Anodic Reactions of Simple Phenolic Compounds

Abstract

A study of the anodic reactions of phenols was made using the techniques of voltammetry, coulometry, and controlled potential electrolysis. It was shown that two entirely different anodic reactions are possible with phenolic compounds. The first is an electrophilic attack on the aromatic nucleus of the nonionized phenol with the irreversible removal of two electrons to give a mesomeric phenoxonium ion. As an example of this process an electrolysis of 2,6-di-tert-butyl-p-cresol in acetonitrile at a platinum anode resulted in the addition of methanol to form 2,6-di-tert-butyl-4-methyl-4-methoxy-cyclohexadienone in 65% yield. The second possible electrode reaction is the reversible removal of one electron from the phenoxide anion to give a phenoxy free radical. This process was illustrated by the electrolysis of the vanillinate anion in acetonitrile at a platinum anode to give dehydrodivanillin in 65% yield. It was found that half-wave potentials for both electrode reactions could be correlated by Hammett-type equations. Brief voltammetric experiments in 50% aqueous isopropanol on a carbon electrode indicated that both reactions could also occur in this system, depending on the pH and on the pKD of the phenol being investigated.Because of the importance of the phenolic hydroxyl group in the field of wood chemistry, an investigation of the anodic reactions of phenols was made using the techniques of voltammetry, coulometry, and controlled potential electrolysis. The literature on the voltammetry of phenols was reviewed recently by Suatoni et al. (1). Although a number of workers have made investigations of the anodic voltammetry of phenolic compounds, only a few papers have attempted to explain the mechanism of the primary electrode reaction, and these revealed two conflicting theories.The first theory, advanced by Hedenberg and Freiser (2), and supported by Ginzberg (3), is that the primary reaction involves the loss of one electron to form a phenoxy free radical. The second theory, proposed by Gaylor et al. (4,5) is that the primary reaction involves the loss of two electrons to give an unspecified intermediate. The latter authors based their admittedly tentative conclusions on the assumption that the reaction of hydroquinone on a graphite electrode in buffered 50% aqueous isopropanol was a two-electron process. However, more recent studies have indicated a strong possibility that, under these conditions, hydroquinone undergoes a one-electron transfer (6,7).If this is the case, the results of Gaylor et al. should be reinterpreted as indicating a one-electron primary electrode reaction in agreement with the results of Hedenberg and Freiser. Nevertheless, the data of Nash, Skauen, and Purdy (8) reveal variations between different phenolic compounds which would seem to preclude the possibility of a single simple mechanism. ExperimentalApparatus.--The polarograph, voltammetric cell, apparatus for constant potential electrolysis, and their use were described in detail in a previous paper (9). Platinum electrodes used in...

Electrocatalytic Properties of Electropolymerized Ni(II)curcumin Complex

“…Consequently, a poorly defined film, with continuous structure is formed on the electrode surface. An analogous reaction path was previously proposed for oxidative polymerization of vanillin porphyrins [13], vanillin [14], guaiacol [15], epidotoxin [16] and eugenol [17].…”

“…Moreover, the procedure for their preparation is usually simple and the properties of the resulting coating can be controlled carefully. Electropolymerization procedure was used successfully for modifying electrode surfaces with Ni(II) cyclam and other similar Ni(II) tetraazamacrocyclic complexes [4] or Ni(II) tetramethyldibezotetraaza (14)anulene [5], and the resulting films showed high catalytic activity toward electrooxidation of alcohols and/or other À OH and À NH 2 containing compounds, respectively.…”

Highly Selective Electrosynthesis of Biphenols on Graphite Electrodes in Fluorinated Media

“…The electrooxidation of vanillin and eugenol on platinum in basic media into the corresponding natural occurring biphenols has been reported. [13,14] Owing to the high costs of platinum, carbon electrode materials are definitely more attractive for technical applications. Unfortunately, the conversion of simple methylated phenols like para-cresol or 2,4-dimethylphenol (1) lead to the formation of the corresponding Pummerers ketone derivative 3 and not to the biphenol.…”

Electrochemical polymerisation of phenol in aqueous solution on a Ta/PbO2 anode

“…Vermillion and Pearl [26] have proposed that the first stage of the anodic oxidation of phenol on PbO 2 occurs according to two different mechanisms depending on the pH. They have suggested that, at pH values below the pK a (9.89), the bielectronic discharge of phenol molecules leads to the formation of phenoxonium cations:…”