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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...

Conventional electrochemical methods have major limitations in measuring the kinetics of a heterogeneous electrochemical process such as localized corrosion. One reason for this is that traditional electrochemical kinetic methods are based on the fundamental Butler-Volmer equation 1,2 which only describes the kinetics of a uniform electrode process. Another reason is that traditional electrochemical techniques usually employ a one piece electrode which can only detect mixed or averaged electrochemical parameters over the whole electrode surface and cannot measure electrochemical parameters from a chosen location of the electrode surface. Although techniques such as the scanning reference electrode method and the scanning vibrating electrode method have been developed to detect localized electrochemical parameters in the solution phase, they do not delineate clearly the areas of cathodes and anodes 3 and they do not measure the kinetics of heterogeneous electrochemical processes.In order to measure local electrochemical parameters and to determine the kinetics of a heterogeneous electrochemical process, a wire beam electrode (WBE), 4-11 has been developed. Preliminary applications were in the study of the electrochemical heterogeneity of organic coatings 5,7,8,10 and localized corrosion processes. 4,6,[9][10][11] The WBE is normally fabricated from a bundle of electrically insulated drawn metal wires embedded in insulating material. However it could be prepared by techniques such as machining, casting, laser cutting, electric arc cutting, or electrodeposition. The surface of the WBE usually simulates the surface shape of a practical workpiece under study. Various previous designs and applications of the WBE can be found in Ref. 4-11. The WBE constitutes a unique integrated multielectrode system which effectively simulates a conventional one piece electrode surface. 4,6,10,11 Similar corrosion patterns were experimentally observed over a WBE and a large area one piece electrode surface when both electrodes were exposed to an identical environment. 4,10 The WBE differs from other sectioned electrodes such as the micromosaic platinum electrode, 12,13 the microcomposite glassy carbon electrode, 14,15 the platinum microelectrode array, 16 and other variously designed multielectrode systems 17-21 since these electrode systems, to our knowledge, were not integrated to simulate a heterogeneous electrode surface and were not designed to study heterogeneous electrochemical processes.The WBE is suitable for studying heterogeneous electrochemical processes. First, each wire in a WBE is an individual addressable electrochemical sensor which can measure local electrochemical parameters. Second, the surface area of each wire in an WBE is much smaller than the total working electrode area, thus electrochemical processes on each corroding wire surface can be assumed to be uniform even if the whole electrode surface is electrochemically nonuniform, an approach which is an analogy to calculus principles. 4 This assumption allows ele...

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A number of esters and amides of the anti-HIV nucleotide analogue 9-[2-(phosphonomethoxy)-ethoxy]adenine (1) have been synthesized as potential prodrugs and evaluated for oral bioavailability in mice. Dialkyl esters 17-20 were prepared via a Mitsunobu coupling of alcohols 8-11 with 9-hydroxypurine 12 whereas (acyloxy)alkyl esters 25-33 and bis-[(alkoxycarbonyl)methyl] and bis(amidomethyl) esters 34-39 were obtained by reaction of 1 with a suitable alkylating agent. Phosphonodichloridate chemistry was employed for the preparation of dialkyl and diaryl esters 42-65, and bis(phosphonoamidates) 66 and 67. Following oral administration to mice, most of the dialkyl esters 17-20 were well-absorbed and then converted to the corresponding monoesters, but minimal further metabolism to 1 occurred. Bis[(pivaloyloxy)methyl] ester 25 displayed an oral bioavailability of 30% that was 15-fold higher than the bioavailability observed after dosing of 1. Methyl substitution at the alpha carbon of the bis[(pivaloyloxy)methyl] ester 25 (33) increased the oral bioavailability of 1 to 74%. Some of the diaryl esters also showed improved absorption properties in comparison with that of 1. In particular, the crystalline hydrochloride salt of diphenyl ester 55 was well-absorbed and efficiently converted to the parent compound with an oral bioavailability of 50%. On the basis of these results as well as the physicochemical properties of the prodrugs and their stability in mouse duodenal contents, the hydrochloride salt of diphenyl ester 55 was identified as the preferred prodrug of 1.

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