hemiacetal (hem´e-as´e-tal)

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 hemiacetal). 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.


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