Study On The Synthesis Of2-Chloro-5-methylpyridine

2-Chloro-5-methylpyridine is a key intermediate for the synthesis of imidacloprid and other nicotinoid insecticides. In this paper, process for the preparation of2-chloro-5-methylpyridine is studied.5-Methyl-3,4-dihydro-2(1H)-pyridone, the intermediate product from imidacloprid production line of Jiangsu Yangnong Chemical Group Co., Ltd. was used as raw material. Chlorination of5-methyl-3,4-dihydro-2(1H)-pyridone with chlorine gave5,6-dichloro-5-methylpiperidin-2-one, which was transformed to the final product2-chloro-5-methylpyridine via elimination of hydrogen chloride, ketone-enol tautomeric aromatization and chloro-substitution reaction of the hydroxyl groups on the pyridine ring.Presently, phosphorus oxychloride was used as chlorination agent in2-chloro-5-methylpyridine workshop on imidacloprid production line of Jiangsu Yangnong Chemical Group Co. Ltd., which led to problems such as low yield of reaction product and produced large amounts of solid waste containing tarry oil. In order to avoid these problems, triphosgene or phosgene was used instead of phosphorus oxychloride in this study. By screening of catalyst and solvent and optimization of process parameters, the yeild of2-chloro-5-methylpyridine increased from the current75%to more than90%. The amount of wastewater generated reduced from10tons per ton of product to3tons per ton of product and the amount of solid residue generated reduced from2tons per ton of product to0.2tons per ton of product. The improved procedure was cleaner, energy-saving and tresulted in less emission and10%of cost saving.Through a large number of experiments a preferred process using solid phosgene as a chlorinating agent was obtained. After removed of the solvent chlorobenzene5-methyl-3,4-dihydro-2(1H)-pyridone from imidacloprid production line was dissolved in chloroform, then chlorine was charged in at45℃for30min, and5,6-dichloro-5-methylpiperidin-2-one was formed. After addition of a small amount of catalyst, the reaction solution was added dropwise to another reaction flask containing chloroform at61℃for2h. Meanwhile, a chloroform solution of triphosgene was added dropwise to the flask, too. The resulted solution was keeped refluxing for6h and gave2-chloro-5-methylpyridine in93%yield. Using phosgene instead of triphosgene under similar conditions gave2-chloro-5-methylpyridine in91%yield. The new process using triphosgene as chlorinating agent was then tested on250kg scale. Average yield of90%was obtained for10batches. This indicated that the new process has good prospects for industrialization.