Reactions of unsymmetrically substituted allyl radicals with methyl radicals

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Abstract
The Hg(63P1) photosensitized decompositions of 3‐methyl‐1‐butene, 2‐methyl‐2‐butene, 3,3‐dimethyl‐1‐butene, and 2,3‐dimethyl‐1‐butene have been used to generate 1‐methylallyl, 1,2‐dimethylallyl, 1,1‐dimethylallyl, and 1,1,2‐trimethylallyl radicals in the gas phase at 24 ± 1°C. From a study of the relative yields of the CH3 combination products, the relative reactivities of the reaction centers in each of these unsymmetrically substituted ambident radicals have been determined. The more substituted centers are found to be the less reactive, and this is ascribed primarily to greater steric interaction at these centers during reaction. Measurement of the ratio of trans‐ to cis‐2‐pentene formed from the 1‐methylallyl radical, combined with published values for this ratio at higher temperatures, enabled the differences in entropy and heat of formation of the trans‐ and cis‐forms of this radical to be calculated as 0.62 ± 0.85 J mol−1 K−1 and ‐ 0.63 ± 0.25 kJ mol−1, respectively, at 298K. Approximate values of the disproportionation/combination ratios for reaction of CH3 with 1,1‐dimethylallyl and 1‐methylallyl have been estimated and used to compute rate constants for the recombinations of tert‐butyl and isopropyl radicals that are in agreement with recently published data.

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