The construction and use of potential–pH diagrams in organic oxidation–reduction reactions

Abstract

Potential-pH diagrams were constructed from literature data for phenol, 2,4,6-tri-t-butylphenol, and hydroquinone (1,4-di hydroxybenzene). The relationship between chemical and electrochemical phenomena is described, and the significance of the results of chemical experiments discussed in relation to the potential-pH diagrams for the case of phenol oxidations.Potential (E)-pH diagrams, sometimes called Pourbaix diagrams, are convenient ways of summarizing equilibrium information about reactions, particularly oxidation-reduction reactions, which take place in solution. The construction of an E-pH diagram has been described by Pourbaix.' Once constructed, it defines the range of potential and pH over which any species of interest is thermodynamically stable. E-pH diagrams can thus be used to predict whether or not two chemical species will react but of course, give no information whatsoever about the rate of reaction. If each of the two species occupy the same area on an E-pH diagram, they will coexist. On the other hand, if they occupy different areas on the E-pH diagram, they are likely to react. Using this principle, Pourbaix has determined the conditions under which all common metals are likely to corrode in water, It is worth pointing out that the electrochemical potential E is directly related to the free energy AG of the system by AG = -nFE where n is the number of electrons involved in the redox reaction and F is the Faraday constant. It follows, then, that an E-pH diagram can be simply transformed into a plot of the free energy of the system as a function of pH. It is not, however, just a matter of scaling, since allowance must be made for the number of electrons n involved in the various electrode reactions.Considering the convenience of Pourbaix diagrams, it is surprising that relatively little use has been made of them in the field of organic chemistry. One of the few examples in the literature is that due to Waters who used a simple two line E-pH diagram to determine the range of pH within which the oxidation of phenol would proceed via a one-electron step as opposed to a two-electron step. Although Waters' paper has been described as important,3g4 little use has been made of E-pH diagrams and few have been published, although in some cases the basic data for their construction exists in the literature. The construction of an atlas of E-pH diagrams comparable with that devised by Pourbaix for the metals would seem desirable.In this paper, we will briefly describe how a potential-pH diagram is constructed, and discuss the sort of approximations that are commonly made during its construction. A simple three-line diagram for phenol will be presented, and the generalization of this diagram to other substituted phenols described. The influence of the solvent on these diagrams will be briefly examined. Finally, we will describe the construction of two more complex E-pH diagrams for the compounds 2,4,6-tri-t-butylphenol in aqueous ethanol (57% w/w) as solvent and 1,4-dihydroxybenzene in water as solvent.Examp...

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Mechanistic Study on the Electrochemical Reduction of 9,10-Anthraquinone in the Presence of Hydrogen-bond and Proton Donating Additives

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