Pentachloropyridine → 2,3,5,6-Tetrachloropyridine

4.6 g (0.07 gram atom) of zinc dust is introduced into a solution, heated to 90° C., of 12.57 g (0.05 mol) of pentachloropyridine in 150 ml of dimethylmethanephosphonate, and there is then added dropwise in the course of 25 minutes, with stirring, a solution of 9.52 g (0.075 mol) of the ammonium salt of methanephosphonic acid monomethyl ester in 44 ml of water. After the addition of the ammonium salt has been completed, the reaction mixture is firstly stirred for 30 minutes, subsequently filtered hot, and the filter residue is washed with 10 ml of dimethylmethanephosphonate. The filtrate is poured into 500 ml of ice water; 12.5 ml of concentrated hydrochloric acid is then added and the mixture is stirred for 2 hours. The crystals which have precipitated are afterwards filtered off, washed with 150 ml of water and dried to yield 8.72 g (80.4% of theory) of crude 2,3,5,6-tetrachloropyridine, m.p. 88° to 89° C., which contains, according to gas-chromatographical analysis, 94.1% of 2,3,5,6-tetrachloropyridine, 2.5% of 2,3,5-trichloropyridine, 1.9% of 2,3,6-trichloropyridine and 0.17% of pentachloropyridine.