Model Compound Studies of the β-O-4 Linkage in Lignin:  Absolute Rate Expressions for β-Scission of Phenoxyl Radical from 1-Phenyl-2-phenoxyethanol-1-yl Radical

• Published in: Journal of organic chemistry 2002-11, Vol.67 (23), p.7937-7945
• Main Authors: Kandanarachchi, Pramod H, Autrey, Tom, Franz, James A
• Format: Article
• Language: English
• Subjects: 08 HYDROGEN; 09 BIOMASS FUELS; ACETOPHENONE; ATOMS; BENZENE; BONDING; Carbohydrate Conformation; Chemistry; Equipment Reuse; Exact sciences and technology; Free Radicals; Free radicals chemistry; GROUND STATES; Half-Life; HYDROGEN; Industrial Waste; Kinetics; Kinetics and mechanisms; LIFETIME; LIGNIN; Lignin - chemistry; Lignin - radiation effects; Models, Molecular; Organic chemistry; Phenols; RADICALS; Reactivity and mechanisms; THIOPHENOLS
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo025581k

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 thiophenol to 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(k abs/M-1 s-1) = (8.88 ± 0.24) − (6.07 ± 0.34)/θ, θ = 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 β-scission of phenoxyl from 1-phenyl-2-phenoxyethanol-1-yl (V):  log(k β/s-1) = (12.85 ± 0.22) − (15.06 ± 0.38)/θ, k β(298 K) ca. (64.0 s-1 in benzene), and log(k β/s-1) = (12.50 ± 0.18) − (14.46 ± 0.30)/θ, k β(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 α-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.