Electrosynthesis : electroreduction of certain oximes

Simple oximes undergo electro reduction with the uptake of four electrons per molecule to form primary amines. The four oximes taken up for study the benzophenone oxime (that carries two phenyl group), salicylaldoxime (that carries a hydroxyl phenyl group and a hydrogen atom), betafurfuraldoxime (that carries a hydrogen atom and a furan nucleus) and cyclohexanone oxime (that carries an alicyclic ring). The electro reduction of the four oximes were carried out at graphite, mercury and deposited nickel black cathodes in (a) aqueous ethanolic hydrochloric acid solution and in (b) aqueous ethanolic ammonium sulphate solution, kept at the pH around 7-8 by the addition of ammonia. Aqueous ethanol hydrochloric acid medium: Benzophenone oxime is highly reducible in this medium at all cathodes. The cyclic voltammograms on these cathodes show a well-defined reduction peak thus supporting the preparative scale results. Salicylaldoxime is also reducible at these three cathodes. Well-defined reduction peak seen in the cyclic voltammograms support this. But its efficiency of reduction is less than that of benzophenone oxime. This is obviously due to the fact that salicylaldoxime carries a (hydroxy) phenyl group and a –H atom, whereas benzophenone oxime has two negative groups – phenyl groups. Beta-furfuraldoxime is reducible at these cathodes in this medium. This is confirmed only from the cyclic voltammetric and polarographic data. The behavior of cyclohexanone oxime is quite different from other three oximes. The striking contrast observed in this oxime may be attributable to its alicyclic nature. The resuction efficiency of this oxime is relatively poor on graphite, mercury and nickel black cathodes and this may be principally due to low reactivity of the oxime and to the hydrolysis that the oxime experiences in the acid medium. The cyclic voltammogram of this oxime on graphite shows no reduction peak at all; and this resembles the base electrolyte, whereas the cyclic voltammograms of benzophenone oxime, salicylaldoxime and beta-furfuraldoxime show well-defined single reduction peak under identical experimental conditions, lending credence to the conclusion that cyclohexanone oxime is insensitive to graphite cathode in this medium. The nature of electrode process involved in the reduction of these oximes at mercury, graphite and nickel black were investigated by applying Nicholson-Shain criteria selectively to the cyclic voltammograms of these oximes (obtained at HMDE, graphite and nickel black cathodes) in aqueous ethanol hydrochloric acid. The reduction of the aromatic oximes at mercury and graphite cathodes appears to be a simple diffusion controlled irreversible charge transfer process. This electrode process is quite conspicuous by absence of chemical reaction following charge transfer. Cyclic voltammograms obtained with multi-cyclus confirm this. The behavior of the oxime at nickel black cathode needs special mention. This is an electrocatalytic type of electrode for which no equations were proposed by Nicholson and Shain. At this cathode, the electrode process is under diffusion control for all the four oximes and it should be emphasized that its behavior resembles a homogeneous catalytic reaction for which Nicholson-Shain criteria are available. It may be a fortutious coincidence and further study is required to confirm this. The reduction efficiency of cyclohexanone oxime at graphite cathode in this medium is very poor, compared to other oximes. Its cyclic voltammogram obtained at graphite cathode, corroborates this. Though the shape resembles the supporting electrolyte, addition of electro-active substance fails to produce even marginal increase in current. This implies that cyclohexanone oxime may be insensitive to graphite cathode in aqueous ethanolic ammonium sulphate medium too. In the case of mercury cathode also, the reduction behavior of cyclohexanone oxime is quite different from other three oximes. High reduction efficiency of these three oximes observed at mercury cathode is principally due to the chemical activity of ammonium amalgam formed in situ. Cyclohexanone oxime, an alicyclic oxime, appears to be less sensitive to amalgam reduction than aromatic oximes. The cyclic voltammetric data also extend support to this conclusion. The cyclic voltammogram of the oxime at HMDE cathode resembles the base electrolyte and unlike other oximes, the addition of electro-active material fails to produce even marginal increase in current