Nitroaromatics hydrogenation

The hydrogenation of nitroacetophenones has been studied and considerable kinetic and mechanistic information obtained. Differences in reaction rate, bonding and selectivity have been observed. The formation of 1-indolinone from 2-NAP was unexpected and revealed the presence of a surface nitrene. This intermediate has not been postulated in nitroaromatic hydrogenation previously. Hydrogenation in the presence of deuterium revealed, as well as a kinetic isotope effect, that it is likely that... 

The major commercial nitroaromatic hydrogenation by catalysis is the hydrogenation of nitrobenzene to aniline. A Cu catalyst is generally employed since its low activity controls the vigorous exothermic reaction and prevents the formation of cyclohexylamine. The hydrogenation of... 

Monolith multiphase chemical reactors are another example of microfluidic multiphase flow applications. The slug flow pattern enhances the mass transfer in the liquid-solid process. There is also low-pressure drop for a given specific contact area. Machado et al (1999) have patented the use of monolith reactors for fast and highly exothermic nitroaromatic hydrogenation. In this process, the product is recycled through the reactors several hundreds of times, and the low-pressure-drop monolith reactor is therefore preferred. 

Substituted aromatics, eg, aLkylbenzenes, sometimes experience attack at the substituent position by NO/ (7). A cyclohexadienyl cation is formed it is unstable and the nitro group migrates on the ring to a carbon atom that is attached to a hydrogen. Loss of the proton results in a stable nitroaromatic. 

Most derivatives of aniline are not obtained from aniline itself, but ate prepared by hydrogenation of their nitroaromatic precursors. The exceptions, for example, /V-a1ky1ani1ines, /V-ary1ani1ines, sulfonated anilines, or the A/-acyl derivatives, can be prepared from aniline and have been discussed. Nitroanilines are usually prepared by ammonolysis of the corresponding chloronitroben2ene. Special isolation methods may be requited for some derivatives if the boiling points are close and separation by distillation is not feasible. Table 6 Hsts some of the derivatives of aniline that are produced commercially. 

Aromatic nitroso compounds usually are considered to be intermediates in the hydrogenation of a nitroaromatic compound to the aromatic hydroxyl-amine or amine. However, nitroso compounds do not accumulate in these reductions, suggesting that they are reduced more easily than are nitro compounds. Catalysts effective for the nitro group should also be effective for nitroso. 

THF and methanol employed as organic modifiers of mobile phase provided a considerable difference in selectivity based on the polar interactions between solutes and the organic solvent molecules in the stationary phase. Acidic compounds, phenols and nitroaromatics, were preferentially retained in the THF-based mobile phase, whereas esters and ketones were preferentially retained in the methanol (a hydrogen-bond donor) containing mobile phase. The system presented here seems to be very practical because any laboratory possessing two sets of HPLC equipment and two C j g columns can attempt similar 2D HPLC by simply changing the mobile phase for the two dimensions. 

Various other reducing methods are employed for the conversion of (3-nitro alcohols to amino alcohols, namely, electrochemical reduction.107 The selective electrohydrogenation of ni-troaliphatic and nitroaromatic groups in molecules containing other groups that are easy to hydrogenate (triple bond, nitrile, C-I) are carried out in methanol-water solutions at Devarda copper and Raney cobalt electrodes (Eq. 6.55).107... 

Group I comprises nitroaromatics with lowest (n, tt )- [or (7t,n )- with considerable (n, 7T )-contribution] excited states, the triplets of which behave electro-philiccilly and tend to abstract hydrogen atoms or electrons and thus typically undergo photoreduction. Compounds which fit this correlation were Nitrobenzene 2-nitro-, 4-nitro- and 4,4 -dinitrobiphenyl, 2-nitrofluorene, 3-nitroaceto-phenone, 1,8- and 1,5-dinitronaphthalene. 

A different mode of reaction, however, is observed in photoreductions of nitroaromatics by aromatic tertiary amines. Irradiation of benzene solutions of N-methylated anilines and either m-chloronitrobenzene or 1-nitronaphthalene results in oxidative demethylation of the amines accompanied with reduction of the nitro compound to the corresponding arylamine 49). The authors suggest that hydrogen abstraction from the methyl group takes place as the primary chemical event. 

In view of the short lifetime ( 10 s), which has been reported for nitrobenzene and might be expected for similar nitroaromatics with low lying (n, 7T ) states, and since intramolecular hydrogen abstractions from benzylic positions take place, lack of oxygen quenching may be inconclusive. 

The required A-arylhydroxylamines may also be generated in situ by treatment of the corresponding nitrobenzenes with mild reductive metals in liquid hydrogen fluoride323 (former reports324-325). Examples of the reduction of nitroaromatic compounds 12 to fluoroaromatic amines 11 arc given below.323... 

Illustrative Example 14.5 Estimating Rates of Reduction of Nitroaromatic Compounds by DOM Components in the Presence of Hydrogen Sulfide... 

Beltran, F.J., Encinar, J.M., and Alonso, M.A., Nitroaromatic hydrocarbon ozonation in water. 2. Combined ozonation with hydrogen peroxide or UV radiation, Indust. Eng. Chem. Res., 37, 32-40, 1998. 

Catalytic reduction of aromatic nitro compounds to the amines is highly exothermic (AH = —548 12 kJ/mol) and has high potential for hazard in the event of cooling- or other process-failure. The total reaction proceeds via nitroso and hydroxylamino intermediates, both of which are reactive and may undergo undesired condensation or disproportionation reactions, and the thermochemistry of all these possibilities was investigated. The reduction or disproportionation of the hydroxylamino intermediate (which is of low thermal stability) is identified as the fastest and most exothermic step (despite which it can frequently be concentrated or trapped) implications for process safety are considered in detail and verified by experiment with typical compounds and intermediates [1]. A calorimetric study of the hazards inherent in hydrogenation of nitroaromatics was made, using nitrobenzene as model compound [2]. Individual incidents of this type are ... 

It seems likely that aromatic amines which are found in liquefaction products have been produced by a combination of thermolysis and hydrogenation. There is no evidence for aromatic amines in coals from either selective oxidation degradations (22) or from direct X-ray Photoelectron Spectroscopy measurements (23). Oxidations would produce very stable nitroaromatics which are not seen. Another possible structure for this formula is phenoxazine(Vb). Such a molecule would not survive high temperature combined with long reaction times. Although annelated thiophene with a pyrrole(VI) would appear to be a likely structure in coal, there is no evidence for its existence in any of the coal derived materials. 

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