Improved synthesis of 3-methoxy-4-hydroxymandelic acid by glyoxalic acid method

The most important industrial process for the synthesis of vanillin is performed in two steps involving an condensation reaction of glyoxalic acid with guaiacol followed by an oxidative decarboxylation of the intermediary 3-methoxy-4-hydroxymandelic acid (MHPA) formed, thereby producing not only vanillin, but also byproducts, which have to be eliminated. In the present study, we focused our efforts on the first step of vanillin synthesis, namely, the condensation reaction governing the yield of vanillin. The factors influencing the stability of glyoxalic acid were preliminarily evaluated to provide significant referential value for suppressing the dismutation of glyoxalic acid in the condensation reaction, and the results indicate that the stability of glyoxalic acid largely depends on the pH, temperature, and holding time of the feed solution. Then the process of the condensation reaction was optimized under the factors affecting the stability of glyoxalic acid, as well as the molar ratio of guaiacol to glyoxalic acid. Under the optimized conditions, the maximum yield of the condensation reaction can reach to 88%.