The thermal decomposition of aromatic compounds - I. Chlorobenzene

Abstract

The thermal decomposition of chlorobenzene has been studied in the temperature region 720 to 800° C. The decomposition is a predominantly homogeneous process and leads to the production of approximately one molecule of hydrogen and one molecule of hydrogen chloride for every molecule of chlorobenzene decomposed. No gaseous carbon compounds are formed, and all the combined carbon and the remaining hydrogen are deposited on the walls of the reaction vessel as a hard, shiny, graphite-like solid. The structure of this deposit was examined by electron diffraction and compared with those produced by pyrolysis of other organic compounds. The thermal decomposition of chlorobenzene is kinetically composite. The partial chain character of the reaction is indicated by the lowering of the decomposition rate to the same limiting value on addition of both nitric oxide and ammonia. It is confirmed by the acceleration produced by an inert gas and by the retardation caused by an increase in the surface. The mechanism of the chain reaction is considered, and a possible chain cycle is proposed. The nature of the fully inhibited reaction is also discussed, and though the evidence is to some extent conflicting it is concluded that it is probably a unimolecular reaction.