Studies on Phenols The Kinetic Isotope Effect

The early experiments of Goldschmidt clearly indicated that phenols are sensitive to radical attack. Not only were fairly stable radicals found in oxidation processes of phenols (Goldschmidt and Schmidt, 1922 Goldschmidt and Stiegerwald, 1924), but the oxidation of hydroquinone to quinone could also be brought about by the stable free radical 2,2-diphenyl-l-picrylhydrazyl (DPPH, Goldschmidt and Renn, 1922). The mechanism of the radical attaok remained unknown for a long time. The kinetic isotope effect played a very important role in its elucidation. 

The first suggestion on the mechanism of the reaction between phenols and peroxy radicals emerged from experiments on the antioxidant effect of phenols, by Bolland and ten Have (1947a, b). These authors found a fair correlation between the increase of chain-terminating efficiency and the decrease of the redox potential of phenols and suggested a mechanism in which the phenolic hydrogen atom is transferred to the peroxy radical, i.e., 

Another description of a common mechanism for radical reactions of phenols and aromatic amines was given by Boozer and Hammond (1954). They suggested the reversible formation of a loose molecular complex in the first step of the reaction between a peroxy radical and phenol, i.e., 

The mechanism of reaction (19) was assumed to be dependent on the structure of the inhibitor (19) would involve the abstraction of the phenolic hydrogen atom. The whole idea was put forward, in part, to account for the failure of deuteriated inhibitors to show a kinetic isotope effect (Hammond et al., 1955). Another argument was that the Hammett equation, correlating the reactivities of the antioxidants, suggested the 

The scheme proposed in (18) and (19) resembles one frequently applied to multi-step reactions (Bodenstein, 1913). If a steady-state concentration is assumed for the complex (and for the chain carrier RO2 radicals) the overall rate of the process consisting of steps (18) and (19) is 

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