Competitive unimolecular reactions at low pressures. The pyrolysis of cyclobutyl chloride

Abstract

AbstractThe thermal decomposition of cyclobutyl chloride has been investigated over the temperature range of 892–1150 K using the technique of very low‐pressure pyrolysis (VLPP). The reaction proceeds via two competitive unimolecular channels, one to yield ethylene and vinyl chloride and the other to yield 1,3‐butadiene and hydrogen chloride, with the latter being the major reaction under the experimental conditions. With the usual assumption that gas‐wall collisions are «strong,» RRKM calculations, generalized to take into account two competing pathways, show that the experimental unimolecular rate constants are consistent with the high‐pressure Arrhenius parameters given by log k1(sec−1) = (14.8 ± 0.3) − (61.1 ± 1.0)/Θ for vinyl chloride formation and log k2(sec−1) = (13.6 ± 0.3) − (55.7 ± 1.0)/Θ for 1,3‐butadiene formation, where Θ = 2.303 RT kcal/mol. The A factors were assigned from previous high‐pressure low‐temperature data of other workers assuming a four‐center transition state for 1,2‐HCl elimination and a chlorine‐bridged biradical transition state for vinyl chloride formation. The activation energies are in good agreement with the high‐pressure results which were obtained with a conventional static system. The difference in critical energies is 4.6 kcal/mol.