Oxidation by molecular oxygen. VI. Iron(III)-catalyzed oxidation of acetoin by oxygen and hydrogen peroxide. Model for some enzymic redox reactions

Abstract

The kinetics, stoichiometries, and mechanisms of the following oxidations of acetoin (3-hydroxy-2butanone) to biacetyl(2,3-butanedione) have been investigated : (1) the Fe(II1)-catalyzed oxidation by 02, (2) the Fe(II1)-catalyzed oxidation by H202, and (3) oxidation by Fe(II1) under anaerobic conditions. In addition, approximate rate constants for the acid-catalyzed enolizations of acetoin have been determined. At 25 O, ionic strength ca. 0.2 M, and pH 1.5-2.3, the rate of 0 2 uptake in reaction 1 follows the expression: rate = (9.1 X 10-7 se~-1)[Fe(III)][acetoin]/[H+], and 2 mol of acetoin reacts with 1 mol of O2 to give 2 mol of biacetyl. Under comparable conditions reaction 2 proceeds faster than reaction 1 and can occur by two different mechanisms, one (2a) which involves Fe(I1) as an intermediate, and another (2b) in which Fe(I1) is not an intermediate. In both cases 1 mol of acetoin reacts with 1 mol of H2O2 to give 1 mol of biacetyl. At 25O, ionic strength ca. 0.2 M and pH 1.6-2.1 the disappearance of H202 in reaction 2b follows the expression: rate = (2.5 X lop3 A4-