Very low-pressure pyrolysis (VLPP) of pentynes. III. Pent-2-yne. Heat of formation and resonance stabilization energy of the 3-methylpropargyl radical

✍ Scribed by Tam T. Nguyen; Keith D. King

• Publisher: John Wiley and Sons
• Year: 1982
• Tongue: English
• Weight: 468 KB
• Volume: 14
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550140603

✦ Synopsis

The thermal unimolecular decomposition of pent-2-yne has been studied over the temperature range of 988-1234 K using the technique of very low-pressure pyrolysis (VLPP). The main reaction pathway is Cd-Ch bond fission producing the resonance-stabilized 3methylpropargyl radical. There is a concurrent process producing molecular hydrogen and penta-lA4-triene presumably via the intermediate formation of cis-penta-1,3-diene. The 1,4-hydrogen elimination from cis-penta-1,3-diene is the rate-determining step in the molecular pathway. This is supported by an independent VLPP study of cis-and transpenta-1,3-diene. RRKM calculations show that the experimental rate constants for C-C bond fission are consistent with the following high-pressure rate expression a t 1100 K: log k l (s-') = (16.0 f 0.3) -(72.6 f 2.0)/0 where 0 = 2.303RT kcal/mol and the A factor was assigned from the results of shock-tube studies of related alkynes. The activation energy leads to AHy,300[CH&'CCH2] = 70.3 and DH&[CH&CCH2-H] = 87.4 kcal/mol. The resonance stabilization energy of the 3methylpropargyl radical is 10.6 f 2.5 kcal/mol, which is consistent with previous results for this and other propargylic radicals.