Discrimination between Free-Radical and Concerted Pyrolysis Mechanisms

Abstract

Derived from quantitative kinetics analysis, an experimental technique that tracks the fate of an isotopic label during pyrolysis of fully protonated reactant in the presence of a fully deuterated solvent allows for discrimination between free-radical and concerted mechanisms. Monitoring the deuterium incorporation (DI) into the product of the reactant or the protium incorporation (PI) into the solvent as a function of the solvent to feed concentration ratio, R, provides the contribution of each mechanism to the overall reaction. At infinite dilution (1/R → 0), DI approaching zero indicates dominance by a concerted mechanism, whereas DI approaching unity suggests dominance by the free-radical mechanism. This work elaborates on the kinetics basis for such a measurement. We also show that tracking PI enables the experimental technique to be applicable even for cases where the deuterated product is not stable and meaningful measurement of DI is thus compromised.