibrium with low concentrations of its hydrate [R-CH(OH)2]. Similarly, in excess alcohol, the aldehyde, its hemiacetal, and its acetal all exist in solution.

Hemiacetal results from addition of alcohol's hydroxyl group to carbon in C=O bond. Acetals are products of substitution reactions catalyzed by acid. Presence of acid improves leaving capacity of hydroxyl group and enables its substitution with alkoxyl group (-OR). Coversion of hemiacetal to acetal is an SN1 reaction.

Ketones give hemiketals and ketals. These do not form as readily as hemiacetals and acetals. To increase yields of ketals or acetals water molecules that form during reaction can be removed.

Hemiacetals and hemiketals are unstable compounds. In some cases, stable cyclic hemiacetals and hemiketals can be readily formed. Glucose, for example, exists as a cyclic hemiacetal.