Dichlorobenzene manufacture

Dichlorobenzene isomers other than m-dichlorobenzene are isomerized to m-dichlorobenzene by heating them to at least 120 DEG C. at 650 pounds per square inch gauge or above in the presence of aluminium chloride and anhydrous hydrogen chloride. A preferred pressure range is 700 to 1350 p.s.i. gauge. m-Dichlorobenzene may be recovered from the initial product of the reaction by addition of an aqueous solution or dispersion of an alkali, e.g. sodium hydroxide, separation, washing and drying of the organic layer and then distillation. Other methods such as fractional crystallization or thermal diffusion may also be used to recover pure m-dichlorobenzene. In examples: (1) commercial o-dichlorobenzene is isomerized by the process of the invention at 130 DEG C. and 700 p.s.i.g. to a product of which the chief constituent is m-dichlorobenzene; (2) to (4) the process of (1) is repeated except that the pressure is 850, 950 and 1350 p.s.i.g., respectively; (5) a comparative experiment in which the process of (1) is performed at 15 p.s.i.g. is carried out, the yield of m-dichlorobenzene being considerably lower than in Examples (1) to (4).