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 group ...

or a

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...

, 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 containing a ...

, in contrast to

acetals In organic chemistry, an acetal is a functional group with the connectivity . Here, the R groups can be organic fragments (a carbon atom, with arbitrary other atoms attached to that) or hydrogen, while the R' groups must be organic fragment ...

or ketals, which are formed when a second alkoxy group has been added to the structure. Cyclic hemiacetals and hemiketals are sometimes called

lactol In organic chemistry, a lactol is the cyclic equivalent of a hemiacetal or a hemiketal. The compound is formed by the intramolecular nucleophilic addition of a hydroxyl group to the carbonyl group of an aldehyde or a ketone. A lactol is often ...

s.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, using ...

and many other aldoses exist as cyclic hemiacetals whereas

fructose Fructose, or fruit sugar, is a ketonic simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbe ...

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, using ...

. The favorability of the formation of a strain-free six-membered ring and the

electrophilicity In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carrie ...

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