Electrochemical oxidation of quaternary ammonium electrolytes: Unexpected side reactions in organic electrochemistry

Quaternary ammonium salts are among the most widely used electrolytes in organic electrochemistry, but there is little known about their unwanted side oxidation reactions. We have, therefore, studied the constant potential oxidation products of quaternary ammonium electrolytes using mass spectrometry, and the proposed oxidation reaction mechanisms were verified by stable isotope labeling studies. Oxidation of tetrabutylammonium at + 2.5 V vs Ag showed oxidation products at m/z 256, 258, and 299 which were shown corresponding to alcohol, ketone, and acetonitrile-derived amide products, respectively. We propose a mechanism based on the anodic oxidation of the side chain, to generate a carbocation, followed by nucleophilic attack by water or acetonitrile. Incorporation of a water molecule and/or an acetonitrile molecule in the structure was verified by mass analysis of the isotopically labeled oxidation products. Oxidation can be avoided by use of quaternary ammonium ions with shorter alkyl chains, where electrostatic repulsion presumably prohibits carbocation generation.

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► Quaternary ammonium electrolytes were oxidized in acetonitrile/water
► Three tetrabutylammonium oxidation products were identified with mass spectrometry.
► The products were formed upon nucleophilic reaction of acetonitrile and water.
► Reaction pathways were verified using isotopically labeled reagents.
► Quaternary ammonium species with short alkyl chains are less sensitive to oxidation.