Electrochemical Characterization of Some Electroactive Nucleophilic Substitution Reactions

Abstract

The substitution reaction of Catechol in presence of Imidazole, L-Arginine and L-Serine have been studied by cyclic voltammetry, controlled potential coulometry and differential pulse voltammetry techniques using Glassy carbon (GC), Gold (Au) and Platinum (Pt) electrodes. The voltammetric studies have been carried out by varying the composition of Catechol, Imidazole, L-Arginine and L-Serine in the buffer solution of different pH, different electrodes and scan rate. Pure Imidazole, L-Arginine and L-Serine are electro-inactive whereas pure Catechol is electro-active. In the second scan of potential, a new anodic peak appears at the lower potential and the oxidation and reduction peak shifted with respect of pure catechol after addition of Imidazole, L- Arginine and L-Serine in catechol solution. At the same time the anodic and cathodic peak current decreases significantly compared with the pure catechol that indicates the participation of reaction of o-benzoquinone with Imidazole, L-Arginine and L-Serine. The reaction products generated from the reaction of catechol in presence of Imidazole, L-Arginine and L-Serine are assumed to be 4-(1H-imidazol-1-yl)cyclohexa- 3,5-diene-1,2-dione- 2-amino-4-(1-(3,4-dioxocyclo hexa-1,5-dien-1-yl)guanidino)-4- oxobutanoic acid and 3-hydroxy-2-((3-hydroxy-4-oxocyclohexa-2,5-dien-1- ylidene)amine)propanoic acid respectively that undergo electron transfer at more negative potentials than the Catechol. The electro –synthesized products generated from Catechol with Imidazole, L-Arginine and L-Serine were isolated. The formation of Catechol-nucleophile adducts are also confirmed by FTIR spectra. The pH effect of catechol in presence of Imidazole, L-Arginine and L-Serine has been investigated in different pH from 3 to 11. From the experiment it has been observed that almost no new anodic peak appeared after repetitive cycling at pH 3-5 (acidic media) and pH 9-11 (basic media). At pH 7, the o-benzoquinone undergoes nucleophilic attack by the amines that is reflected from voltammetric new anodic peak, A0 appeared after repetitive cycling. The slopes of the peak potential, Ep vs pH plot has determined graphically as the anodic peaks of Catechol-Imidazole (60 mV/pH), Catechol-Arginine (56mV/pH) and Catechol-Serine (61.5 mV/pH) at 0.1V/s.