Hemiacetal

A hemiacetal or a hemiketal is a compound that results from the addition of an alcohol to an aldehyde or a ketone, respectively. The Greek word hèmi, meaning half(semi), refers to the fact that a single alcohol has been added to the carbonyl group, in contrast to acetals or ketals, which are formed when a second alkoxy group has been added to the structure. A hemiacetal or a hemiketal is a compound that results from the addition of an alcohol to an aldehyde or a ketone, respectively. The Greek word hèmi, meaning half(semi), refers to the fact that a single alcohol has been added to the carbonyl group, in contrast to acetals or ketals, which are formed when a second alkoxy group has been added to the structure. 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 can be a hydrogen. Hemiketals are regarded as hemiacetals where none of the R-groups are H, and are therefore 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 with 5- and 6-membered rings. 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. Hemiacetals can be synthesized in a number of ways: