5677-58-7Relevant articles and documents
The surprisingly high reactivity of phenoxyl radicals
Foti,Ingold,Lusztyk
, p. 9440 - 9447 (2007/10/02)
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) k5293K = 1.1 × 109 M-1 s-1 vs k1303K = 3.2 × 106 M-1 s-1 in a similar nonpolar medium, and with ubiquinol-10 in the same solvent mixture k5293k = 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.
