Hydroxylamine accumulation

As remarked above, hydroxylamines are problematic, because of their potential strongly exothermic decomposition [26]. They are also known to be highly carcinogenic and are therefore hazardous after interrupted or incomplete hydrogenation [27]. Hydroxylamine accumulation can also lead to low product quality because reaction with the nitroso compound gives colored azo or azoxy condensation products (as mentioned above). The maximum concentration of hydroxylamines can vary and is notoriously difficult to predict product quality can, therefore, differ from batch to batch. The suppression of hydroxylamine accumulation is therefore a topic of industrial importance. In a recent publication we reported that the addition of vanadate and other metal salts can drastically reduce accumulation of hydroxylamines in the hydrogenation of nitroarenes catalyzed by commercial Pt and Pd catalysts [20]. 

The addition of promoters, especially vanadium salts, had a dramatic effect on the course of the hydrogenation of several nitroarenes. Occasionally the maximum level of hydroxylamine accumulation was reduced from > 40 % to < 1 % with NH4VO3 or V/Pd/C. Salts of Fe, Co, Cu, and T1 also reduced the hydroxylamine level but were less effective. Other metals salts had no effect. 

Imines are easily reduced and rarely accumulate (62,83). Hydroxylamines are reduced relatively slowly and can be obtained in good yield platinum in acidic media appears to be the preferred system (6,27,54,58,65). Best yields are obtained from oximes of aliphatic carbonyls aromatic oximes give amines. 

Aromatic nitro compounds are hydrogenated very easily aliphatic nitro compounds considerably more slowly. Hydrogenations have been carried out successfully under a wide range of conditions including vapor phase (S9). Usually the goal of reduction is the amine, but at times the reduction is arrested at the intermediate hydroxylamine or oxime stage nitroso compounds never accumulate, although their transient presence may appreciably influence the course of reaction. In practice, nitro compounds often contain other reducible functions that are to be either maintained or reduced as well. 

Hydroxylamines ordinarily do not accumulate in the reduction of aromatic nitro compounds for, with some exceptions, most systems in competition will reduce the hydroxylamine function preferentially. Nonetheless, systems have been found that afford the intermediate aromatic hydroxylamine in excellent yield. With hydrogen gas as a reductant and platinum-on-carbon or -on-alumina and about I wt % of DMSO based on nitro compound as a modifier, aromatic hydroxylamines can be formed in 90% yield under mild conditions. The reduction slows markedly after absorption of the second mole of hydrogen and should be stopped at this stage (80). 

It is now well established that hydroxylamine is an intermediate, the oxygen atom being derived from dioxygen (equation 93).1539 The formation of hydroxylamine and its subsequent oxidation to nitrite are affected differently by various inhibitors, while certain inhibitors cause an accumulation of hydroxylamine. This suggests that the two processes are independent. The presence of dioxygen is essential for hydroxylamine to be oxidized to nitrite, otherwise N20 and NO are formed. 

In principle, the nitroso, hydroxylamine, and/or amine products of nitro reduction might undergo coupling to form azoxy, azo, and/or hydrazo dimers, but no evidence for these products has been found under the conditions that have been studied to date. One reason that these dimers do not accumulate may be that they are rapidly reduced by Fe°. In fact, Fe° reduces azo groups to amines [Eq. (12)] very rapidly [111-113], and this reaction may prove to be useful in the remediation of wastewaters contaminated with azo dyes. 

Schmidt, I., Look, C., Bock, E., andjetten, M. S. M. (2004a). Ammonium and hydroxylamine uptake and accumulation in Nitrosomonas. Microbiology-Sigm 150, 1405—1412. 

Aromatic hydroxylamines are frequently produced during the hydrogenation of aryl nitro groups. Generally, these are undesired intermediates because when they are present in excessive amounts, a potentially explosive situation caused by the exothermic disproportionation of the hydroxylamine can result. 2 This is usually not a problem, but care should be exercised to prevent the accumulation of large amounts of the hydroxylamine, particularly when the... 

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