Electrochemical Investigations of Benzil in Nonaqueous Media

Introduction

    The electrochemical behavior of the aromatic diketone benzil has been studied in a number of solvents (1-4).  The benzil system is particularly interesting because of its ability to undergo various reactions accompanying the reduction process.   Most of the solvent systems studied to date have involved proton-donating properties to some degree, so the use of the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate ( BMPY TfO ) provides a useful contrast to the other solvents. 

Experimental

     All electrochemical experiments were carried out in a Vacuum Atmospheres glovebox.  Cyclic voltammograms were obtained on a PAR 283 potentiostat using PowerSuite software.  Potentials are reported with respect to an Ag/AgCl reference electrode. 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate (BMPY TfO) was prepared by a literature method (5).  Benzil was obtained from Aldrich Chemical Company.

Results and Discussion

     As can be observed from Figure 1(a),  benzil undergoes electrochemical reduction in BMPY TfO in two successive, reversible electron-transfer steps. Figure 1(b) shows the voltammetric behavior for only the first process.  In contrast, reduction in other nonaqueous solvents such as acetonitrile and dimethylformamide (2,3) involves considerable irreversibility for the second process.  The low proton-donating ability of BMPY TfO seems largely to account for this effect.  The ionic liquid BMPY TfO, then, serves as a suitable solvent for observing relatively uncomplicated voltammetric behavior of benzil, presenting the possibility of studying the effect of added proton donors in this solvent.

References

1. H. E. Stapelfeldt and S. P. Perone,  Analytical Chemistry,  41(4), 623 (1969).
2. M. D. Ryan and D. H. Evans,  J. Electroanal. Chem.,  67, 333 (1976).
3. M. Chandrasekaran,  M. Noel and V. Krishnan,  J. Chem. Soc., Perkin Trans.2, 979 (1992).
4. G. T. Cheek, ECS Transactions, 6(6), 47 (2007).
5. L. Crowhurst, N. L. Lancaster,  J. M. P. Arlandis, and T. Welton,  J. Am. Chem. Soc., 126, 11549 (2004).

Figure 1.  Cyclic voltammetry of  40 mM benzil in  BMPY TfO at glassy carbon ( 1 mm diameter ).

                  Scan rate  100 mV/s

(a)   Scan to -2.50 V                [ black curve ]        

(b)   Scan reversal at -1.50 V  [ red    curve ]