Solvent Effects on Electron-Transfer Equilibria

Examples of solvent effects on electron-transfer equilibria between organic species i.e. redox and disproportionation reactions) are rather scarce. 

A nice example is the disproportionation reaction of the l-ethyl-4-(meth-oxycarbonyl)pyridinyl radical (43), which leads to the ion pair (43a)l(43b) [215, 216]. 

As expected, an increase in solvent polarity shifts this equilibrium from the left to the right-hand side due to the better solvation of the ion pair as compared to the neutral radical [215]. 

From equilibrium constants for transfer from water to vapour, determined by dynamic vapour pressure measurements, the hydrophilic character of 1,4-benzoquinone and 1,4-hydroquinone has been estimated [227]. 1,4-Benzoquinone is about 3.2 orders of magnitude less strongly solvated by water than is 1,4-hydroquinone. Because 1,4-hydroquinone is so much more strongly solvated than 1,4-benzoquinone, its reducing 

The redox properties of tetraphenylporphyrin have been studied in dimethyl sulfoxide (DMSO) and more polar water/DMSO mixtures. Increasing solvent polarity [i.e. inereasing water content) shifts the reduction potentials to more positive values and makes the reduetion step more favourable, since a better solvated anion is formed from a neutral speeies [291]. 

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