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RCH(OH)OR′, a product of the addition of an alcohol to an aldehyde (an acetal is formed by the addition of an alcohol to a h.). In the aldose sugars, the h. formation is internal and labile, brought about by the 4-OH or 5-OH attack on the carbonyl O, yielding the furanose or pyranose structures; the h. forms of the sugars are involved in all polysaccharides, as glycosyls or glycosides. SEE ALSO: hemiketal, acetal.

hemi·ac·e·tal .hem-ē-'as-ə-.tal n any of a class of compounds characterized by the group C(OH)(OR) where R is an alkyl group and usu. formed as intermediates in the preparation of acetals from aldehydes or ketones