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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.
D-ribose in itself is a hemiacetal and in equilibrium with the pyranose 3. In aqueous solution ribose is 75% pyranose and 25% furanose and a different acetal 4 is formed. Selective acetalization of carbohydrate and formation of acetals possessing atypical properties is achieved by using arylsulfonyl acetals.
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.
The anomeric centre in hemiacetals is the anomeric carbon C-1; in hemiketals, it is the carbon derived from the carbonyl of the ketone (e.g. C-2 in D-fructose). In aldohexoses the anomeric reference atom is the stereocenter that is farthest from the anomeric carbon in the ring (the configurational atom, defining the sugar as D or L).
The β-D-glucopyranosyl group which is obtained by the removal of the hemiacetal hydroxyl group from β-D-glucopyranoseIn organic chemistry, a glycosyl group is a univalent free radical or substituent structure obtained by removing the hydroxyl (−OH) group from the hemiacetal (−CH(OH)O−) group found in the cyclic form of a monosaccharide and, by extension, of a lower oligosaccharide.
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 .
Hemithioacetal functional group. In organic chemistry, hemithioacetals (or thiohemiacetals) are organosulfur compounds with the general formula R−CH(−OH)−SR’.They are the sulfur analogues of the acetals, R−CH(−OH)−OR’, with an oxygen atom replaced by sulfur (as implied by the thio-prefix).
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 ).