Hemiacetal

Hemiacetals and hemiketals are compounds that are derived from aldehydes and ketones respectively. The Greek word hèmi means half. These compounds are formed by formal addition of an alcohol to the carbonyl group.

When the alcohol group is replaced by a second alkoxy group, an acetal or a ketal, respectively, is formed.

The general formula of a hemiacetal is R¹R²C(OH)OR, where R¹ or R² is often hydrogen and R (bonded to O) is not hydrogen.
While in the IUPAC definition of a hemiacetal R¹ or R² may or may not be a hydrogen, in a hemiketal none of the R-groups is an H. Hemiketals are regarded as hemiacetals that have no R-groups being H, thus a subclass of the hemiacetals.

Hemiacetals and hemiketals are generally unstable compounds. In some cases however, stable cyclic hemiacetals and hemiketals, called lactols, can be readily formed, especially when 5- and 6-membered rings are possible. In this case an intramolecular OH group reacts with the carbonyl group. Glucose and many other aldoses exist as cyclic hemiacetals whereas fructose and similar ketoses exist as cyclic hemiketals. Examples of hemiketals which are stable are OH-CH2-CH2-CH2-C(=O)-CH3,OH-CH2-(CH2)4-C(=O)-CH3.

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

Hemiacetals and hemiketals may be thought of as intermediates in the reaction between alcohols and aldehydes or ketones, with the final product being an acetal or a ketal:

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

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