Nitrobenzene hydroxylation

The oxidation of benzene to phenol and 1,4-dihydroxybenzene (Figure 2.11a) (Hyman et al. 1985), both side chain and ring oxidation of ethyl benzene, and ring-hydroxylation of halogenated benzenes and nitrobenzene (Keener and Arp 1994). 

Hence the 30 % that are lost could be on the support and slowly react with 1-hexanol to form HA. However when aniline is reacted there is no significant loss of material, which suggests that aniline cannot interact directly with the surface hydroxyls. This suggests that the interaction between aniline and the support hydroxyls is not as simple as shown above, rather it is more likely that the reaction operates via a spillover mechanism involving an intermediate in the nitrobenzene hydrogenation sequence rather than aniline. The alkylation reaction between aniline and 1-hexanol takes place on the metal function, therefore the reaction with the missing aniline associated with the support will be slow as it requires a reverse spillover and a diffusion across the support surface. 

For quite sparingly soluble substances, formic acid, pyridine, bromobenzene, nitrobenzene, and occasionally also phenol, ethyl benzoate, aniline, and dioxan are used. A distinct relation exists between the constitution of solute and solvent, and is expressed by the old rule similia similibus solvuntur. Thus, as is well known, substances containing hydroxyl (e.g. sugars, carboxylic acids) are soluble in water, whereas hydrocarbons are more soluble in benzene and petrol ether than, for example, in alcohols. 

It is interesting to note the effect of charge on hydroxyl loss from ortfto-substituted nitrobenzene ions. The loss of OH is observed from many orffto-substituted nitrobenzene molecular anions66. This ortho-ziiecl is often operative in apparent analogy with the corresponding cations. The results can be rationalized if an intramolecular proton transfer is... 

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. 

McLuckey, S. A. Glish, G.L. The Effect of Charge on Hydroxyl Loss From Ortho-Substituted Nitrobenzene Ions. Org. Mass Spectrom. 1987, 22, 224-228. 

Witte, F., Urbanik, E., and Zetzsch, C. Temperature dependence of the rate constants for the addition of hydroxyl radical to benzene and to some monosubstituted aromatics (aniline, bromobenzene, and nitrobenzene) and the unimolecular decay of the adducts. Part 2. Kinetics into a quasi-equilibrium, J. Phys. Chem., 90(14) 3251-3259, 1986. 

Nitrobenzenes with an ortho or a para hydroxyl or amino function form an exceptional group of compounds in which the nitro function can readily be reduced to amino in alkaline solution. Heterolytic cleavage of the nitrogen-oxygen bond in the phenylliydroxylamine intermediate is promoted by any 2- or 4-substituent which can donate a lone pair of electrons. Further reduction steps then lead to the overall... 

Photoexcited nitrobenzene may be used for benzylic hydroxylation (at C-9) of 17/3-acetoxy-3-methoxyoestra-l,3,5(10)-triene. The photochemistry of the 17/3-nitro-steroid (217) is markedly solvent dependent, the major products being in ether the 17-desnitro-compound (218), in propan-2-ol the hydroxylamine (219), and in EtOH-NaOEt the hydroxamic acid (220) and the cyclopropane (221). The hydroxamic acid (220) is probably formed through the oxaziridine (Scheme 7). Although there are analogies to this in the photochemistry of nitrones and oximes, the photoreduction of a nitroalkane in propan-2-ol to an alkyl-hydroxylamine appears to have no precedent. Further studies of photochemistry of conjugated... 

With nitrobenzene, reduction to an intermediate stage results in the formation of a complex which, as is also the case with the initial complexes formed with ferricyanide, benzoquinone, and benzaldehyde, cannot react in the manner shown in the first conclusion. These species require the presence of added alkali, which apparently effects the displacement of reduced substrate by hydroxyl anion to yield hydroxypentacyanocobaltate(III). All of the substrates mentioned have been found to undergo catalytic hydrogenation when added to the catalyst system in less than stoichiometric quantities in the presence of alkali. 

Starting from nitrobenzene, p-aminophenol can be generated at lead cathodes via the intermediate hydroxyl amine, which rearranges under the reaction conditions. This reaction is also unprecedented by other methods [19] ... 

For Hammett correlations for rate constants of substituted benzenes reacting with hydroxyl radicals under experimental conditions of pH 9, the elementary hydroxyl radical rate constants (l/m sec 108) containing toluene, dimethoxy-benzene, benzene, and nitrobenzene correlate with aresonance as follows ... 

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