The surprisingly high reactivity of phenoxyl radicals

Rate constants have been measured in nonaqueous media for hydrogen atom abstraction by the phenoxyl radical from some biologically important phenols and related compounds. Although the thermochemistry for these reactions must be very similar to the thermochemistry for H atom abstraction from the same substrate by a peroxyl radical, the phenoxyl rate constants, k 5, are ca. 100-300 times greater than the (already well-known) peroxyl rate constants, k 1. For example, with ?-tocopherol in benzene/di-tert-butyl peroxide (1:3, v/v) k 5 293K = 1.1 × 10 9 M -1 s -1 vs k 1 303K = 3.2 × 10 6 M -1 s -1 in a similar nonpolar medium, and with ubiquinol-10 in the same solvent mixture k 5 293k = 8.4 × 10 7 M -1 s -1, while the corresponding value for k 1 is 3.5 × 10 5 M -1 s -1. The greater reactivity of the phenoxyl radical has been traced to the fact that the Arrhenius preexponential factors are much larger than for the corresponding peroxyl radical reactions, i.e., A 5 ~ 10 2A 1. For example, with ?-naphthol log(A 5/M -1 s -1) = 8.9 and E 5 = 2.2 kcal/mol vs log(A 1/M -1 s -1) = 6.4 and E 1 = 1.7 kcal/mol. The preexponential factors for H-atom donors more reactive than ?-naphthol are even greater; for example, with ?-tocopherol in CH 3CN/di-tert-butyl peroxide (1:2, v/v) log(A 5/M -1 s -1) = 10.0 and E 5 = 2.0 kcal/mol, and with ubiquinol-0 in benzene/di-tert-butyl peroxide (1:3, v/v) log(A 5/M -1 s -1) = 10.5 and E 5 = 3.5 kcal/mol. The role that intermediate hydrogen-bonded complexes between the reacting radical and the phenolic hydrogen donor may play in these reactions is discussed, and it is pointed out that our results are likely to be relevant to in vivo radical chemistry.