The pyrolysis of 1.2-dichloroethane in a static system has been studied by g.1.c. analysis of the products and by pressure measurements in the temperature range 340-51 5 "C, surface :volume ratio range (1 -32-37.4) cm-l, 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 k 3 kcal mol-l). 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-I. It is suggested that the autocatalysis is due to a heterogeneous initiation process involving adsorbed chlorine.THE pyrolysis of 1,2-dichloroethane has previously been studied under homogeneous and heterogeneous 5-7 conditions. Under static conditions it is widely accepted that the reaction is homogeneous and of the first order and shows many characteristics of a chain reaction, such as inhibition by propene, acceleration by oxygen and chlorine, and the presence of induction periods. To account for these features, Howlett suggested the mechanism of reactions ( 1)-( 4).In order to fit the(3)observed overall rate expression, i.e., equation (5), it was necessary to assume an unreasonably low pre-k1s-l = 1010.81exp (-47,00O/RT) (5) exponential factor of 1O1O s-l for reaction (3). The corollary of a long-lived C,H3C1, radical proved to be a convenient explanation for the observed induction periods, which were attributed to the slow build-up of this radical to its stationary-state concentration.Both Barton and Howlett (who used vessels with s/v 2.7 and 8.5 cm-l) and Howlett (who varied s/v from 2.2 to 22) found the rate of reaction to be little affected by packing the reaction vessel. Accordingly, Howlett suggested that steps (1) and (4) as well as ( 2) and (3) were homogeneous.Kapralova and Semenov4 have since shown, using differential calorimetry and large reaction vessels, that the reaction is isolated in a zone near the vessel walls. This implies that the reaction is initiated and in part terminated a t the vessel walls.

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