ABSTRACT
Arrhenius rate expressions were determined for beta-scission of phenoxyl radical from 1-phenyl-2-phenoxyethanol-1-yl, PhC*(OH)CH2OPh (V). Ketyl radical V was competitively trapped by thiophenol to yield PhCH(OH)CH2OPh in competition with beta-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(k(abs)/M(-1) s(-1)) = (8.88 +/- 0.24) - (6.07 +/- 0.34)/theta, theta = 2.303RT, was determined by competing hydrogen atom abstraction with radical self-termination. Self-termination rates for PhC*(OH)CH3 were calculated using the Smoluchowski equation employing experimental diffusion coefficients of the parent alcohol, PhCH(OH)CH3, as a model for the radical. The hydrogen abstraction basis reaction was employed to determine the activation barrier for the beta-scission of phenoxyl from 1-phenyl-2-phenoxyethanol-1-yl (V): log(k beta)/s(-1)) = (12.85 +/- 0.22) - (15.06 +/- 0.38)/theta, k beta (298 K) ca. (64.0 s(-1) in benzene), and log(k beta /s(-1)) = (12.50 +/- 0.18) - (14.46 +/- 0.30)/theta, k beta (298 K) = 78.7 s(-1) in benzene containing 0.8 M 2-propanol. B3LYP/cc-PVTZ electronic structure calculations predict that intramolecular hydrogen bonding between the alpha-OH and the -OPh leaving group of ketyl radical (V) stabilizes both ground- and transition-state structures. The computed activation barrier, 14.9 kcal/mol, is in good agreement with the experimental activation barrier.
