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 aCl values, a possible reason for which is outlined.