Radical mechanisms nitrobenzene reduction

It was found that the action of OH radicals on nitrobenzene or benzoic acid produced all the three isomeric phenolic compounds. OH radicals produced, for example, by reaction (1) can be used to generate other free radicals or atoms from inorganic or organic compounds. These reactions have been studied more recently by Kolthoff and Medalia (13), Merz and Waters (46), Stein and Weiss (45), and others. Reaction (1) also proved useful in the study of the so-called active oxalic acid (47). The free radical mechanism of hydrogen peroxide has been discussed also in connection with the mechanism of the action of the enzymes catalase and peroxydase, the prosthetic groups of which are iron porphyrin complexes which presumably also undergo oxido-reduction processes in the course of their catalytic activity (48). 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

Ito T, Shinohara H, Hatta H, Nishimoto S-l (1999) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers product and laser flash photolysis studies on the oxidative splitting mechanism. J Phys Chem A 103 8413-8420 ItoT, Shinohara H, Hatta H, Fujita S-l, Nishimoto S-l (2000) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers. 2. Conformational effects on the reductive splitting mechanism. J Phys Chem A 104 2886-2893 ItoT, Shinohara H, Hatta H, Nishimoto S-l (2002) Stereoisomeric C5-C5 -linked dehydrothymine dimers produced by radiolytic one-electron reduction of thymine derivatives in anoxic solution structural characteristics in reference to cyclobutane photodimers. J Org Chem 64 5100-5108 Jagannadham V, Steenken S (1984) One-electron reduction of nitrobenzenes by a-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction. J Am Chem Soc 106 6542-6551... 

On continuous cycling of the potential through the two reduction processes, the magnitude of the current associated with the first reduction process gradually drops whereas that due to the second process increases. This behaviour is expected for an ECE process in which the p-iodonitrobenzene is reduced irreversibly to the nitrobenzene radical anion. Further, the addition of iodide anions to the solution decreases the rate of formation of nitrobenzene as evidenced by noting that the rate at which the peak current associated with the p-iodonitrobenzene reduction decreases less rapidly on successive scans through both reduction processes. The dependence on the iodide concentration in bulk solution suggests that the C step of the ECE mechanism actually consisted of processes (51) and (52),... 

Hydridic cluster anions are also important in the homogeneous WGSR cata-lysis.t The mechanism of WGSR reactions occurring on Ru3(CO)i2 with an acid cocatalyst (CF3COOH) has been fully elucidated.The radical anion [Fe3(CO)ii], obtained in the phase-transfer-catalyzed reaction of Fe3(CO)i2 with OH , catalyzes the reduction of nitrobenzene to aniline. 

The reduction of 1-bromo-4-nitrobenzene (Lviii) in [C4mpyr] [N(Tf)2] proceeds via a DlSPl type mechanism, were Lviii is reduced by one electron to form a radical anion (Eq. 15.54) [50]. This step is followed by a debromination step, which also involves the fast scavenge of a proton from the solvent system to yield nitrobenzene (Eq. 15.55). Due to the aprotic nature of this ionic liquid, the protonation step should involve water (represented as AH in Eq. 15.55) as the proton donor molecules, which may be present in the IL system as impurities. Einally, the product of this reaction is then reduced further to its radical anion through a disproportionation reaction (Eq. 15.56). 

The reduction of nitrobenzene to aniline is a typical oxidation-reduction reaction in which tin metal, Sn°, is oxidized to stannic ion, Sn, in the form of stannic chloride, SnCl4 hydrochloric acid serves as the source of protons. A plausible mechanism of this reaction is outlined in Scheme 21.3. Generally, the reduction occurs by a sequence of steps in which an electron is first transferred from a tin atom to the organic substrate to give an intermediate radical ion that is then protonated. The oxygen atoms on the nitro group are eventually removed as water molecules. It is left as an exercise to write a balanced equation for the overall reaction and to provide a mechanism for the reduction of N-phenylhydroxylamine (11) into aniline (see Exercises 13 and 14 at the end of this section). 

---