Synthesis
General procedure for the synthesis of 2,3,4,5-tetrachloropyridine from 2-chloropyridine: 300 g of 2-chloropyridine and 15 g of loaded catalyst Sn-AC were added to a 500 mL glass reaction flask equipped with a thermometer and a reflux condenser.The reaction mixture was heated to 110 °C, and chlorine was passed through the reaction at a flow rate of 200 mL/min at a pressure of 0.1 MPa for 20 hours. After completion of the reaction, the temperature was lowered to below 30 °C and the loaded catalyst was removed by filtration. The content of 2,3,4,5-tetrachloropyridine in the filtrate was detected by HPLC and 217 g of product was obtained. The filtrate was transferred to another glass reaction flask, and 800 mL of methanol-water mixed solvent (4:1, v/v), 79 g of pyridine and 217 mg of Pd-loaded activated carbon catalyst (5% Pd/C, purchased from Xi'an Kelley New Materials Co., Ltd.) were added. After displacing the reaction system with hydrogen for three times, the reaction system was heated to 50 °C and hydrogen was passed at a flow rate of 200 mL/min under 0.5 MPa pressure. At the same time, 5% Na2CO3 aqueous solution was added dropwise to maintain the pH of the reaction system at 8-10. When the HPLC analysis showed that the content of 2,3,4,5-tetrachloropyridine was less than 5%, the passage of hydrogen was stopped and cooled to room temperature. The catalyst was removed by filtration, the filtrate was evaporated under reduced pressure to remove methanol, and the precipitated white solid was collected by filtration. The filter cake was washed three times with 100 mL of 5% hydrochloric acid and dried to give 465 g of white solid.HPLC analysis showed that the crude product contained 5.8% dichloropyridine, 89.8% 2,3,5-trichloropyridine and 4.4% 2,3,4,5-tetrachloropyridine. The crude product was distilled under reduced pressure to give 414 g of 2,3,5-trichloropyridine in 86.2% yield and >99% purity.
