The Effect of Substituents on the Half-Wave Potential of the Polarographic Reduction of Pyridine N-Oxide Derivatives

Polarograms of twenty-three pyridine N-oxide derivatives substituted at γ- and β-positions were extensively measured at various pH’s. The half-wave reduction potential, E1⁄2, was expressed by the form −E1⁄2=a[pH]+b. The wave height due to the reduction of the →O group decreased with an increase in the pH in a pH region over ∼6. This phenomenon was common to all the compounds studied here; it was thought to be useful in identifying an aromatic N-oxide group by the polarographic method. The E1⁄2 plot against Hammett’s σ value of the substituent satisfied a linear relation well, and the slope of Hammett’s ρ value was positive. This indicates that the reduction process is nucleophilic. The linear relation was explained qualitatively on the basis of the MO theory by applying the second-order perturbation method.