Competitive Unimolecular Decomposition Processes in Chemically Activated Systems

Abstract

A general formulation of the rates of competitive unimolecular reactions is given, and various relationships between the rates and between the energetics of competing processes are described. The relative and absolute rates of decomposition, $$\begin{array}{c}\mathrm{C}–\mathrm{C}–\mathrm{C}–\mathrm{C}\mathrm{̇–}\mathrm{C}–{\mathrm{C}}^{*}\to {\displaystyle {lim}_{k_E^p}}·{\mathrm{CH}}_3\text{+1}{\mathrm{-C}}_5{\mathrm{H}}_{10}\\ \to {\displaystyle {lim}_{k_E^b}}·{\mathrm{C}}_2{\mathrm{H}}_5\text{+1}{\mathrm{-C}}_4{\mathrm{H}}_8,\end{array}$$ have been measured at pressures from 0.004 to 90 mm Hg for vibrationally excited 3‐hexyl radicals, chemically activated by the addition of hydrogen atoms to trans−3‐hexene at 300°C. Use of a quantum‐statistical model for k_E in the general relations provides good theoretical agreement with the experimental data at all pressures. The results can also be used to lead to either of two alternative conclusions: that D₀(CH₃–H) — D₀(C₂H₅–H) is 5.4 kcal mole⁻¹, or that the difference in critical energies for addition of methyl to 1‐pentene, E₀^m, and of ethyl to 1‐butene, E₀^e, is E₀^m—E₀^e= −0.4 kcal.