Studies on the Mechanism of Oxime and Semicarbazone Formation

Abstract

At neutral pH hydroxylamine and semicarbazide react rapidly with a number of aldehydes and ketones to form addition compounds which lack the ultraviolet and infrared absorption of the original carbonyl compound. These addition compounds undergo a slow, acid-catalyzed dehydration to form oxime or semicarbazone. The decrease in the rate of oxime formation at acid pH appears to be Due to a transition to a rate-limiting attack of free nitrogen base on the carbonyl compound and is not dependent on general acid catalysis. Dehydration of the addition complex, addition of water to the oxime or semicarbazone and addition of semicarbazide to the carbonyl group are subject to both specific and general acid catalysis, while the addition of the stronger base, hydroxylamine, to the carbonyl group is specific acid-catalyzed to only a small extent. © 1959, American Chemical Society. All rights reserved.