Hidden chemistry in phenoxyl radical (C6H5O•) coupling reaction mechanism revealed

A novel hidden reaction of the phenoxyl radical (C6H5O center dot) with a specific daughter is found to significantly alter its hitherto accepted coupling reactions' scheme. Transient characterizations and mechanistic evaluations in highly acidic to strongly alkaline aqueous medium reveal this concurrent reaction competing favorably in nanosecond-microsecond time-scale with the five distinct C6H5O center dot + C6H5O center dot reactions, which produce various phenolic endproducts as reported earlier (M. Ye and R. H. Schuler, J. Phys. Chem. 1989, 93, 1898). Presently, only the symmetric 4,4'-dioxo transient precursor, O=C6H5-H5C6=0 that leads to the stable 4,4'-biphenol product, gets partially oxidized by a fraction of remaining C6H5O center dot. The resulting secondary transient (C12H9O2)-C-center dot radical is generated at diffusion-controlled rate, k > 5.0 x 10(9) M-1 s(-1), and follows an independent chemistry. Consequently, when the previously reported five coupled end product distribution ratios were appropriately updated, the respective fractional values revealed a closer match for the symmetric 2,2'- and 4,4'-biphenols with their suggested coupling reaction branching probabilities based on the atomic spin-density distributions in the C6H5O center dot radical (P. Neta, R. W. Fessenden, J. Phys. Chem., 1974, 78, 523). Results also suggest that in the remaining fraction, differential solvation in aqueous medium of various orientation-related encounter complexes (C6H5O center dot center dot center dot C6H5O)formed during coupling favors rearrangement only toward 2,4'-biphenolic product, at the cost of 2- and 4-phenoxyphenolic species. Copyright (C) 2009 John Wiley & Sons, Ltd.