ABSOLUTE RATE CONSTANTS FOR THE BETA-SCISSION REACTION OF THE 1-PHENYL-2-PHENOXYPROPYL RADICAL - A MODEL FOR RADICAL REACTIONS OF LIGNIN

Absolute rate expression for beta-scission of the phenoxy radical from the 1-phenyl-2-phenoxypropyl radical, forming cis- and trans-beta-methylstyrene, were determined by competition of beta-scission (k-beta) with abstraction of hydrogen from trimethylstannane (k(abs)). Relative rates (k-beta/k(abs)) were converted to absolute rates (k-beta)) by using a rate expression determined for abstraction of hydrogen atom from tributylstannane by the phenylethyl radical: log [k(abs)/(M-1s-1)] = (9.31 +/- 0.30) - (7.11 +/- 0.49)/theta, where theta = 2.303RT kcal/mol (errors are 2-sigma). The resulting expressions for beta-scission are log (k-beta,(trans)/s-1) = (13.45 +/- 0.26) - (16.94 +/- 0.52)/theta and log (k-beta,(cis)/s-1) = (13.41 +/- 0.3) - (19.3 +/- 0.75)/theta. The basis rate expression for abstraction of hydrogen from tributylstannane by the phenylethyl radical 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/theta. Combining the Arrhenius parameters with the enthalpy change for beta-scission leads to activation barriers for addition of phenoxy radical to trans- and cis-beta-methylstyrene of 5.2 and 6.6 kcal/mol, respectively.