Study On The Synthesis Of Azo Dyes As Raw Materials Of Ultraviolet Absorbents

Some monoazo dyes have a nitro group in the ortho position of the nitrogen-nitrogen double bond.Such azo dyes can be reduced to form benzotriazole ultraviolet absorbers.Azo dyes are usually synthesized by diazo coupling method.In most cases,diazonium salts prepared by diazotization of aniline compounds have poor thermal stability at room temperature.Therefore,the diazotization reaction is often carried out at a low temperature of 0-5 ℃,which requires higher reaction conditions or equipment conditions.It is known from the literature that the stability of diazonium salts is greatly affected by other substituents in the benzene ring of aniline compounds,while the stability of diazonium salts is enhanced by the nitro group with strong electronattracting power.In this paper,diazonium salts were prepared at room temperature,and then coupled with phenols to obtain five dyes: 2-nitro-2’-hydroxy-5’-methylazobenzene,4-chloro-2-nitro-2’-hydroxy-3’-tert-butyl-5’-methyl-azobenzene,4-chloro-2-nitro-2’-hydroxy-3’,5’-ditert-butyl-azobenzene,2-nitro-2’-hydroxy-3’,5’-ditert-amylazobenzene and 2-nitro-2’-hydroxy-5’-tert-octyl-azobenzene.They are raw materials for the synthesis of benzotriazole UV absorbers UV-P,UV-326,UV-327,UV-328 and UV-329 respectively.2-nitro-2’-hydroxy-5’-methyl-azobenzene(the raw material of UV-P)was synthesized by nitrososulfuric acid method and sodium nitrite solution method respectively.It was found that the diazonium salt prepared by nitrososulfuric acid method did not decompose at room temperature,so the subsequent process optimization experiment chose nitrososulfuric acid method to prepare diazonium salt.The effects of the p H of coupling reaction,the amount of sodium nitrite,the amount of concentrated sulfuric acid,surfactant and the ratio of raw materials on the dye yield were studied.In the scale-up experiment,the dye yield reached 88%.The synthesis of the other four dyes need to use surfactants.The dispersibility of surfactant is affected by a large amount of concentrated sulfuric acid,so sodium nitrite solution method is used to prepare diazonium salt.The synthesis conditions of the four dyes were also optimized,the effects of the amount of sodium nitrite,the amount of concentrated sulfuric acid,the ratio of raw materials,the type of surfactant,the amount of surfactant and the p H of coupling reaction on the dye yield were studied.In the scale-up experiment after optimizing the process conditions,the yield of 4-chloro-2-nitro-2’-hydroxy-3’-tert-butyl-5’-methyl-azobenzene(the raw material of UV-326)reached88%,the yield of 4-chloro-2-nitro-2’-hydroxy-3’,5’-ditert-butyl-azobenzene(the raw material of UV-327)reached 90%,the yield of 2-nitro-2’-hydroxy-3’,5’-ditert-amylazobenzene(the raw material of UV-328)reached 93%,and the yield of 2-nitro-2’-hydroxy-5’-tert-octyl-azobenzene(the raw material of UV-329)reached 85%.Compared with the traditional synthesis of azo dyes,diazo coupling reaction was carried out at room temperature to synthesize azo dyes,which reduced the requirements of reaction equipment and had positive significance in reducing energy consumption.