Di:chloro-acenaphthene prepn. by acenaphthene chlorination - using catalyst mixt. of sulphur, sulphur halide or divalent sulphur cpd. and titanium tetra:chloride

of formula (I) is prepd. by reacting acenapthene (II), which a chlorinating agent (III), in (III):(II) mol. ratio 2:1-2.5:1, in the presence of (a) S, S halide or divalent S cpds.M and (b) TiCl4, as catalysts. (I) is a raw material for the prodn. of dyes, plant protection agents and pharmaceuticals. By oxidn. with HNO3 or chromic acid, (II) can be converted to 4,5-dichloronaphthalic acid anhydride, which can further be converted to optical brighteners. Pure (I) is obtd. in high yields.