Mechanism of aromatic substitution by free radicals

Abstract

hydrogen (reactions 35, 28, 32, 33 and 26). A decrease in the concentration of radicals again will favor the reactions of phenyl radical with solvent and decrease the importance of the reactions of phenyl radical with its descendants, benzyl and adduct radicals. Consequently the yields of products of reaction of benzyl and adduct radicals with themselves and with each other will increase relative to the products of reaction of these radicals with phenyl radical. As these latter reactions become of small impor­ tance, the distribution of products from the remaining reactions should become nearly independent of concentration. Two complications must be considered. First, benzyl radical results from abstraction of hydrogen from toluene not only by phenyl radical, but probably also to some extent by benzoyloxy radical. This circumstance, while it would complicate a quantitative kinetic expression, should not affect the qualitative conclusions. The existence of a separate source of benzyl radical should not alter the direction in which the yield of any product changes with decreasing concentration. A second complication is the reactions of adduct radical by coupling with or loss of hydrogen to benzoyloxy radical (reaction 27). This well may be a process of some importance at high concentration. The importance of this reaction too must decrease with di­