[Cl:1][C:2]1[N:7]=[C:6]([Cl:8])[C:5]([Cl:9])=[C:4](Cl)[C:3]=1[Cl:11].C[N+](C)(C)C.COP(C)(=O)O>CC(C)P([O-])(=O)[O-].O.[Zn]>[Cl:8][C:6]1[C:5]([Cl:9])=[CH:4][C:3]([Cl:11])=[C:2]([Cl:1])[N:7]=1A suspension of 12.8 g (0.05 mol) of pentachloropyridine and 4.1 g (0.062 gram atom) of zinc dust in 120 ml of dimethylmethanephosphonate is heated with stirring to 80° C. There is then added dropwise within 15 minutes a solution of 13.76 g (0.075 mol) of the tetramethylammonium salt of methanephosphonic acid monomethyl ester in 30 ml of water. The mixture is subsequently filtered hot, the filter residue is washed with 30 ml of dimethylmethanephosphonate, and the filtrate is poured into 600 ml of ice water containing 12.5 ml of concentrated hydrochloric acid. The resulting white crystal suspension is stirred for 30 minutes; it is then filtered, and the filter residue is washed with water and dried to yield 9.95 g (90% of theory) of crude 2,3,5,6-tetrachloropyridine, m.p. 87.5° to 89° C., which contains, according to gas-chromatographical analysis, 96.9% of 2,3,5,6-tetrachloropyridine, 1.6% of 2,3,5-trichloropyridine, 0.9% of 2,3,6 -trichloropyridine and 0.6% of pentachloropyridine.
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