Electrochemical Oxidation of the Phenolic Benzotriazoles UV‐234 and UV‐327 in Organic Solvents

Abstract

The electrochemical behavior of selected phenolic benzotriazoles (BZTs), namely 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐bis(1‐methyl‐1‐phenylethyl)phenol and 2,4‐di‐tert‐butyl‐6‐(5‐chlorobenzotriazol‐2‐yl)phenol (commercial names UV234 and UV327, respectively) were examined with cyclic voltammetry (CV) and controlled potential electrolysis (CPE) in acetonitrile and dichloromethane solutions. CV indicated that both phenolic BZTs undergo a chemically irreversible oxidation process at approximately Ep°x=+1.0 V vs. Fc/Fc+ (where Ep°x is the anodic peak potential and Fc=ferrocene) to form compounds that cannot be electrochemically converted back to the starting material on the voltammetric timescale. In basic conditions, cyclic voltammetry experiments indicated that the corresponding phenolates (prepared by reacting the phenols with equiv. mols of n‐Bu4NOH) were oxidized at Ep°x∼−0.2 V vs. Fc/Fc+ via a one‐electron diffusion controlled process with anodic (ip°x) to cathodic (ipred) peak current ratios (ip°x/ipred)≫1, suggesting that the produced phenoxyl radicals decomposed rapidly via a chemical step. However, electron paramagnetic resonance (EPR) experiments performed on the bulk electrolyzed solutions of the phenolates after one‐electron bulk oxidation indicated long lifetimes of the UV234. and UV327. phenoxyl radicals. Therefore, the long timescale CPE and spectroscopic (UV‐vis and EPR) studies provided good evidence of a reversible dimerization mechanism between the phenoxyl radicals, which explained the apparent discrepancy with the short timescale CV experiments.

An Electrochemical o-Phthalaldehyde Sensor Using a Modified Disposable Screen-Printed Electrode with Polyacrylate Hydrogel for Concentration Verification of Clinical Disinfectant

“…This result indicates that the commercially available disinfectant Cidex-OPA formula contains other electroactive substance(s), such as benzotriazole and D&C Green Dye #5, which were mentioned in a use instruction claimed by the manufacturer. These electrochemical characteristics of benzotriazole are also supported by another report [ 35 ]. Reductive additives are generally adopted in OPA disinfectant solutions to prolong storage time and maintain OPA quality, which is a habitual art in commercial products because the OPA reagent oxidizes at room temperature, making its disinfecting quality hard to maintain.…”

Electrochemical and Spectroscopic Characterization of Oxidized Intermediate Forms of Vitamin E

“…The potential axis in Figure 1 , and most subsequent figures that include voltammograms are referenced to the ferrocene 0/+ redox couple (Fc/Fc + ), which is the standard internal reference used for voltammetry in non-aqueous solvents [ 70 ]. The chemically irreversible feature of the CV of 2,4,6–tri– tert –butylphenol is typical of most phenols that form highly reactive oxidized compounds during the forward scan and are unable to be reduced back to the starting material regardless of the scan rate [ 69 , 71 , 72 , 73 , 74 , 75 , 76 ]. The rare exception to this is phenols that undergo intramolecular hydrogen bonding of the phenol hydrogen atom with an amine, which can result in stabilization of the initially formed cation radical and allow the appearance of a partially chemically reversible CV [ 77 ].…”

“…When a strong organo-soluble base is added to the solution in equal molar amounts, such as tetraethylammonium hydroxide [ 69 , 71 , 72 , 74 , 75 , 76 ], the phenol starting material is completely deprotonated to form the phenolate anion (TO − ) [ 74 ]. Due to its negative charge, the phenolate is much easier to oxidize than the phenol by approximately 1.5 V, to form the phenoxyl radical by heterogeneous one-electron transfer as shown in the blue pathway in Scheme 4 .…”

“…Phenolate anions are most easily prepared by reacting the phenolic starting materials with strong base [ 69 , 74 , 76 ]. The phenolates of the tocopherols can exist in solution for long periods but due to their very low oxidation potentials (~−0.9 V vs. Fc/Fc + ) [ 74 ] undergo spontaneous reaction with O 2 [ 114 ], therefore, need to be stored in in oxygen free environment.…”

“…The author gratefully acknowledges the many academic collaborators, post-doctoral fellows, graduate, and undergraduate students who have worked on aspects of this study and named in references [ 49 , 50 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 74 , 75 , 76 , 80 , 81 , 84 , 85 , 86 , 87 , 88 , 115 , 128 ].…”