Hemiacetal

Encyclopedia : H : HE : HEM : Hemiacetal

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

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

Reactions

Synthesis

In organic synthesis, hemiacetals can be prepared in a number of ways:

Nucleophilic addition of an alcohol to a carboxyl group
Nucleophilic addition of an alcohol to a resonance stabilized hemiacetal cation
Partial hydrolysis of an acetal

Hemiacetals can be converted into acetals:

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 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 S_N1 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, for example, exist as a cyclic hemiacetals.

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.