The mechanism of the reaction between t-butyl hydroperoxide and 5,10,15,20-tetra(N-methyl-4-pyridyl)porphyrinatoiron(III) pentachloride in aqueous solution

Abstract

The kinetics of the reaction of t-butyl hydroperoxide with 5,10,15,20-tetra(N-methyl-4-pyridyl)-porphyrinatoiron(III) pentachloride in aqueous solution have been investigated using 2,2′-azinobis(3-ethylbenzthiazoline-6-sulphonate)(ABTS) as a one-electron trap for the active oxidants. The reaction shows first-order dependence on the concentration of the hydroperoxide and of the iron(III) porphyrin. The measured second-order rate constants (k₂) increase linearly with the ionic strength of the medium. However, large changes in the concentrations of the buffers, while maintaining a constant ionic strength, have only small effects on k₂. The log k₂/pH profile of the reaction, from pH 4–10.2, is complex and in acidic solution the k₂ values show a dependence on the nature of the buffering species.

The products from the catalysed reaction of t-butyl hydroperoxide in the presence and absence of ABTS have been determined and the accountability of the oxidant is excellent. With ABTS present the major product is t-butyl alcohol whilst in its absence the yield of this alcohol is very small and acetone, methanol, formaldehyde and t-butylmethyl peroxide predominate. The product distribution is not dependent on the pH of the reaction medium but it is sensitive to the presence of dioxygen. In particular, the yields of methanol and t-butylmethyl peroxide are highest under anaerobic conditions whereas in air these yields decrease with a concomitant increase in formaldehyde.

The mechanisms of peroxide bond cleavage by the iron(III) porphyrin are discussed. The results are shown to be in agreement with a homolytic process generating an oxoiron(IV) porphyrin and a t-butoxyl radical rather than a heterolytic step to give an oxoiron(IV) porphyrin π radical cation and t-butyl alcohol.