In organic chemistry, a hemiacetal or a hemiketal has the general formula R1R2C(OH)OR, where R1, R2 is hydrogen or an organic substituent. They generally result from the addition of an alcohol (a compound with at least one −OH group) to an aldehyde (R−CH=O) or a ketone (R2C=O), although the latter are sometimes called hemiketals. Most sugars are hemiacetals.
Nomenclature
According to the IUPAC definition, in R1R2C(OH)OR R1 and R2 may or may not be a hydrogen. In a hemiketal, neither R-group 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.[1] The Greek prefix hèmi means half, 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.[2]
Cyclic hemiacetals and hemiketals are sometimes called lactols.[3] They often form readily, especially when they are 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.
Formation
Formation of hemiacetals
Formation of hemiketals
Solutions of simple aldehydes in alcohols mainly consist of the hemiacetal. The equilibrium is easily reversed and dynamic. The equilibrium is sensitive to steric effects.[4]
Acetalization of aldehydes and ketones
Carbonyl compound
alcohol solvent
%hemiacetal
acetaldehyde
methanol
97
acetaldehyde
ethanol
91
propionaldehyde
methanol
95
bromoacetone
methanol
47
Structures of some readily isolable hemiacetals and hemiketals. Chloral and ethyl glyoxalate illustrate the stabilizing influence of electron-withdrawing groups. The cyclopropanone case illustrates the effect of ring-strain.[5] The two cases on the right illustrate the effect of ring-closure.[4]
Hemiacetals in nature
Arguably, the most common hemiacetals are sugars, for example glucose. The favorability of the formation of a strain-free six-membered ring and the electrophilicity of an aldehyde combine to strongly favor the acetal form.
Left, glucose, a cyclic hemiacetal. Right a lactol of fructose, a cyclic hemiketal.
Reactions
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:
R2C=O + R'OH ⇌ R2C(OH)(OR')
R2C(OH)(OR') + R'OH ⇌ R2C(OR')2 + H2O
Usually, the second reaction is unfavorable. In the presence of a dehydrating agent, it proceeds.
