Rapid equilibrium steps kinetic consequences

Generalization Kinetic Consequences of Rapid-Equilibrium Steps... 

Our recent studies on phenol oxidation by oxoiron(IV) and oxomanganese(IV) porphyrins in aqueous solution (pH 7.6) have shown that the initial step in these reactions is H atom abstraction to generate a phenoxyl radical (Scheme 3). e kinetic isotope effect measured in the present study indicates that H atom abstraction also occurs in the rate determining step of the oxidation of azonaphthol dyes by 1. However, although the structures of azonaphthol dyes are normally shown as azo compounds, in aqueous solution they are in a rapid dynamic equilibrium with their hydrazone tautomers the latter isomer being the dominant species (Scheme 4). This complicates kinetic studies on the dyes, since the substrate is in effect a mixture of two compounds. Consequently one or both the tautomers may be the active form of the substrate providing the H atom for the oxidant. It is important to note that, irrespective of which tautomer is the reactive substrate, H atom abstraction leads to a common azonaphthoxyl radical intermediate and subsequent reactions of this species should be independent of the initial tautomerism (Scheme 5). 

The reaction of methylcobalamin with mercury(n) is extremely rapid and has been studied by stopped flow kinetics. However, methyl transfer from methylcobalamin to tetrachloropalladate(n) is slower. In both methyl transfer reactions there are two kinetically distinct steps. The initial step involves a rapid association of the metal species with methylcobalamin, resulting in a base-on, base-off pre-equilibrium, the electrophilic metal system competing with the cobalt(m) for the lone pair of electrons on the benzimidazole nitrogen. There follows a slower demethylation step in which the electrophilic metal species attacks the Co—C bond in the uncomplexed methylcobalamin. The more rapid transfer with mercury(n) is a consequence of the greater electrophilicity of mercury(n) than palladium(ii), and the mercury(n) produces slightly more of the base-off species. ... 

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