A hemiacetal or a hemiketal has the general formula R¹R²C(OH)OR, where R¹ or R² is hydrogen or an organic substituent. They generally result from the addition of an alcohol to an

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

or a ketone, although the latter are sometimes called hemiketals. Most sugars are hemiacetals.

Nomenclature

According to the

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

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. The Greek prefix ''hèmi'' means half, refers to the fact that a single alcohol has been added to the

carbonyl group In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...

, in contrast to acetals or ketals, which are formed when a second alkoxy group has been added to the structure. Cyclic hemiacetals and hemiketals are sometimes called lactols.IUPAC Gold Boo
lactols
/ref> 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 Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

and many other aldoses exist as cyclic hemiacetals whereas fructose and similar

ketoses A ketose is a monosaccharide containing one ketone group per molecule. The simplest ketose is dihydroxyacetone, which has only three carbon atoms. It is the only ketose with no optical activity. All monosaccharide ketoses are reducing sugars, be ...

exist as cyclic hemiketals.

Formation

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. RobustHemiacetals

Hemiacetals in nature

Arguably, the most common hemiacetals are sugars, for example

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

. 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. {{clear

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.

References

Functional groups Hemiacetals