High Acid or Base Concentrations Near the Electrode

Locally High Acid or Base Concentrations Near the Electrode 

To raise this problem is a favourite remark from members of the audience at meetings on organic electrochemistry, and the answer is a never-failing I don t know, and I know of no way of finding out . However, since pH-control in the layer near the electrode appears to be crucial in, e.g., cathodic metal deposition, several methods have actually been developed for the direct measurement of pH near 

A reaction that has been discussed in terms of high acid concentrations is the nitration that takes place upon electrolysis of a suspended aromatic hydrocarbon in dilute (1-2 m) nitric acid (for summaries, see Allen, 1958 Fichter, 1942 Tomilov, 1961 Tomilov and Fiochin, 1963). The proposal has been that nitric acid would be formed in high concentration in the anode region, thus causing nitration in an ordinary electrophilic substitution reaction. This is an entirely reasonable idea, as it has since been shown (Cauquis and Serve, 1970) that nitrate ion is oxidized to dinitrogen pentoxide using nitromethane as a solvent (59). In an aqueous solution this 

1 Although less likely, a direct mechanism cannot be completely excluded. Volt-ammetric studies of anthracene in molten tetrabutylammonium nitrate at 150°C indicate that the initial electron transfer is from anthracene (Woodhall and Davies, 1969). It is difficult, however, to formulate a satisfactory mechanism for direct nuclear nitration, since aryl nitrates are known to exhibit different chemical behaviour (cf. Nyberg, 1971d). 

ECE mechanism (Parker, 1969a see p. 25). In this particular case, a series of benzylic cations were found to show anomalous reactivities, in that the less reactive ones were selective toward the weaker nucleophile, acetonitrile, contrary to predictions based on concepts from homogeneous solution chemistry.1 2 Moreover, in a certain region of water concentrations the competition ratio between the 

---