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In organic chemistry, a hemiacetal is a functional group the general formula R 1 R 2 C(OH)OR, where R 1, R 2 is a hydrogen atom or an organic substituent. They generally result from the nucleophilic addition of an alcohol (a compound with at least one hydroxy group ) to an aldehyde ( R−CH=O ) or a ketone ( R 2 C=O ) under acidic conditions.
In contrast to variations of R, both R' groups are organic fragments. If one R' is a hydrogen, the functional group is instead a hemiacetal, while if both are H, the functional group is a ketone hydrate or aldehyde hydrate. Formation of an acetal occurs when the hydroxyl group of a hemiacetal becomes protonated and is lost as water.
In organic chemistry, a lactol is a functional group which is the cyclic equivalent of a hemiacetal (−CH(OH)O−) or a hemiketal (>C(OH)O−). The compound is formed by the intramolecular , nucleophilic addition of a hydroxyl group ( −OH ) to the carbonyl group ( C=O ) of an aldehyde ( −CH=O ) or a ketone ( >C=O ).
The resulting molecule has a hemiacetal or hemiketal group, depending on whether the linear form was an aldose or a ketose. The reaction is easily reversed, yielding the original open-chain form. In these cyclic forms, the ring usually has five or six atoms.
A glycosidic bond is formed between the hemiacetal or hemiketal group of a saccharide (or a molecule derived from a saccharide) and the hydroxyl group of some compound such as an alcohol. A substance containing a glycosidic bond is a glycoside .
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions.The same functional group will undergo the same or similar chemical reactions regardless of the rest of the molecule's composition.
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It undergoes rapid polymerization, including forming a hemiacetal cyclic dimer. Under alkaline conditions, it undergoes a rapid aldol condensation. Hydroxyacetone can be produced by degradation of various sugars. In foods, it is formed by the Maillard reaction. It reacts further to form other compounds with various aromas. [6]