The Redox Chemistry of 4-Benzoyl-N-methylpyridinium Cations in Acetonitrile with and without Proton Donors:  The Role of Hydrogen Bonding

Abstract

In anhydrous CH 3 CN, 4-benzoyl-N-methylpyridinium cations undergo two reversible, well-separated (∆E 1/2 ∼ 0.6 V) one-electron reductions in analogy to quinones and viologens. If the solvent contains weak protic acids, such as water or alcohols, the first cyclic voltammetric wave remains unaffected while the second wave is shifted closer to the first. Both voltammetric and spectroelectrochemical evidence suggest that the positive shift of the second wave is due to hydrogen bonding between the two-electron reduced form of the ketone and the proton donors. While the one-electron reduction product is stable both in the presence and in the absence of the weak-acid proton donors, the two-electron reduction wave is reversible only in the time scale of cyclic voltammetry. Interestingly, at longer times, the hydrogen bonded adduct reacts further giving nonquaternized 4-benzoylpyridine and 4-(R-hydroxybenzyl)pyridine as the two main terminal products. In the presence of stronger acids, such as acetic acid, the second wave merges quickly with the first, producing an irreversible two-electron reduction wave. The only terminal product in this case is the quaternized 4-(Rhydroxybenzyl)-N-methylpyridinium cation. Experimental evidence points toward a common mechanism for the formation of the nonquaternized products in the presence of weaker acids and the quaternized product in the presence of CH 3 CO 2 H.

A Comparative Electrochemical Study of Diffusion in Room Temperature Ionic Liquid Solvents versus Acetonitrile

“…This compares to values from previous reports under the same conditions of 0.99 AE 0.10 and 0.79 AE 0.02. [3,12] A larger discrepancy was seen in the case of the dication, where the ratio of diffusion coefficients,…”

“…A generic complication of using a system with two successive electron transfers, however, is the possible (thermodynamically favoured) conproportionation reaction between TMPD and TMPD 2 + that could occur in the diffusion layer adjacent to the electrode surface and affect any voltammetry were it not accounted for. [11,12] To circumvent this problem, use of solutions of neutral TMPD was restricted to the study of TMPD and TMPD + C only, while a solution of bulk TMPD + C was employed separately to interrogate the ions TMPD + C and TMPD 2 + . As such, the two species susceptible to conproportionation were never found in bulk solution together and hence this potential side-reaction avoided.…”

The extreme migrational enhancement of faradaic current at microelectrodes: experimental studies on sodium (6,8-diferrocenylmethylthio)octanoate electrooxidation

“…These two independent methods gave the following average value of DFcMTO À diffusivity: (1.7 ± 0.3) · 10 À11 m 2 /s. It is known that the successive heterogeneous transfer of two electrons is accompanied by a comproportionation reaction between the final product and the substrate of the electrode process [11,[15][16][17]. In our case the possible homogeneous reaction, that proceeds in the transport layer leading to the generation of formally uncharged intermediate species, can be represented schematically as…”

Alteration of diffusional transport by migration and natural convection. Theoretical and direct experimental evidences upon monitoring steady-state concentration profiles at planar electrodes

“…W Ã B and W Ã C depend on the local composition of the solution at the position where the detection is performed. They can be deduced from previous theory [26] predicting the limiting currents in default of supporting electrolyte at second waves of classical EE mechanisms [28,29] (i.e., for electrochemical reactions A z+ + 2ne À ! A (zÀ2n)+ , see Appendix B).…”