Competitive Unimolecular Decomposition in Chemically Activated Radical Systems. Isopropyl Radical Rupture

The competitive unimolecular decomposition of vibrationally excited 5‐methyl‐hexyl‐3· (5MH3·*) and 3,4‐dimethyl‐pentyl‐2· (34DMP2·*) radicals was studied in chemical activation systems in which H atoms were added to suitable olefins. The relative rate of isopropyl versus methyl‐radical rupture from each radical yields $D_0\mathrm{°(i}\mathrm{Pr}–\mathrm{H}\text{)\hspace{0.17em}=\hspace{0.17em}92.8}{\mathrm{kcal\; mole}}^{\text{−1}}$ . Absolute rates of decomposition of 5MH3·*, 2MH3·*, and 34DMP2·* are compared to values obtained from the theoretical model and show good agreement The reactions of isopropyl radicals with 5MH3·, 2MH3·, and 34DMP2· radicals are discussed briefly; disproportionation‐recombination ratios are obtained and compare favorably with values calculated according to an empirical prescription given previously. The measurement of the relative rates of decomposition of 5MH3·* and of 2MH3·* in the pseudocompetitive reaction system, H+trans‐2‐methylhexene‐3, provides a relation between the energetics of the 5MH3· radical and the 2MH3· radical and supports $D_0\text{°(5}\mathrm{MH}\text{3·–}\mathrm{H}{\mathrm{)\hspace{0.17em}=\hspace{0.17em}D}}_0\text{°(2}\mathrm{MH}\text{3·–}\mathrm{H})$ . A summary of bond dissociation energies determined by the present method is given.