Rate constants for reduction of substituted methylperoxyl radicals by ascorbate ions and N,N,N',N'-tetramethyl-p-phenylenediamine

Abstract

Absolute rate constants ( k ) for reduction of substituted methylperoxyl radicals by ascorbate ions and by TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine) in aqueous solutions have been determined by pulse radiolysis. The rate constants vary from lo6 to lo9 M-I s-l, increasing as the electron-withdrawing capacity of the substituent on the peroxyl group increases. Linear correlations are observed between log k and the Taft substituent constants u* for a wide variety of substituents, but not all substituents fit the same line. In the case of ascorbate as reductant, the points for peroxyl radicals that contain halogens on the a-carbon lie on a different line ( p * = 0.41) than that for the other substituents ( p * = 1.25). In the case of TMPD there are also two families of peroxyl radicals: those comprising the electron-donating groups Me through t-Bu ( p * = 5.6) and those containing electron-withdrawing substituents ( p * = 0.64).