Chemically activated 3‐methyl‐l‐butene and 2‐methyl‐l‐butene from photolysis of diazomethane–isobutene–neopentane–oxygen mixtures

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AbstractAn experimental study of the decomposition kinetics of chemically activated 2‐methyl‐l‐butene and 3‐methyl‐l‐butene produced from photolysis of diazomethane‐isobutene‐neopentane‐oxygen mixtures is reported. The experimental rate constants for 3‐methyl‐l‐butene decomposition were 1.74 ± 0.44 × 108 sec−1 and 1.01 ± 0.25 × 108 sec−1 at 3660 and 4358 Å, respectively. 2‐Methyl‐l‐butene experimental decomposition rate constants were found to be 5.94 ± 0.59 × 107 sec−1 at 3660 Å and 3.42 ± 0.34 × 107 sec−1 at 4358 Å. Activated complex structures giving Arrhenius A‐factors calculated from absolute rate theory of 1016.6 ± 0.5 sec−1 for 3‐methyl‐l‐butene and 1016.2 ± 0.4 sec−1 for 2‐methyl‐l‐butene, both calculated at 1000°K, were required to fit RRKM theory calculated rate constants to the experimental rate constants at reasonable E0 and E* values. Corrected calculations (adjusted E0 values) on previous results for 2‐pentene decomposition gave an Arrhenius A‐factor of 1016.45 ± 0.35 sec−1 at 1000°K. The predicted A‐factors for these three alkene decompositions giving resonance‐stabilized methylully radicals are in good internal agreement. The fact that these A‐factors are only slightly less than those for related alkane decompositions indicates that methylallylic resonance in the decomposition products leads to only a small amount of tightening in the corresponding activated complexes. This tightening is a significantly smaller factor than the large reduction in the critical energy due to resonance stabilization.