Thermal unimolecular decomposition of 1-ethylcyclohexa-1,4-diene, 1,2-dimethylcyclohexa-1,4-diene and bicyclo[4,3,0]nona-1(6), 3-diene

Abstract

The pyrolysis of 1-ethylcyclohexa-1,4-diene has been studied from 316 to 379°C in the gas phase at pressures of around 10 Torr. Under these conditions, in an aged reaction vessel, the reaction was strictly first order, and the products, hydrogen and ethylbenzene were formed quantitatively. The rate constants yielded the Arrhenius equation : log k₁/s^–1= 13.12±0.09–45 870±250 cal mol^–1/2.303 RT i.e., log k₁/s^–1= 13.12±0.09–191 930±1030 J mol^–1/2.303 RT

The pyrolysis of 1,2-dimethylcyclohexa-1,4-diene in the temperature range 296 to 354°C was found to yield only hydrogen and o-xylene. At pressures between 0.25 and 5.4 Torr which was the range studied, the reaction was strictly first order and the rate constants obtained yielded the Arrhenius equation: log k₂/s^–1= 12.54±0.03–43 720±80 cal mol^–1/2.303 RT i.e., log k₂/s^–1= 12.54±0.03–182 920±320 J mol^–1/2.303 RT.

The pyrolysis of bicyclo[4,3,0]nona-1⁽⁶⁾, 3-diene yielded hydrogen and bicyclo[4,3,0]nona-1,3,5-triene (1,2-dihydroindene) as the only products. In the temperature range 346 to 408°C and at pressure above 0.8 Torr the decomposition was first order and the rate constants obtained yielded the Arrhenius equation: log k₃/s^–1= 12.95±0.16–48 580±470 cal mol^–1/2.303 RT i.e., log k₃/s^–1= 12.95±0.16–203 260±1960 J mol^–1/2.303 RT.

The results obtained in this work are compared with those obtained for the pyrolyses of other cyclohexa-1,4-dienes.