VERY LOW-PRESSURE PYROLYSIS OF 4-PHENYL-1-BUTYNE - PROPARGYL RESONANCE ENERGY

The unimolecular decomposition of 4-phenyl-1-butyne has been studied over the temperature range of 850-1150 K by means of the very low-pressure pyrolysis (VLPP) technique. The decomposition takes place via C-C fission to yield resonance-stabilized propargyl and benzyl radicals. The experimental rate constants are in conformity with the following expression for the high-pressure Arrhenius parameters obtained by application of RRKM theory: log (k/s-1) = (14.6 ± 0.4) - (60.3 ± 2.0)/θ, where θ = 2.303RT kcal/mol. From the experimental activation energy the following thermochemical quantities at 300 K were deduced: DH°[PhCH2-CH2CCH] = 62.4, ΔHf°(HCCĊH2,g) = 81.5 and DH°[HCCCH2-H] = 89.3, all in kcal/mol. The resonance stabilization energy of the propargyl radical was calculated to be 8.7 ± 2.5 kcal/mol, in excellent agreement with earlier pyrolysis results. © 1979 American Chemical Society.