Pentachloropyridine → 2,3,5,6-Tetrachloropyridine

12.76 g (0.05 mol) of pentachloropyridine is dissolved, by heating at 90° C., in 80 ml of dimethylmethanephosphonate. After the addition of 4.1 g (0.063 gram atom) of zinc dust, there is added dropwise to the clear solution in the course of 20 minutes, with vigorous stirring, a solution of 4.26 g (0.08 mol) of ammonium chloride in 15 ml of water. After the addition of the ammonium chloride solution has been completed, stirring is continued for 40 minutes. The reaction mixture obtained is then filtered hot, and the filter residue is washed with 10 ml of dimethylmethanephosphonate. The filtrate is poured into 500 ml of ice water; there is then added 12.5 ml of concentrated hydrochloric acid and the mixture is stirred for 2 hours. The 2,3,5,6-tetrachloropyridine, which has precipitated in the form of white crystals, is subsequently washed with 150 ml of water and dried. In this manner is obtained 10.0 g (90.8% of theory) of 2,3,5,6 -tetrachloropyridine having a melting point of 89° to 90° C. According to gas-chromatographical analysis, the product contains 97.0% of 2,3,5,6-tetrachloropyridine, 0.5% of 2,3,5-trichloropyridine, 0.6% of 2,3,6-trichloropyridine and 1.6% of pentachloropyridine.