Hemiacetals and hemiketals are compounds that are derived from respectively aldehydes and ketones. 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.
Formula and formation
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. 
Cyclic hemiacetals and hemiketals
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.
In organic synthesis, hemiacetals can be prepared in a number of ways:
Hemiacetals and hemiketals may be thought of intermediate between alcohols and aldehydes or ketones, and acetals and ketals:
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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