Pyrolysis of Ethylene in Shock Waves

The pyrolysis of ethylene, highly diluted by neon, was studied in the temperature range 1710°—2170°K with reflected shock pressures of 225 to 1600 torr. The products are C2H2 and H2 with some diacetylene at higher temperatures. The rate law is −d[C2H4]/dt=k32[C2H4][Ne]12 over a threefold ethylene and sixfold neon pressure range and the rate constant is given by log10k32=(0.01±0.19)−(50 500±1700)/2.303RT (cc/molecule)12sec−1. In experiments with equimolar C2H4+C2D4 mixtures the hydrogen formed initially contains as much HD as H2 although at the same temperatures the reaction H2+D2→2HD is found to be relatively slow. In experiments with C2H4+D2 mixtures as much HD as H2 is formed in pyrolysis. Some relatively slow isotopic exchange between ethylenes and between C2H4 and D2 occurs, forming mainly C2H3D in the latter case. An unexpectedly high sensitivity of the mass spectrometer to hydrogen formed in the pyrolysis suggested that it may be formed in vibrationally excited states. A number of diagnostic experiments could neither confirm nor reject this possibility. Both a simple unimolecular reaction C2H4+M→C2H4*→C2H2+ H2*+M (where H2* is a vibrationally excited molecule) and several free-radical chain mechanisms of the Rice—Herzfeld type can describe the observed kinetics, although in all cases the relatively low observed activation energy is very difficult to explain. Thus no clear-cut identification of the most probable reaction mechanism is possible.