Oxidation of Nucleophilic Substrates and Lewis Bases

Br0nsted Acids. Carboxylic acids, phenols, and alcohols are electrochemi-cally reduced by means of their Br0nsted acidity at a reduction potential that is a direct measure of their acidity (p/fj in a given solvent (see Chapter 8)  

R = Me, PhCH2, CH2=CH, HOCH2CH2, CH3CH=CH] are unique in their ability to undergo two reversible one-electron reductions in almost any medium (Table 12.3).15 This property makes the viologens especially useful mediators between one-electron and two-electron processes. This property also is the basis for the herbicidal activity of methyl viologen (paraquat), which disrupts the electron-transport chain in green-plant photosynthesis. 

All molecules with nonbonding electron pairs (e.g., H20, / OH, ROR, / NH2, / SH, RSR) are, by definition, Lewis bases with a degree of nucleophilicity. Their electrochemical oxidation potential is a measure of (1) the ease of removal for one of the electron pair of electrons and (2) relative nucleophilicity (the less positive the potential, the more nucleophilic). Aromatic molecules with Lewis base substituents are easier to oxidize than the aliphatic forms of the substituents (e.g., PhOMe, +1.75 V vs. SCE MeOH, +2.5 V vs. SCE) because the aromatic ring provides a means for delocalizing the positive chaige and electron spin that would result from electron removal (in the case of PhOMe, there are five additional hydrogen atoms to share the positive chaige and six 

Because aliphatic alcohols can be viewed as organic water (but with a greater basicity and a weaker O—H bond), they are almost as difficult to oxidize  

Other aliphatic bases (amines and thiols) are oxidized by similar pathways their electrochemistry is discussed to a limited extent in Chapter 11. 

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Catechols and Hydroquinones. Just as quinones are ideal examples of electrophilic substrates, their fully reduced form (catechols and hydroquinones) illustrates the electrochemical oxidation of aromatic nucleophilic substrates (Lewis bases). Figure 12.3a, b illustrates the oxidation of 3,5-di+m-butyl-catechol (DTBCH2) via an irreversible two-electron process (ECEC) to give the o-quinone (DTBQ) 12... 

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