Electrochemical Investigations of Aromatic Hydrocarbons in Adiponitrile

Introduction

     Recent work from this laboratory has shown that adiponitrile (NCCH₂CH₂CH₂CH₂CN) / tetraethylammonium tetrafluoroborate (TEA BF₄) is a versatile solvent system for organic electrochemistry (1,2).  The extraordinary 7-Volt potential range noted in earlier work (3) has been confirmed and applied to measurement of water (1) and electrodeposition of metals (4).  The +5 V vs Ag/AgCl potential limit allows another 2 volts for study, compared to that of most other solvents.  The present paper addresses the usefulness of the positive potential region for oxidation of aromatic hydrocarbons.

Experimental

     Adiponitrile was purchased from Sigma-Aldrich, and tetraethylammonium tetrafluoroborate was purchased from Southwestern Analytical Chemicals.  Cyclic voltammograms were obtained on a PAR 283 potentiostat using PowerSuite^TMsoftware.  Potentials are referenced against a Ag/AgCl reference electrode (1).

 Results and Discussion

     A cyclic voltammogram of 10 mM naphthalene in adiponitrile / 0.2 M TEA BF₄ is shown in Figure 1.  The reversible reduction of naphthalene is observed at -2.5 V, followed by an irreversible oxidation process at +1.74 V vs Ag/AgCl.  Scan reversal at +2.00 V shows no reduction current associated with this oxidation process at 100 mV/s.  Continuation of the potential scan to +5.0 V, as shown, results in considerable oxidation current up to the positive potential limit. This charge passage, however, significantly lessens the peak currents for the redox process at -2.5 V on the following sweep, implying some electrode filming in the highly positive potential range. Naphthalene oxidation has also been studied in acetonitrile (5), and the results were similar to those in the present study although the positive scan limit in acetonitrile was only +1.75 V vs SCE. In the acetonitrile study (5), the irreversible nature of naphthalene oxidation was attributed to rapid coupling of naphthalene radical cations.  The results of the present study show that the entire positive potential range in adiponitrile is available for study of the representative aromatic hydrocarbon naphthalene.  It is also intended to present results for other aromatic systems.  

References

1.  G. T. Cheek, ECS Trans., 58(46), 45-51 (2014).

2.  G. T. Cheek, ECS Trans., 61(30), 19-24 (2014).          

3.  H. Duncan, N. Salem, and Y. Abu-Lebdeh, J. Electrochem. Soc., 160(6), A838-A848.

4.  G. T. Cheek, ECS Trans., 61(24), 9-14 (2014).  

5.  M. N. Cortona, N. Vettorazzi, J. J. Silber, and L. Sereno, J. Electroanal. Chem., 394, 245-251 (1995).

Figure 1.  Cyclic voltammogram of 10 mM naphthalene at glassy carbon in adiponitrile / 0.2 M tetraethylammonium tetrafluoroborate.

               Initial negative-going sweep from 0.00 V vs Ag/AgCl; 100 mV/s