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

✍ Scribed by Warren S. Staker; Keith D. King; Tam T. Nguyen

• Publisher: John Wiley and Sons
• Year: 1992
• Tongue: English
• Weight: 620 KB
• Volume: 24
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550240903

✦ Synopsis

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 c5-c6 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 10'60'03 exp(-300.4 % 12.6 k J rnol-l/RT) s-' for bond fission and 10'32c04 exp(-247.7 2 8.4 k J mol-'/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 AH,%,o [H~CCHCCCHZ] = 391.9, DH& [HzCCHCCCH?-H] = 363.3, and a resonance stabilization energy of 56.9 ? 14.0 k J mol-' for the 3-ethenylpropargyl radical, based on a value of 420.2 k J mol-' for the primary C -H bond dissociation energy in alkanes. Comparison with the revised value of 46.6 k J mol-' for the resonance energy of the unsubstituted propargyl radical indicates that the ethenyl substituent (CHz=CH) on the terminal carbon atom has only a small effect on the propargyl resonance energy.