The thermal dehydrochlorination of some substituted hydrocarbons

Abstract

After a historical survey of the kinetics of gaseous dehydrochlorination reactions, a critical account of developments in the theory of unimolecular reactions is given. This latter has considerable bearing on the choice of compounds examined here. The construction and operation of both static and dynamic apparatus are described. The preparation and purification of the materials are detailed, and their physical constants are compared with previous determinations. Full details of the kinetics of the thermal decomposition of allyl chloride, trichloro-ethylene, and the 1:2-dichloroethylenes, in a static apparatus with carbon coated walls, are reported. In all cases the effects of changes in the surface/volume ratio of the reactor and the addition of inhibitors such as propylene and n-hexane have been carefully investigated. In addition, the effects of the "inert" gases, n-pentane and diethyl ether, on the unimolecular decompositions of the chloroethylenes have been studied. A series of flow experiments, primarily for the large scale collection and analysis of reaction products has also been performed. It is deduced that the pyrolysisof allyl chloride is predominantly a non-chain, but partially heterogeneous reaction, while the decompositions of all the chloroethylenes are homogeneous, simultaneous chain and molecular processes. Having shown that the results justify the assumption that the second elimination of hydrogen chloride from a polychlorohydrocarbon is slow compared with the first, a mechanistic interpretation of the kinetics is given. A possible explanation of the unexpected heterogeneity, detectable chain component, and low efficiency of the unimolecular process found in the pyrolysis of allyl chloride is given. A reasonable chain mechanism which explains the kinetics of the free radical decomposition of trichloroethylene is postulated, and the finer points of the mechanism confirmed by a quantitative treatment of the observed induction periods. Finally, the unimolecular decompositions of ail the chloroethylenes are discussed in connection with Slater's treatment of unimolecular reactions.