Polarography and Electron Paramagnetic Resonance of Nitrobenzene Derivatives

Abstract

Electron paramagnetic resonance (EPR) spectra of electrolytically generated nitrobenzen derivative anion radicals and the polarographic behavior of nitrobenzene derivatives are reviewed. Complete reduction by the electrolysis outside the cavity is desirable, because many of the radicals undergo rapid electron exchange reactions with their precursors that broaden the lines in the spectrum. Intra muros technique with the mercury pool cathode within the microwave cavity has the advantages of simplicity and ease of operation. Tetraalkyammonium salt must be employed for the supporting electrolyte which are less likely to form tight complexes with the anion radicals than alkali ions. Dissolved oxygen must be removed completely, because it plays a remarkable negative role in diminishing the proton hyperfine lines by the magnetic dipole-dipole interaction. The polarographic half wave potentials of nitrobenzene derivatives are correlated to the nitrogen coupling constants of nitrobenzene derivative anion radicals. Twisting of nitro group to the ring plane reduces the conjugation of the nitro group with the ring. Consequently, coupling constant at the nitro group increases with the decrease of the hydrogen coupling constant at the ortho, meta and para position of the benzene ring. Fraenkel proposes localized solvent complexes with the functional group of the radical ion to explain the solvent effect which nitrogen coupling constant increases markedly and hydrogen coupling constants are practically unchange. The strange phenomena of line broadening and anisotropy in EPR spectra of nitrobenzene derivative anion radicals are found in the presence of phenol derivatives or lithium ion. Very strong exchange interaction between electrolytically generated benzonitrile negative ion and the parent benzonitrile is described. The electron interchange reactions between highly reactive radicals (for instance, nitrobenzene negative ion) and normal ground state substance (for instance, paranitrobenzenitrile) are discussed. Several examples of unusual electrode reaction are described, such as the formation of unsubstituted nitrobenzene anion radical from the electrolysis of iodonitrobenzene or ortho-bromonitrobenzene.