The pyrolysis of alkyl chlorides on the surface of Pyrex glass

Abstract

A study has been made of the surface-catalysed dehydrochlorination of a selection of gaseous chlorinated hydrocarbons on Pyrex glass. For primary chlorides at 413° the rate sequence: ethyl<n-propyl<isobutyl<neopentyl was observed for 200 mm initial pressure of reagent. Ethyl chloride decomposes at 400-450° according to a first-order law, and the overall Arrhenius activation energy for the heterogeneous reaction is 24.1 kcal/mole; the rate is slightly depressed by the addition of propene and slightly enhanced by hydrogen chloride. Catalysis by hydrogen chloride was observed to be much more marked in the case of isopropyl chloride decomposition at 190-250°, but reproducible behaviour for this reaction was not readily attained. The catalytic action of Pyrex glass is accounted for by the polar surface assisting in the separation of charges within the reacting molecule, emphasis being placed upon the polarization of the carbon-chlorine bond as the important rate-determining factor.