Abstract
The autoxidation of benzaldehyde in glacial acetic acid catalyzed by cobalt salts has been studied by kinetic and analytical methods. In the initial phase of the reaction the oxygen reacts quantitatively with the aldehyde to form perbenzoic acid, but as the reaction proceeds the peracid concentration falls below that of the oxygen absorbed due to (
a
) the catalyzed decomposition of the perbenzoic acid, and (
b
) the reaction of the perbenzoic acid with benzaldehyde to give benzoic acid. The relative contributions of these reactions to the decay of the peracid concentration has been determined. The initiating reaction has been shown to be the interaction of the cobaltic ion with the aldehyde according to Co
3+
+ C
6
H
5
CHO → Co
2+
+ C
6
H
5
CO·. (1) Direct measurements of the rate of this reaction agreed with the value of the rate of initiation determined by an analysis of the kinetics of the rate of the inhibited oxidation. The overall rate of oxidation may be fully explained by the following kinetic scheme: C
6
H
5
CO· + O
2
→ C
6
H
5
COOO· (2) C
6
H
5
COOO· + C
6
H
5
CHO → C
6
H
5
COOOH + C
6
H
5
CO· } (propagation), (3) 2C
6
H
5
COOO → inert products (termination). (4) The chain length and the activation energies of the elementary reactions have been determined. The oxidation was inhibited by hydroquinone, diphenylamine and
β
-napthol and retarded by benzoquinone. The rate of the retarded oxidations satisfied the relation rate of oxidation =
k'
/
k"
+ [benzoquinone]· By considering the reaction which is responsible for the retardation, C
6
H
5
COOO· + C
6
H
4
O
2
→ inert products, (5) in conjunction with the normal termination reaction (4) it is shown that at retarder concentrations below 10
-3
M radical-radical termination is the more important reaction, but at 10
-1
M retarder concentration radical-radical interaction contributes only 1% to the termination process.
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