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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.
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.
Acetals are stable compared to hemiacetals but their formation is a reversible equilibrium as with esters. As a reaction to create an acetal proceeds, water must be removed from the reaction mixture, for example, with a Dean–Stark apparatus , lest it hydrolyse the product back to the hemiacetal.
The runners-up used are also cyclic acetals with 1,2‑hydroxythiols or dithioglycols – the so-called O,S– or S,S-acetals. Ethylene glycol 1,3‑Propadiol. Overall, trans-acetalation plays a lesser role in forming protective acetals; they are formed as a rule from glycols through dehydration.
Hemiacetals (1 C, 2 P) K. Ketals (1 C, 11 P) Pages in category "Acetals" The following 17 pages are in this category, out of 17 total.
If reaction is between the C-4 hydroxyl and the aldehyde, a furanose is formed instead. [1] The pyranose form is thermodynamically more stable than the furanose form, which can be seen by the distribution of these two cyclic forms in solution. [2] Formation of pyranose hemiacetal and representations of β-D-glucopyranose
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).