Absolute Rate Constants for the β-Scission Reaction of the 1-Phenyl-2-phenoxypropyl Radical: A Model for Radical Reactions of Lignin

Article abstract of DOI:10.1021/jo00006a043

Absolute rate expressions for β-scission of the phenoxy radical from the 1-phenyl-2-phenoxypropyl radical, forming cis- and trans-β-methylstyrene, were determined by competition of β-scission (k_β) with abstraction of hydrogen from trimethylstannane (k_abs).Relative rates (k_β/k_abs) were converted to absolute rates (k_β) by using a rate expression determined for abstraction of hydrogen atom from tributylstannane by the phenylethyl radical: log abs/(M^-1s^-1)> = (9.31 +/- 0.30) - (7.11 +/- 0.49)/θ, where θ = 3.303RT kcal/mol (errors are 2?).The resulting expressions for β-scission are log (k_β,trans/s^-1) = (13.45 +/- 0.26) - (16.94 +/- 0.52)/θ and log (k_β,cis/s^-1) = (13.41 +/- 0.3) - (19.3 +/- 0.75)/θ.The basis rate expression for abstraction of hydogen from tributylstannane by the phenylethylradical was determined in a competition of abstraction (k_abs) with self-termination (k_t), using the Smoluchowski expression for self-reaction of phenylethyl radical: log <2k_t/(M^-1s^-1)> = 11.93 - 3.112/θ.Combining the Arrhenius parameters with the enthalpy change for β-scission leads to activation barriers for addition of phenoxyradical to trans- and cis-β-methylstyrene of 5.2 and 6.6 kcal/mol, respectively.