Hydrochloric Acid → 2,3,5,6-Tetrachloropyridine
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[Cl:1][C:2]1[N:7]=[C:6]([Cl:8])[C:5]([Cl:9])=[C:4](Cl)[C:3]=1[Cl:11].C(=O)([O-])[O-].[NH4+].[NH4+].Cl>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]=1[Cl:1][C:2]1[N:7]=[C:6]([Cl:8])[C:5]([Cl:9])=[C:4](Cl)[C:3]=1[Cl:11].C(=O)([O-])[O-].[NH4+].[NH4+].Cl>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]=11.84 g (0.0281 gram atom) of zinc dust is introduced into a solution, heated to 80° C., of 5.1 g (0.02 mol) of pentachloropyridine in 35 ml of dimethylmethanephosphonate. A solution of 2.44 g (0.0254 mol) of ammonium carbonate in 10 ml of water is subsequently added dropwise at a temperature of 90° to 95° C. in the course of 70 minutes with vigorous stirring. The reaction mixture is then stirred into 250 ml of ice water; there is added 5 ml of concentrated hydrochloric acid and the mixture is stirred for a further 10 minutes, and is subsequently extracted twice with 100 ml of ether each time. The combined extracts are washed with 70 ml of water, dried over sodium sulfate, filtered, and evaporated to dryness to yield as residue 4.05 g (92% of theory) of 2,3,5,6-tetrachloropyridine having a melting point of 86° to 88° C. According to gas-chromatographical analysis, the product contains 93.4% of 2,3,5,6-tetrachloropyridine, 2.1% of 2,3,5-trichloropyridine, 2.2% of 2,3,6-trichloropyridine and 0.8% of pentachloropyridine.
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