The kinetics of the interaction of peroxy radicals. I. The Tertiary Peroxy Radicals

Existing data on the self‐reactions of tertiary peroxy radicals R O 2 has been reanalyzed and corrected to deduce Arrhenius parameters for both termination and nontermination paths. For R = t ‐Butyl, these are log k t ( M −1 sec −1 ) = 7.1 ‐ (7.0/θ) and log k nt ( M −1 sec −1 ) = 9.4 ‐ (9.0/θ), respectively, different from those recommended by other authors. The higher magnitudes observed for termination processes of tertiary peroxy radicals like those of cumyl and 1,1‐diphenylethyl have been discussed in terms of a much greater cage recombination of cumyloxy radicals as contrasted with t ‐butoxy radicals. It is shown that for benzyl peroxy radicals, the R —O bond dissociation energy is sufficiently low (18–20 kcal) that reversible dissociation into R ˙ + O 2 opens a competing second‐order path to fast recombination R ˙ + R O → R OO R . This path is probably not important for cumyl peroxy radicals under usual experimental conditions but can become important for 1,1‐diphenyl ethyl peroxy radicals at (O 2 ) < 10 −3 M . At very low R O concentrations (<10 −5 M ), in the absence of added O 2 , an apparent first‐order disappearance of R O can occur reflecting the rate determining breaking of the cumyl—O bond followed by the second step above. The thermochemistry of R O is used to show that the reaction of R 2 O 4 → 2 R O + O 2 must be concerted and cannot proceed via R O which is too unstable and cannot form even from R O ˙ + O 2 .