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The kinetics of the interaction of peroxy radicals. I. The Tertiary Peroxy Radicals
Author(s) -
Nangia Prakash S.,
Benson Sidney W.
Publication year - 1980
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550120104
Subject(s) - radical , chemistry , thermochemistry , arrhenius equation , dissociation (chemistry) , bond dissociation energy , activation energy , kinetics , photochemistry , medicinal chemistry , computational chemistry , organic chemistry , physics , quantum mechanics
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 ˙ 2bond 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 ˙2 2→ 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 ˙ 2concentrations (<10 −5 M ), in the absence of added O 2 , an apparent first‐order disappearance of R O ˙ 2can occur reflecting the rate determining breaking of the cumyl—O ˙ 2bond followed by the second step above. The thermochemistry of R O ˙ nis 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 ˙ 3which is too unstable and cannot form even from R O ˙ + O 2 .