[S:1]([Cl:4])([Cl:3])=[O:2].[Cl:5][C:6](=[C:10]([Cl:14])[CH:11]([Cl:13])[Cl:12])[C:7](O)=[O:8].S(=O)=O.[ClH:18]>CN(C)C=O>[S:1]([Cl:4])([Cl:3])=[O:2].[Cl:5][C:6](=[C:10]([Cl:14])[CH:11]([Cl:13])[Cl:12])[C:7]([Cl:18])=[O:8]An agitated flask is charged with 800 g (6.72 mole) of thionyl chloride together with 2 ml of dimethylformamide as accelerator; the mixture is heated under agitation to 50° C. and, within 2 hours, 350 g (1.56 mol) of crystalline 2,3,4,4-tetrachloro-2-butenoic acid is added in incremental portions. During this step a strong evolution of gas occurs. After the addition has been completed, the mixture is heated for 1-2 hours to boiling (79° C.) until the evolution of sulfur dioxide and hydrogen chloride has ceased (bubble counter at the cooler outlet). Thereafter the reaction mixture is distilled over a 10 cm column, thus obtaining 604 g (5.07 mol) of unreacted thionyl chloride and 359 g (1.48 mol) of 2,3,4,4-tetrachloro-2-butenoic acid chloride; b.p. 102° C./26 mbar; nD20 =1.5478. Yield: 95% of theory.
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