Green Technology for the Esterification of Carboxylic Acids

Dr. G. V. Subbaraju

The well-known general Fisher esterification of carboxylic acids, by alcohols is an equilibrium process and requires large volumes of alcohol and is catalyzed by strong and corrosive mineral acids. In some cases, the water generated is removed continuously to drive the equilibrium towards the product. In addition, the alcohols like methanol and ethanol generate genotoxic, mono methyl or ethyl sulphates impurities. The other general method that involves the use of acid chlorides, poses a greater environmental burden with the liberation of HCl gas, SO2, gas etc. In view of these disadvantages, a green reagent, dimethyl carbonate (DMC) was introduced for base catalyzed alkylation or esterification reactions. The uniqueness of DMC lies in the fact that it is non-toxic and gives CO2 and methanol as by-products. However, DMC is inherently not as reactive as dimethyl sulphate or methyl halides. As a consequence, higher temperatures (160-3000C) under base catalysis are essential.

We have developed a simple, efficient, cost-effective, green and chemoselective eserification carboxylic acids with DMC or DEC (diethyl carbonate) under acid catalysis in mild and solvent-free conditions. The products were obtained in nearly quantitative yield and in high purity (The products does not require further purification). The technology developed is scalable (tested in 100kg batches) and applicable to variety of aromatic and aliphatic carboxylic acids. In the case of phenolic carboxylic acids like salicylic acid, p-hydroxybenzoic acid, the reactions were found to be chemoselective with the phenolic hydroxyl groups intact.  Because of solvent-free conditions, the reactor capacities increase many fold. Disadvantages of base catalysis like the use of high temperatures, autoclaves, expensive bases like DBU in stoichiometric amounts could be avoided. 

The details of many successful examples will be presented in this case study.

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