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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.
Acid catalyzed acetal formation from the corresponding hemiacetal. Acetals, as already pointed out, are stable tetrahedral intermediates so they can be used as protective groups in organic synthesis. Acetals are stable under basic conditions, so they can be used to protect ketones from a base. The acetal group is hydrolyzed under acidic conditions.
Used in a more general sense, the term X,Y-acetal also refers to any functional group that consists of a carbon bearing two heteroatoms X and Y. For example, N , O -acetal refers to compounds of type R 1 R 2 C(OR)(NR' 2 ) (R,R' ≠ H) also known as a hemiaminal ether or Aminal , a.k.a. aminoacetal.
The latter reagent in itself is an acetal and therefore the reaction is actually a cross-acetalisation. Kinetic reaction control results from 2-methoxypropene as the reagent. 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.
Because of the greater stability of thioacetals, the equilibirum lies on the side of the acetal. In contradistinction to the O,O‑acetal case, it is not needed to remove water from the reaction mixture in order to shift the equilibrium. [65] S,O-Acetals are hydrolyzed a factor of 10,000 times faster than the corresponding S,S-acetals. Their ...
Sustituyo con textos editables. 01:23, 28 August 2015: 680 × 400 (40 KB) Ltsc2335: Reverted to version as of 19:05, 15 January 2007 wrong image: ... Acetals Category
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