Development of a Novel Electrolytic System for Electroorganic Synthesis Based on Ionic Liquids; A Comparative Kinetic Study of Organic Electron Transfer Mediators

With the importance of “green and sustainable” chemistry continuing to grow and be appreciated, electroorganic synthesis has gained greater attention as one of the most environmentally friendly approaches for organic synthesis (1). However, it does require the use of large amounts of supporting electrolyte in order to provide sufficient ionic conductivity. Frequently this causes separation and waste problems following the electrolysis and creates unwanted difficulties when researchers seek to apply electroorganic synthesis to green and sustainable chemistry.

   In an effort to overcome these problems, researchers have explored the use of ionic liquids (ILs) as a surrogate for conventional supporting electrolytes. However, the main limitation in the performance of neat ILs arises due to their high viscosity and low conductivity, which limit the ion mobility, the mass transfer and possibly heterogeneous electron transfer rate of electroactive species in the ILs. This is mainly due to ion association, leading to the formation of ion pairs, which do not contribute to the overall conductance of the electrolytic medium (2).

   Recently, researchers have reported the enhanced self-diffusion coefficient and conductivity of the electrolyte for a dye-sensitized solar cell, and attributed it to a disruption of ionic interaction between the IL’s cation and anions when in the presence of carbon nanoparticles (3). Inspired by this finding, we have designed, synthesized and are testing a new recyclable “polymeric ionic liquid (PIL) and super P® carbon black composite” as a surrogate for conventional supporting electrolytes (Scheme 1). Our “composite” combines the features of a PIL to serve as an electrolyte, and the properties of the Super P^®carbon black to generate a dispersion (4). Hence, it enables one to perform an electrolysis without additional supporting electrolyte, and to efficiently recover and reuse the composite in subsequent electrolyses.

   Refer to Scheme 1.below

   In the first part of this presentation, a variety of electrochemical oxidations of aromatic alcohols will be described using the “composite” in an effort to demonstrate its reusability as well as the nature of the workup procedure for the reuse of the composite.

   In addition to the formation of a composite and its use in preparative scale electroorganic synthesis, we have chosen a unified approach to synthesis wherein analytical electrochemical investigations focusing upon the kinetics of mass transport and heterogeneous electron transfer rates have been incorporated in an effort to establish a comprehensive view of the role played by the composite. 

   In the second part of the presentation, electrochemical kinetic investigations of commonly used organic electron transfer mediators (i.e., triarylamine (5), triarylimidazole (6), and TEMPO (7)) in a composite dispersion will be presented and the results will be compared with data obtained in other media including a neat ionic liquid and a traditional organic solvent-electrolyte system. By using cyclic voltammetry, the diffusion coefficient of organic electron transfer mediators will be estimated from the gradient of the linear plot of anodic peak current vs. the square root of scan rate based on the Randles-Sevcik formulae for a reversible process (8), and the data obtained will be compared with the values obtained by pulsed gradient spin echo NMR (9). In addition, the heterogeneous electron-transfer rate constant will be determined by (a) chronoamperometry, analyzed by the Cottrell equation and (b) cyclic voltammetry, analyzed by the method of Nicholson (10).

ACKNOWLEDGEMENTS

We are grateful to NSF Partnership for International Research and Education-Electron Chemistry and Catalysis at Interfaces (PIRE-ECCI) for a fellowship to SJY, and to Amgen for their support of education.

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