Kinetics of the thermal unimolecular decomposition of hex‐1‐ene‐3‐yne. Heat of formation and resonance stabilization energy of the 3‐ethenylpropargyl radical

The thermal unimolecular decomposition of hex‐1‐ene‐3‐yne (HEY) has been investigated over the temperature range 949–1230 K using the technique of very low‐pressure pyrolysis (VLPP). One reaction pathway is the expected C 5 C 6 bond fission to form the resonance‐stabilized 3‐ethenylpropargyl radical. There is a concurrent process producing molecular hydrogen which probably occurs via the intermediate formation of hexatrienes and cyclohexa‐1,3‐diene. RRKM calculations yield the extrapolated high‐pressure rate parameters at 1100 K given by the expressions 10 16.0±0.3 exp(−300.4 ± 12.6 kJ mol −1 / RT ) s −1 for bond fission and 10 13.2+0.4 exp(−247.7 ± 8.4 kJ mol −1 / RT ) for the overall formation of hydrogen. The A factors were assigned from the results of previous studies of related alkynes, alkenes, and alkadienes. The activation energy for the bond fission reaction leads to Δ H   f 300 °[H 2 CCHCCĊH 2 ] = 391.9, DH   300 °[H 2 CCHCCCH 2 H] = 363.3, and a resonance stabilization energy of 56.9 ± 14.0 kJ mol −1 for the 3‐ethenylpropargyl radical, based on a value of 420.2 kJ mol −1 for the primary CH bond dissociation energy in alkanes. Comparison with the revised value of 46.6 kJ mol −1 for the resonance energy of the unsubstituted propargyl radical indicates that the ethenyl substituent (CH 2 CH) on the terminal carbon atom has only a small effect on the propargyl resonance energy. © John Wiley & Sons, Inc.