Ionic liquids as a shuttle for releasing and capturing hydrogen chloride: a new way to utilize waste HCl generated in organic reactions

Abstract

Herein, an ionic liquid, 1-[1-(ethoxycarbonyl)-2-oxopropyl]-3-methylimidazolium chloride (1,3-(CO)₂IL-HCl), was synthesized and used as a donor of hydrogen chloride (HCl) to participate in an organic reaction. Interestingly, the simultaneously generated 1-[1-(ethoxycarbonyl)-2-oxopropyl]-3-methylimidazolium inner salt (1,3-(CO)₂IL) can then capture HCl from another reaction that produces HCl as a waste. This allowed us to use IL as a shuttle to transfer waste HCl from the first reaction to the second one that needs HCl as one of the reactants. In a proof-of-concept experiment, a ring-opening reaction of glycidyl phenyl ether with released HCl was selected; and the waste HCl comes from an alcoholysis of benzoyl chloride with ethanol. 1,3-(CO)₂IL can be used as a shuttle for transferring HCl five times without evident decline of its performance. Mechanism investigation revealed that the interconversion of keto and enol forms in 1,3-(CO)₂IL and a moderate pK_a value are keys to ensuring the release and capture of HCl.