Very low‐pressure pyrolysis (VLPP) of pentynes. II. 4‐methylpent‐2‐yne and 4,4‐dimethylpent‐2‐yne. Heats of formation and resonance stabilization energies of methyl‐substituted propargyl radicals

Abstract

AbstractStudies of the unimolecular decomposition of 4‐methylpent‐2‐yne (M2P) and 4,4‐dimethylpent‐2‐yne (DM2P) have been carried out over the temperature range of 903–1246 K using the technique of very‐low pressure pyrolysis (VLPP). The primary reaction for both compounds is fission of the CC bond adjacent to the acetylenic group producing the resonance‐stabilized methyl‐substituted propargyl radicals, CH3CĊH(CH3) from M2P and CH3CCĊ(CH3)2 from DM2P. RRKM calculations were performed in conjunction with both vibrational and hindered rotational models for the transition state. Employing the usual assumption of unit efficiency for gas‐wall collisions, the results show that only the rotational model with a temperature‐dependent hindrance parameter gives a proper fit to the VLPP data over the entire experimental temperature range. The high‐pressure Arrhenius parameters at 1100 K are given by the rate expressions log k2 (sec−1) = (16.2 ± 0.3) − (74.4 ± 1.5)/θ for M2P and log k3 (sec−1) = (16.4 ± 0.3) − (71.4 ± 1.5)/θ for DM2P where θ = 2.303RT kcal/mol. The A factors were assigned from the results of recent shock‐tube studies of related alkynes. Inclusion of a decrease in gas‐wall collision efficiency with temperature would lower both activation energies by ∼1 kcal/mol. The critical energies together with the assumption of zero activation energy for recombination of the product radicals at 0 K lead to DH0[CH3CCCH(CH3)CH3] = 76.7 ± 1.5, ΔHf0[CH3CCCH(CH3)] = 65.2 ± 2.3, DH0[CH3CCCH(CH3)H] = 87.3 ± 2.7, DH0[CH3CCC(CH3)2CH3] = 72.5 ± 1.5, ΔH[CH3CCĊ(CH3)2] = 53.0 ± 2.3, and DH0[CH3CCC(CH3)2H] = 82.3 ± 2.7, where all quantities are in kcal/mol at 300 K. The resonance stabilization energies of the 1,3‐dimethylpropargyl and 1,1,3‐trimethylpropargyl radicals are 7.7 ± 2.9 and 9.7 ± 2.9 kcal/mol at 300 K. Comparison with results obtained previously for other propargylic radicals indicates that methyl substituents on both the radical center and the terminal carbon atom have little effect on the propargyl resonance energy.