Benzonitriles electroreduction

D. The chronoamperometric results can also be used to ascertain the number of electrons involved in the formation of benzonitrile from p-chloro-benzonitrile. In order to translate the chronoamperometric data into a meaningful n value, a compound is selected that has a diffusion coefficient very similar to that of p-chlorobenzonitrile and that gives a stable, known product upon electroreduction. Tolunitrile, which satisfies these criteria, is known to be reduced to its radical anion at a diffusion-controlled rate. Since this one-electron process gives a value of 168 pA s1/2- M x cm 2 for it1/2/CA, the corresponding value of 480 pA s1/2 A/ 1 cm-2 for the reduction of p-chlorobenzonitrile to benzonitrile anion radical must represent an overall three-electron process. When we subtract the one electron that is required to reduce benzonitrile to its radical anion from this total, we immediately conclude that two electrons are involved in cleavage of the carbon-chlorine bond in p-chlorobenzonitrile. A scheme that is consistent with these data is described by Equations 21.1 to 21.6. 

The efficiency of electron-transfer reduction of Cgo can be expressed by the selfexchange rates between Coo and the radical anion (Ceo ), which is the most fundamental property of electron-transfer reactions in solution. In fact, an electrochemical study on Ceo has indicated that the electron transfer of Ceo is fast, as one would expect for a large spherical reactant. This conclusion is based on the electroreduction kinetics of Ceo in a benzonitrile solution of tetrabutylammonium perchlorate at ultramicroelectrodes by applying the ac admittance technique [29]. The reported standard rate constant for the electroreduction of Ceo (0.3 cm s ) is comparable with that known for the ferricenium ion (0.2 cm s l) [22], whereas the self-exchange rate constant of ferrocene in acetonitrile is reported as 5.3 x 10 s , far smaller than the diffusion limit [30, 31]. 

Chung, T.D. and F.C. Anson (2001). Catalysis of the electroreduction of 02by cobalt 5,10,15,20-tetraphenylporphyrin dissolved in thin layers of benzonitrile on graphite electrodes. 7. Electroanal. Chem. 598, 115-122. 

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