Electron spin resonance studies. Part XXIII. The one-electron reduction of some α-halogenonitroalkanes and some subsequent reactions

Abstract

E.s.r. spectra are reported for the radicals observed during the reduction of some α-chloronitroalkanes, R¹R²CCl·NO₂, by radicals derived from alcohols or ethers by abstraction of an α-hydrogen atom, by the radicals, ·CO₂H and ·CO₂^–, and by titanium(III) ions complexed with D-tartaric acid. Depending on the conditions (pH, reducing agent, and nature of R¹ and R²), the following radicals can be detected: the α-chloroalkylalkoxynitroxide R¹R²CCl·N-(O·)OR³(R³. = reducing agent); the radical ·CR¹R²·NO₂; and the radical anion R¹R²C(NO₂)·CR¹R²(NO₂)[graphic omitted]. Evidence is adduced from these and other observations for the following steps in the reduction: formation of the radical anion, R¹R²CCl·NO₂[graphic omitted], which, if not rapidly trapped by a carbonium ion, loses chloride ion to give the radical ·CR¹R²·NO₂; the reduction of this radical to the aci-anion R¹R²C:NO₂^– or its conjugate acid R¹R²C:NO₂H; reaction of the aci-anion with the radical ·CR¹R²NO₂; and reaction of the acid R¹R²C:NO₂H with other radicals to give protonated nitroalkane radical anions. The factors which determine the relative importance of each step are discussed. The one-electron reduction of some α-bromonitroalkanes follows a generally similar course.

The assignment of some of the spectra to protonated nitroalkane radical anions was substantiated by the result of an experiment with deuteriated water; there is a significant splitting from the hydroxy-proton in these radicals. The chlorine-containing radicals have unusually large a_Cl values, a possible reason for which is outlined.