- Kinetics of Decomposition and Interconversion of 3-Methylbut-1-yne and 3-Methylbuta-1,2-diene. Resonance Stabilization Energies of Propargylic Radicals
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The thermal unimolecular reactions of 3-methylbut-1-yne (MBT) and 3-methylbuta-1,2-diene (MBTD) have been studied over the temperature range 940-1222 K by using the technique of very low-pressure pyrolysis (VLPP).Both compounds decompose via C3-C4 bond fission producing the resonance-stabilized radicals, 1-methylpropargyl from MBT and 1-methylallenyl from MBTD.In addition, interconversion between the two reactants takes place via structural isomerization.RRKM calculations, incorporating competing pathways and a temperature-dependent gas/wall collision efficiency, show that the experimental rate constants are consistent with the following high-pressure specific rate expressions at 1100K : log(k1/s-1) = (16.3 +/- 0.3) - (71.6 +/- 1.0)/Τ for bond fission and log(k2/s-1) = (13.2 +/- 0.6) - (60.5 +/- 1.0)/Τ for isomerization for MBT, log(k3/s-1) = (16.3 +/- 0.3) - (75.3 +/- 1.0)/Τ for bond fission and log(k4/s-1) = (13.2 +/- 0.6) - (63.8 +/- 1.0)/Τ for isomerization for MBTD, where Τ = 2.303RT kcal/mol.The A factor for bond fission was assigned from the results of recent shock-stube studies of related alkynes, and the A factor for isomerization was adopted from that previously reported for the analogous allene propyne interconversion.The results lead to ΔH0f300 = 72.4, DH0300 = 85.0, ΔH0f300 = 74.3, and DH0300 = 91.4 kcal/mol.The resonance stabilization energies are 10.0 +/- 2.2 and 7.8 +/- 2.2 kcal/mol for the 1-methylpropargyl and 1-methylallenyl radicals, respectively, in reasonable agreement with previous results for other propargylic radicals.The activation energy for the isomerization MBTD -> MBT is similar to that previously reported for the conversion of allene to propyne, and the equilibrium constants for the interconversion MBT MBTD calculated from therate date show good agreement with the values estimated from thermodynamic data over the experimental temperature range.
- Nguyen, Tam T.,King, Keith D.
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p. 3130 - 3136
(2007/10/02)
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