Electro-oxidation of 1,2-dihydroxybenzene in Presence of Alanine, Phenylalanine and Leucine at Different pH Media

Abstract

The electro-oxidation of 1,2-dihydroxybenzene generates o-benzoquinone (Michael acceptor) and its reaction in presence of different concentration of L-Alanine, LPhenylalanine and L-Leucine (nucleophiles) has been investigated. The overall study has been carried out in buffer solution of different pH (5, 7, 9 and 11) by using Cyclic Voltammetry (CV), Controlled Potential Coulometry (CPC) and Differential Pulse Voltammetry (DPV) techniques at different electrodes (GC, Pt and Au) and scan rates (0.05-0.5 V/s). Cyclic voltammogram of electro-active 1,2-dihydroxybenzene in pure buffer solution (5-11) shows one anodic and corresponding cathodic peak within a quasireversible process. Pure L-Alanine, L-Phenylalanine and L-Leucine are electro-inactive having no peak in the potential range of investigation (-0.6 to 0.9V). Addition of different composition of L-Alanine (10-150 mM), L-Phenylalanine (2-100 mM) and L-Leucine (30-200 mM) in fixed 2 mM of 1,2-dihydroxybenzene solution, in the second scan of potential a new anodic peak (A0) arises at the more negative potential with respect to the pure 1,2-dihydroxybenzene. The anodic (A1) and cathodic peak (C1) current intensity of 1,2-dihydroxybenzene also decreases significantly. This indicates the participation of 1,4-Michael addition reaction of o-benzoquinone with L-Alanine, LPhenylalanine and L-Leucine to produce 2-((3,4-dihy droxyphenyl)amino)propanoic acid, 2-((3,4-dihydroxyphenyl)amino)-3-phenylpropanoic acid and 2-((3,4-dihydroxyphenyl) amino)-4-methyl-pentanoic acid adducts. The 1,4-Michael addition reaction of 1,2-dihydroxybenzene is strongly influenced by the concentration of nucleophiles. The electrooxidation of 2 mM 1,2-dihydroxybenzene is mostly favorable in 50 mM of L-Alaline, 20 mM of L-Phenylalanine and 100 mM of L-Leucine respectively. The effect of pH on 1,2-dihydroxybenzene in presence of different nucleophiles has been studied by varying pH ranging from 5 to 11. In acidic pH media (pH <7), no new anodic peak arises after repetitive cycling due to protonation of amine group. But in the neutral (pH=7) and basic media (pH >7), o-benzoquinone undergoes nucleophilic attack by the amine part of amino acids and the maximum peak current is observed at pH 7. The slope value of 1,2-dihydroxybenzene-Alanine, 1,2-dihydroxybenzene-Phenylalanine and 1,2-dihydroxybenzene-Leucine adducts have been calculated (70.5 mV/pH for first anodic peak A1), (59 mV/pH for first anodic peak A1) and (68.5 mV/pH for first anodic peak A1) at 0.1 V/s respectively. These values indicate that the nucleophilic substitution reactions are preceded via 1e−/1H+ process. It is also suggested that during the course of reaction, electron and proton are released simultaneously from the 1,2-dihydroxybenzene-Amino acid adducts. The nature of voltammogram, peak position and current intensity for the studied systems are different for different electrodes and the voltammetric response of GC electrode is better than Au and Pt electrodes. The effect scan rates on cyclic voltammogram of 1,2-dihydroxybenzene in presence of LAlanine, L-Phenylalanine and L-Leucine have also been studied. The peak current of both the anodic and the corresponding cathodic peaks increases with the increase of scan rate. The nearly proportionality of the anodic and corresponding cathodic peak suggests that the peak current of the reactant at each redox reaction is controlled by diffusion process with some chemical complications. The current function, Ip/v1/2 vs scan rates (v) of 1,2-dihydroxybenzene-Amino acid adducts are found to be decreased exponentially with increasing scan rate which suggests that the behavior of reaction mechanism is Electron transfer-Chemical reaction-Electron transfer (ECE) type for all the studied system. The products obtained by bulk electrolysis have also been analyzed by FTIR spectra.