Pairing and Double-Layer Effects

In the electroreduction of aromatic hydrocarbons, nitro compounds, and quinones in aptotic solvents, the first step is the transfer of an electron from the electrode to form a radical anion. Once the radical anion is formed, electron repulsion will decrease the facility with which a second electron transfer occurs. But solvation and ion pairing diminish the effect of electron repulsion and tend to shift the reduction potential for the addition of the second electron to more 

There is strong evidence67 that at least two types of ion pairs exist. All cations in solution tend to be surrounded by solvent molecules. A small ion generates in its vicinity a more powerful electrostatic field than a large ion of the same charge thus smaller ions tend to produce more rigid solvation shells than large ions. 

In the case of anthracene, which forms large anions with delocalized charge, the ion-pairing effect is related to the size of the solvated cation the interaction increases as the solvated cation becomes smaller. 

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