Selective Hydrodechlorination of 3,4,5,6-Tetrachlorophthalic Anhydride: Preparation of 3,4,6-Trichlorophthalic Acid

Treatment of tetrachlorophthalic anhydride (I) with zinc dust and aqueous sodium hydroxide produces 3,4,6-trichlorophthalic acid (II) which under similar reaction conditions yields predominantly a mixture of 3,6-dichlorophthalic acid (III) and 3-chlorophthalic anhydride (IV).