| Abstract
| - Arrhenius rate expressions were determined for β-scission of phenoxyl radical from 1-phenyl-2-phenoxyethanol-1-yl, PhC•(OH)CH2OPh (V). Ketyl radical V was competitively trapped by thiophenolto yield PhCH(OH)CH2OPh in competition with β-scission to yield phenoxyl radical and acetophenone. A basis rate expression for hydrogen atom abstraction by sec-phenethyl alcohol, PhC•(OH)CH3, from thiophenol, log(kabs/M-1 s-1) = (8.88 ± 0.24) − (6.07 ± 0.34)/θ, θ = 2.303RT, wasdetermined by competing hydrogen atom abstraction with radical self-termination. Self-terminationrates for PhC•(OH)CH3 were calculated using the Smoluchowski equation employing experimentaldiffusion coefficients of the parent alcohol, PhCH(OH)CH3, as a model for the radical. The hydrogenabstraction basis reaction was employed to determine the activation barrier for the β-scission ofphenoxyl from 1-phenyl-2-phenoxyethanol-1-yl (V): log(kβ/s-1) = (12.85 ± 0.22) − (15.06 ± 0.38)/θ,kβ(298 K) ca. (64.0 s-1 in benzene), and log(kβ/s-1) = (12.50 ± 0.18) − (14.46 ± 0.30)/θ, kβ(298 K) =78.7 s-1 in benzene containing 0.8 M 2-propanol. B3LYP/cc-PVTZ electronic structure calculationspredict that intramolecular hydrogen bonding between the α-OH and the −OPh leaving group ofketyl radical (V) stabilizes both ground- and transition-state structures. The computed activationbarrier, 14.9 kcal/mol, is in good agreement with the experimental activation barrier.
|
