Competitive Unimolecular Decomposition in Chemically Activated Radical Systems

Abstract

The study of the competitive unimolecular decomposition of chemically activated alkyl radicals has been extended. Relative and absolute rates of decomposition for the series of radicals 3-methyl—butyl-2, 3-methyl—pentyl-2, 3-ethyl—pentyl-2, 2-methyl—pentyl-3, 2-methyl—hexyl-3, and 3-methyl—hexyl-3, each of which possess from two to four carbon—carbon rupture reaction channels, have been measured. A quantum-statistical model for the absolute decomposition rates provided good theoretical agreement with the experiments. The measurement of relative rupture rates provided stringent conditions on the permissible values of the differences in critical energies ΔE0 for the competing decomposition modes. They also allowed the formulation of a consistent prescription for making the vibrational assignment for the decomposition complexes of the six radicals. The results are compatible with D0∘(CH3–H)−D0∘(C2H5–H)=5.7 kcal mole−1 and differences between critical heats of addition of methyl and ethyl radicals to olefins, Ec(Me, olefin) — Ec(Et, olefin), which vary systematically from −0.2 to 0.9 kcal mole−1.