The thermal decomposition of 1,2-dichIoroethane to give vinyl chloride and hydrogen chloride has been reinvestigated at 703 K and the major features of this free-radical chain reaction found previously have been confirmed. A direct comparison of the pulsed KrF laser-induced process (248 nm) in the same static reactor at this temperature showed that a substantial increase in rate occurs even at low pulse-repetition rates (2 Hz).In an extended study of the laser-induced reaction at temperatures where thermal reaction is negligible (476-577 K), quantum yields of vinyl chloride formation have been measured to be between 3000 and 13000. Quantum yields decreased slightly with increasing pressure and showed an inverse square-root dependence on initial chlorine atom concentration. The major by-products were identified as ethene, ethyne, 1,2,3,4-tetrachlorobutane and 1,1,2-trichloroethane.At 577 K and 150 Torr, the quantum yield of ethene production is close to unity, indicating that under these conditions reaction (2) is essentially complete.

It has recently been suggested [ll that when l,l,l-trichloroethane is irradiated with a continuous-wave COP laser in the presence of SF6 sensitizer, the decomposition occurs in a molecular manner, viz.rather than by a radical-chain mechanismas has been proposed as the main component of the thermally induced decomposition [21. The evidence for this is largely based on the following observations:(1) No induction period was observed in the laser-induced SF6-sensitized decomposition of l,l,l-trichloroethane although induction periods are a feature of the thermal decomposition of this and certain other alkyl chlorides. (2) Upon addition of propene, the laser-induced SF,-sensitized reactions of l,l,l-trichloroethane and 1-chloropropane were retarded in a similar way although the latter is unaffected by propene when undergoing thermal decomposition. (3) When ethyl chloride and l,l,l-trichloroethane were irradiated together with SF6 no ethane, methane, or chloroform was detected. We have considered these arguments and carried out some similar experiments with two molecules (chloroethane and 1,l-dichloroethane) whose thermal decomposition is known to proceed molecularly [3,41 and two molecules (1,2-dichloroethane and l,l,l-trichloroethane) known to decompose thermally via a radical chain mechanism [2,51.

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