The pyrolysis of 1,2-dichloroethane

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.