Study On The Synthesis Of 3,6-dichlorosalicylic Acid

3,6-dichlorosalicylic acid, namely 2-hydroxy-3,6-dichlorobenzoic acid, is a kind of important herbicide intermediate. It is also a key intermediate for the production of herbicide dicamba. Dicamba is a benzoic acid-based herbicide with the uptake and conduction effect,and it has significant control effect on annual and perennial broadleaf weeds. But to wheat, maize, millet, rice and other cereal crops is safe. It has been widely applied both at home and abroad at present, and the market demand is in good condition. So how to synthetic 3,6-dichlorosalicylic acid efficiently and simply is worth studying.This paper reviews various synthesis methods of 3,6-dichlorosalicylic acid. After analyzing the correlated literature, the route of 3,6-dichlorosalicylic acid is determined. We choose the cheap and easy gained 2,5-dichloronitrobenzene as initial raw materials, through the hydrazine hydrate reduction, 2,5-dichloroaniline is gained and through the diazotization reaction, we can obtain the intermediate 2,5-dichlorophenol, then the target product 3,6-dichlorosalicylic acid is through carboxylation reaction by high temperature and pressure. The key factors effecting the above reactions are studied through single factor experiments, and optimum condition is gained.The results show that in the process of hydrazine hydrate reduction reaction, the yield of 2,5-dichloroaniline is 96.4% and the purity of GC is 99.81% when the mole ratio of material is n(2,5-dichloronitrobenzene):n(80% hydrazine hydrate)=1:1.8, the mole ratio of FeO(OH) to 2,5-dichloronitrobenzene is 0.25, with 95% ethanol as solvent and the quality ratio is m(2,5-dichloronitrobenzene):m(95% ethanol)=1:1.9, the catalyst can be used for seven times, the catalytic activity is still good and the solvent can be recycled; The yield of 2,5-dichlorophenol is 84.6% and the purity of GC is 99.37% when the mole ratio is n(2,5-dichloroaniline):n(sodium nitrite)=1:1.1, n(2,5-dichloroaniline):n(92% sulfuric acid)=1:6, the temperature of hydrolysis is 140℃ and the time of hydrolysis is 2.5h in the process of diazotization hydrolysis reaction; Finally, in the carboxylation reaction of 2,5-dichlorophenol, when the mole ratio is n(2,5-dichlorophenol):n(potassium hydroxide)=1:1.2, the mole ratio of K2CO3 to 2,5-dichlorophenol is 1.0, the pressure is 5.5MPa, the temperature is 145℃ and the stirring rate is 500r/min, the time is 9 hours, we can obtain the target product 3,6-dichlorosalicylic acid, the yield of it is 80.7% and the purity of HPLC is 99.24%.In conclusion, the yield(calculated by 2,5-dichloronitrobenzene) of 3,6-dichlorosalicylicacid is 65.8%, respectively, and its quality satisfies the commercial requirement. The synthetic method is simple and economic, and the target product has high purity and easy be separated. The Fourier Transform Infrared Spectrometer and the 1H-MNR spectra are consistent with the characters of the 3,6-dichlorosalicylic acid.