A hemiacetal is a functional group, or a compound containing this functional group, in the form of:
where R and R' are any alkyl groups.
A hemiacetal can react with an alcohol under acidic conditions to form an acetal, and can dissociate to form an aldehyde and an alcohol.
In organic synthesis, hemiacetals can be prepared in a number of ways:
- Nucleophilic addition of an alcohol to a carbonyl group of an aldehyde
- Nucleophilic addition of an alcohol to a resonance stabilized hemiacetal cation
- Partial hydrolysis of an acetal
Hemiacetals can be converted into acetals:
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 the alcohol's hydroxyl group to the carbon in the C=O bond. Acetals are products of substitution reactions catalyzed by acid. The presence of acid improves the leaving capacity of the hydroxyl group and enables its substitution with an alkoxyl group (-OR). The conversion of a hemiacetal to an 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 formed during the reaction can be removed.
Hemiacetals and hemiketals are generally unstable compounds. In some cases however, stable cyclic hemiacetals and hemiketals can be readily formed. Glucose and many other sugars exist as cyclic hemiacetals.