Electrochemical reactions of organic compounds in liquid ammonia. II. Nitrobenzene and nitrosobenzene

The electrochemical behavior of nitrobenzene and nitrosobenzene in anhydrous liquid ammonia was investigated by cyclic voltammetry and controlled potential coulometry. In the absence of added protonating agents, nitrosobenzene and nitrobenzene are both reversibly reduced in two one-electron transfer steps to yield the stable radical anion and stable dianion species. In the presence of the weak acid isopropyl alcohol, the dianion of nitrosobenzene adds a single proton to form the anionic species, which can be reversibly oxidized back to parent compound. The dianion of nitrobenzene also adds a single proton and then rapidly decomposes with the loss of hydroxyl ion to neutral nitrosobenzene, which undergoes further reduction and protonation. The overall reduction process consists of the addition of a single electron to yield a stable radical anion followed by addition of three electrons and two protons to yield the protonated dianion of nitrosobenzene. I n the presence of strong acid (ammonium ion), nitrosobenzene is reduced in a single two-electron reduction process to yield phenylhydroxylamine. Nitrobenzene is reduced in two steps, involving the addition of one and three electrons, to yield the same final product, phenylhydroxylamine. Estimates of the equilibrium or rate constants for several of these reactions associated with the electrode reactions are given. The mechanism of the electrochemical reduction of nitrobenzene to phenylhydroxylamine and aniline has received considerable attention over the past 25 years.'-I4 Most of this work involved the use of aqueous solutions containing alcohol or an ether to aid in the dissolution of the relatively insoluble organic compound. Attempts a t elucidating the reduction mechanism were made by correlating changes in electrochemical behavior of the system with changes in pH, which was adjusted through the use of various buffer systems. Some studies have also been undertaken in nonaqueous solvent systems with addition of proton sources of varying proton-donating strength. In aqueous solution, nitrobenzene is reduced to phenylhydroxylamine in a single fourelectron reduction step a t all pH values. At pH values less than 4.7, nitrobenzene is assumed to be pre-protonated giving the species C ~ H S N O ~ H ~ ~ + , while analysis of the polarographic wave shows that the rate-determining step involves the addition of two electrons and a single p r o t ~ n . ~ . ~ The mechanism of reduction is given as: C6H5N02H22* + 2e+ H' C,H,NOH' + H?O (slow) C,H,NOH+ + 2e+ 2H' C,H,NH,OH+ (fast) The reduction product of the rate-determining step (C6HsNOH+) is reducible a t a less negative potential than the starting compound (CsHsNO*H2*+), which explains why this intermediate has never been detected during the course of an experiment. A second wave a t a more negative potential occurs in acid solution corresponding to reduction of the protonated phenylhydroxylamine species CbHsNHzOH+, to yield aniline in a single two-electron transfer step: Smith. Bard / Nitrohenzrne and Nitrosohenzene