Process for producing 4,4′-dichlorodiphenyl sulfoxide

A process for producing 4,4′-dichlorodiphenyl sulfoxide comprising:

• (I) reacting thionyl chloride, chlorobenzene and aluminum chloride in a molar ratio of thionyl chloride:chlorobenzene:aluminum chloride of 1:(6 to 9):(1 to 1.5) at a temperature in the range from 0 to below 20° C., forming an intermediate reaction product and hydrogen chloride;
• (II) mixing aqueous hydrochloric acid and the intermediate reaction product at a temperature in the range from 70 to 110° C. to obtain an organic phase comprising 4,4′-dichlorodiphenyl sulfoxide and an aqueous phase;
• (III) cooling the organic phase comprising the 4,4′-dichlorodiphenyl sulfoxide to a temperature below the saturation point of 4,4′-dichlorodiphenyl sulfoxide to obtain a suspension comprising crystallized 4,4′-dichlorodiphenyl sulfoxide, wherein cooling of the organic phase is carried out in a gastight closed vessel by
• (i) reducing the pressure in the gastight closed vessel;
• (ii) evaporating chlorobenzene;
• (iii) condensing the evaporated chlorobenzene by cooling;
• (iv) returning the condensed chlorobenzene into the gastight closed vessel;
• (IV) solid-liquid-separation of the suspension to obtain a residual moisture containing solid 4,4′-dichlorodiphenyl sulfoxide comprising crystallized 4,4′-dichlorodiphenyl sulfoxide and mother liquor.