Photo-heterolysis and -homolysis of substituted diphenylmethyl halides, acetates, and phenyl ethers in acetonitrile: Characterization of diphenylmethyl cations and radicals generated by 248-nm laser flash photolysis

Article abstract of DOI:10.1021/ja00175a028

Para-substituted diphenylmethyl halides, acetates, and ethers RPh(R′Ph)CH-X (R, R′ = CF₃ to OCH₃), upon photolysis with ～250-nm light in acetonitrile solutions, undergo homolysis and heterolysis of the C-X bond to give the radicals, RPh(R′Th)CH^? (abbreviated as C^?), and the cations, RPh(R′Ph)CH⁺ (C⁺). Whereas the quantum yields for homolysis (0.2-0.4) are rather independent of the nature of the substituent on the benzene ring, those for heterolysis increase with increasing electron-donator strength from ≤0.07 for CF₃ to 0.3 for OMe. The cation:radical ratios are also dependent on the nucleofugal properties of X. For the halides, the observed heterolysis:homolysis ratios correlate with the pK_a values of the conjugate acids HX and not with the electron affinities of X^?. In acetonitrile, heterolysis is much less endothermic than homolysis. Homolysis and heterolysis can also be effected indirectly by reaction with triplet acetophenone (produced by 308-nm photolysis). Unless stabilized by one or more MeO, the cations decay predominantly by reaction with acetonitrile to give nitrilium ions. However, since this reaction is reversible (shown for the benzhydryl cation), the nitrilium ion contributes only to an insignificant degree to the formation of the final (cation-derived) products, which result from reaction with trace water (main product, benzhydryl alcohol; minor, benzhydrylacetamide). The rate constants for addition of C⁺ to CH₃CN are in the range 3.5 × 10⁵ to 3.8 × 10⁷ s^-1 for the cations with R = R′ = Me to R = H, R′ = CF₃. The rate constants for reaction of C⁺ with halides (ion recombination) are ～2 × 10¹⁰ M^-1 s^-1 (diffusion control). The radicals C^? disappear by dimerization and disproportionate, for which a complete mass balance has been achieved by product analysis for the case of the benzhydryl system. At laser-pulse powers > 10 mJ electronically excited radicals, C^?*, are additionally formed in many cases, via absorption of a light quantum by ground-state C^?.