Лако-красочные материалы — производство

Electrochemical methods have been used extensively to elucidate the mechanism of reduction of tetrazolium salts. In aprotic media, the first step is a reversible one-electron reduction to the radical 154 as confirmed by ESR spectroscopy.256,266 As shown in Scheme 26, this radical can then dispro­portionate to the tetrazolium salt and the formazan anion (166) or take up another electron to the formazan dianion (167). The formation of the dianion through a direct reduction or through the intermediate tetrazolyl anion (168) has also been proposed.272 28 1,294 In aqueous solutions, where protonation/deprotonation equilibria contribute to the complexity of the reduction process, the reduction potentials are pH dependent and a one — electron wave is seldom observed.

The reduction potentials, measured by scanning voltammetry, are substitution dependent.282 As shown in Table 12 for a series of triaryl tetrazoliums (169), the reduction becomes easier when electron-withdrawing substituents are present. This is in agreement with the polarographic data on a series of benzothiazolyl tetrazolium salts (170).283 With bis tetrazolium

Table 12. Reduction Potentials
of Tetrazolium Salt (169)

X Y E (mV)a

‘Versus Ag/AgCI.

salts in nonaqueous aprotic solvents, a series of one-electron steps have been identified leading to both the “monoformazans” and the “diformazans” 164 and 165 in Scheme 25.285 In aqueous and micellar media, two two-electron steps and one four-electron step have also been observed.284 — 288