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A hemiacetal or a hemiketal have 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 In , alcohol is an that carries at least one (−OH) bound to a atom. The term alcohol originally referred to the primary alcohol (ethyl alcohol), which is and is the main alcohol present in s. An important class of alcohols, of which ...
to an
aldehyde Chemically, an aldehyde is a compound containing a functional group In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same fu ...

aldehyde
or a
ketone In chemistry, a ketone is a functional group with the structure R2C=O, where R can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond). The simplest ketone is acetone (R = R' = methyl) ...
, 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 OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the authoritative power over chemist ...
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 Organic chemistry is a branch of chemistry that studies the structure, properties and reactions of organic compounds, which contain carbon in covalent bonding.Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemist ...

carbonyl group
, in contrast to
acetals
acetals
or
ketals
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 compound, cyclic equivalent of a hemiacetal or a hemiketal. The compound is formed by the Intramolecular reaction, intramolecular nucleophilic addition of a hydroxyl group to the carbonyl group of an alde ...
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 with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

Glucose
and many other exist as cyclic hemiacetals whereas
fructose Fructose, or fruit sugar, is a ketonic simple sugar Monosaccharides (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a cou ...

fructose
and similar
ketoses A ketose is a monosaccharide Monosaccharides (from Greek '' monos'': single, ''sacchar'': sugar), also called simple sugars, are the simplest form of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many o ...
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.


Hemiacetals in nature

Arguably, the most common hemiacetals are sugars, for example
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

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. {{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