The surprisingly high reactivity of phenoxyl radicals

Article abstract of DOI:10.1021/ja00100a005

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, k₅, are ca. 100-300 times greater than the (already well-known) peroxyl rate constants, k₁. For example, with α-tocopherol in benzene/di-tert-butyl peroxide (1:3, v/v) k₅^293K = 1.1 × 10⁹ M^-1 s^-1 vs k₁^303K = 3.2 × 10⁶ M^-1 s^-1 in a similar nonpolar medium, and with ubiquinol-10 in the same solvent mixture k₅^293k = 8.4 × 10⁷ M^-1 s^-1, while the corresponding value for k₁ is 3.5 × 10⁵ 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., A₅ ～ 10²A₁. For example, with α-naphthol log(A₅/M^-1 s^-1) = 8.9 and E₅ = 2.2 kcal/mol vs log(A₁/M^-1 s^-1) = 6.4 and E₁ = 1.7 kcal/mol. The preexponential factors for H-atom donors more reactive than α-naphthol are even greater; for example, with α-tocopherol in CH₃CN/di-tert-butyl peroxide (1:2, v/v) log(A₅/M^-1 s^-1) = 10.0 and E₅ = 2.0 kcal/mol, and with ubiquinol-0 in benzene/di-tert-butyl peroxide (1:3, v/v) log(A₅/M_-1 s^-1) = 10.5 and E₅ = 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.