Selective chlorination of a 1-(2-fluorophenyl)-1,2,4-triazole

A process for the chlorination of 1-(2-fluorophenyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazole (the "2-fluorophenyl compound") in the 4-position of the phenyl ring which comprises

• a) adding 0.8 to 1.6 molar equivalents of chlorine to a stirred slurry of 1-(2-fluorophenyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazole in a solvent selected from acetonitrile, N,N-dimethylformamide, nitromethane, and nitrobenzene, at a temperature in the range of ambient to 50.degree. C., at a rate such that the temperature does not exceed 50.degree. C., and continuing the stirring at 30 to 50.degree. C. for from one to ten hours;
• b) for a period of from one to six hours, while maintaining the temperature at 30 to 50.degree. C., reducing the pressure in the reaction vessel so that the solvent refluxes and most of the hydrogen chloride by-product is driven off;
• c) optionally, purging the reaction vessel with nitrogen to reduce the hydrogen chloride concentration in the reaction mixture to below 1%;
• d) repeating steps a, b, and c two times;
• e) recovering 1-(4-chloro-2-fluorophenyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazole.