Polarographic reduction of p–substituted 1-phenyl-3-aminocarbonylpyridinium salts

Abstract

The substitution effect of different groups (H, NO2, COOH, Br, Cl, NHCOCH3, CH3, OCH3, OH, and N(C2H5)2) on the polarographic behaviour of p–substituted 1-phenyl-3-aminocarbonylpyridinium cations has been investigated, in particular on their half-wave potentials in aqueous phosphate buffers pH 6·65 (10% DMF) and in anhydrous solutions of dimethylformamide with 0·05 mol l-1 (n-C4H9)4N+BF4- as supporting electrolyte. The half-wave potentials of the reduction wave which corresponds to the uptake of a single electron (wave B) and to the formation of the primary radical, obey a Hammett correlation in a similar way as it is in the case of 1-benzyl-3-aminocarbonylpyridinium cations. The slope ρπ,R in the Hammett plot equals 0·093 V for 10% DMF and 0·179 V for anhydrous DMF and compares thus with this slope obtained with the 1-benzyl derivatives where 0·095 V was found for water and 0·127 V for anhydrous acetonitrile. The transfer of the substitution effect from the substituent in the para position on the benzene nucleus to the heterocyclic ring in thus equally active in both substances and depends more strongly on the solvent than on the structure of cations of both types. The low sensitivity in both series towards a change in the substituent is explained by the fact that during the uptake of the electron the benzene and the pyridine nucleus are not even approximately coplanar. This is why the π-overlap between the two nuclei is considerably restricted. The analysis of sampled dc-polarographic waves has confirmed that the one-electron uptake is followed by a chemical reaction, most probably a dimer formation or a reaction of the primary product with the starting substance.