Convenient Esterification of Carboxylic Acids by S_N2 Reaction Promoted by a Protic Ionic-Liquid System Formed in Situ in Solvent-Free Conditions

Abstract

The reaction of esterification of benzoic acid with benzyl chloride was chosen as a model reaction to study the esterification by S_N2 promoted by tertiary amine as deprotonating agent. The use of ionic liquid (IL) 1,3-dimethylimidazolium methanesulfonate [MMIm][OMs] as reaction medium has proven to give quantitative yield of the ester, but interestingly the reaction does occur even in solvent-free conditions, where the acid + the amine form a liquid system (a protic IL) in situ. This last methodology was extended to several carboxylic acids in conditions of atom economy (i.e., without excess of any reagent), giving moderately good yields of esters (54–78%) recovered by weight in pure form.

GRAPHICAL ABSTRACT

Keywords:

• Esterification
• methanesulfonate anion
• nucleophilic substitution
• protic ionic liquids
• solvent-free reaction
• triethylamine

Notes

^a Temperature = 90 °C; reaction time = 2 h; amine/benzyl chloride/benzoic acid = 1.2:1.1:1.0; IL = 0.3 g; molecular solvent = 3 mL.

^a Isolated yields; all mixtures of these acids are liquid at temperature = 90 °C; reaction temperature = 90 °C; reaction time = 2 h; amine = benzyl chloride = benzoic acid = 1 mmol. The structures of esters were identified through the comparison of ¹H NMR spectra data with those reported in literature.

^b Yield after purification by chromatography column.