The pyrolysis of 1,2-dichloroethane

Abstract

The pyrolysis of 1,2-dichloroethane in a static system has been studied by g.l.c. analysis of the products and by pressure measurements in the temperature range 340–515 °C, surface:volume ratio range (1·32–37·4) cm^–1, and with initial pressures from 0·3 to 300 Torr, in reaction vessels coated with pyrolytic carbon films.

The major reaction is dehydrochlorination to vinyl chloride, but ethylene is also a primary reaction product. The ethylene yield is very small ( < 2% of the vinyl chloride) under conditions of low surface:volume ratio. It is concluded that the ethylene is produced by a concurrent heterogeneous dechlorination of 1,2-dichloroethane.

At low surface:volume ratio the vinyl chloride produced is closely paralleled by the pressure increase and the reaction is found to have a high order (2·4–2·8) and activation energy (73 ± 3 kcal mol^–1). The reaction is inhibited by additions of vinyl chloride, ethylene, and propene, and accelerated by added hydrogen chloride, oxygen, and chlorine. The radical-chain mechanism previously suggested by Barton and Howlett is discussed and modified to account for the experimental observations.

The pressure–time curves obtained from experiments at high surface:volume ratio indicate that under these conditions the reaction is autocatalytic. The maximum rate corresponds to an order of 1·5 and an activation energy of 33·0 kcal mol^–1. It is suggested that the autocatalysis is due to a heterogeneous initiation process involving adsorbed chlorine.