Ion-pairing effects on the reduction of nitroarenes in propan-2-ol solutions: An electrochemical investigation

The electrochemical reduction of 1-chloro-4-nitrobenzene in propan-2-ol was studied by means of cyclic voltammetry at a glassy-carbon electrode with LiSCN, NaSCN, KSCN, and Buⁿ<inf>4</inf>NSCN as support electrolytes. With Buⁿ<inf>4</inf>NSCN a two-wave process obtains, corresponding to an overall four-electron reduction (a reversible one-electron process followed by an irreversible three-electron process). With KSCN, NaSCN, and LiSCN the second wave is progressively shifted to more anodic potentials, to the point where it merges into the first peak, which is unaffected by the nature of the cation. Additions of 18-crown-6 to the solutions containing Na⁺ and, particularly, K⁺ result in shifts of the second wave to more negative potentials. The resulting voltammetric curves closely resemble that observed with Buⁿ<inf>4</inf>NSCN. Additions of 18-crown-6 to solutions of Li⁺ have no effect at all on the cyclic voltammetry curves. The observed phenomena are explained in terms of ion-pairing effects, and suggest that the electrochemical reduction of 1-chloro-4-nitrobenzene in propan-2-ol proceeds via formation of the radical anion, followed by a second electron transfer to give the dianion. The relevance of these observations to the mechanism of reduction promoted by alkoxide ions is briefly discussed.