Fischer Esterification of Benzoic Acid and Polyhydric Alcohols Catalyzed by Basic Ionic Liquids of Bisimidazolium Tungstates

Fischer esterification referred to the reaction of carboxylic acids and alcohols, which was generally catalyzed by Bronsted or Lewis acidic catalysts. Basic ionic liquids with the nucleophilic anion were generally considered incapable of catalyzing this reaction due to sufficiency of electrons or the lack of ionizable protons. Herein, bisimidazolium tungstates were found be able to catalyze Fischer esterification of ethylene glycol (EG) and benzoic acid relying on the nucleophilic WO42- anion. The mechanism studies indicated that C6H5COO- and H-WO42- anionic intermediate was formed between benzoic acid and WO42- via hydrogen-bond interaction under the influence of hydroxyl of EG. This intermediate could further release H atom to accelerate the reaction speed of Fischer esterification, which was similar catalytic process to that of Bronsted acidic catalysts. Furthermore, other nucleophilic anions (CO32-, OH-, OAc-, C6H5COO-) had little catalytic capacity in the reaction. This suggested that the strength of anionic nucleophilic ability played an important role in the esterification reaction.