Study On Synthesis Of Menthol From Thymol And Turpentine

Menthol is one of the most popular perfumes in the world. Its mint flavor and cool property are widely applied in many products. Over the past years, menthol always comes from natural plant such as mentha haplocalyx and mentha piperita. However, it cannot meet requirements from industrialization in recent years. Turpentine, as abundant renewable resources, is extensive fine chemicals to synthesize perfumes and other fine chemicals. In this paper, our research work was focused on methods to synthesize menthol, with thymol and turpentine as raw material respectively.In the first route, the menthol is prepared from thymol by catalytic hydrogenation reaction. With Raney nickel as catalyst, menthol can be synthesized through hydrogenation reaction with thymol at 150℃in 4 MPa H2 pressure, in high conversion rate. Menthyl acetate was obtained in reaction of the result compound with acetic anhydride, with yield up to 90%.With Raney nickel as catalyst, pinane was gained by hydrogenation reaction of turpentine at 150℃in 3 MPa H2 pressure, with yield up to 90%. Dihydro-myrcene was obtained from pinane in yield 40% at 430℃. We also have much research work in preparation of citronellal from dihydromyrcene in two ways in this paper. In one case, citronellyl boron was get from reaction of dihydro-myrcene with sodium borohydride and malonic acid in THF at 60℃. Then, citronellol in 30.0% content is prepared from the citronellyl boron in oxidized and hydrolyzed reaction with hydrogen peroxide and sodium hydroxide under air respectively. Citronellal in 94.5% was obtained from citronellol, under the complex [CuBr2(2,2'-bipyridine)] from 2,2,6,6-tetramethyl-piperidyl1oxy, CuBr2, sodium hydroxide and 2,2'-bipyridine. In another case, citronellal was also prepared with one pot process from dihydro-myrcene in the above catalysts. The result of gas analysis shows that the content of citronellal is up to 21.4%. Then, menthol was achieved from citronellal by two-step reaction, with the yield of the compound of each step is to 90%.With the above result in synthsis of menthol, it will be favor of our next reseach work in reaction and lipase-catalyzed resolution.