Tetrachlorophthalic anhydride → 3,4,6-trichlorophthalic Acid

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Reaction Conditions

Reaction Steps

This compound was prepared by modifications to the literature method of Syn. Lett., 1990, 339. A mixture of 3,4,5,6-tetrachlorphthalic anhydride (19) (Aldrich Chemical Co., 100 g, 0.35 mmol) and NaOH (50.0 g, 1.25 mmol) in water (1000 mL) was stirred at 50-60° C. (bath) for 45 min under a nitrogen atmosphere. Zinc dust (70.0 g, 1.07 mmol) was then added portionwise over 10 min, and the mixture was stirred at 70-80° C. for a further 6 h. The reaction was cooled to room temperature and filtered through a bed of Celite, and the filter and residue was washed successively with 0.1N NaOH (2×100 mL) and H2O (2×100 mL). The combined filtrate was acidified with conc. HCl to pH≦1, and the colourless precipitate was collected by filtration and washed with 0.1N HCl (3×100 mL). The damp solid was stirred with EtOAc (600 mL) and acidified with conc. HCl until all the solids had dissolved. The EtOAc layer was separated and the aqueous portion further extracted with the same solvent (2×100 ml, The combined EtOAc solution was dried (Na2SO4), filtered, and evaporated under reduced pressure give 3,4,6-trichlorophthalic acid (20) (83.5 g, 89%) as a colourless solid; m.p. (without recrystallisation) 151-153° C. (lit. m.p. 150-153° C.), 1H NMR identical to literature.

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