Hydroxylamines reduction reactions

The base was being prepared by distilling a mixture of hydroxylamine hydrochloride and sodium hydroxide in methanol under reduced pressure, and a violent explosion occurred towards the end of distillation [1], probably owing to an increase in pressure above 53 mbar. It explodes when heated under atmospheric pressure [2], Traces of hydroxylamine remaining after reaction with acetonitrile to form acetamide oxime caused an explosion during evaporation of solvent. Traces can be removed by treatment with diacetyl monoxime and ammoniacal nickel sulfate, forming nickel dimethylglyoxime [3], An account of an extremely violent explosion towards the end of vacuum distillation had been published previously [4], Anhydrous hydroxylamine is usually stored at 10°C to prevent internal oxidation-reduction reactions which occur at ambient temperature [5], See other REDOX REACTIONS... 

Perhaps the most important application of redox chemicals in the modern laboratory is in oxidation or reduction reactions that are required as part of a preparation scheme. Such preoxidation or prereduction is also frequently required for certain instrumental procedures for which a specific oxidation state is essential in order to measure whatever property is measured by the instrument. An example in this textbook can be found in Experiment 19 (the hydroxylamine hydrochloride keeps the iron in the +2 state). Also in wastewater treatment plants, it is important to measure dissolved oxygen (DO). In this procedure, Mn(OH)2 reacts with the oxygen in basic solution to form Mn(OH)3. When acidified and in the presence of KI, iodine is liberated and titrated. This method is called the Winkler method. 

IV. HYDROXYLAMINES THROUGH REDUCTION REACTIONS A. Hydroxylamines through Reduction of Nitre Compounds... 

Zinc in the presence of ammonium chloride reduces primary, secondary and tertiary aliphatic nitro compounds but yields of hydroxylamines are moderate and formation of coupling products is common. Zinc with or without ammonium chloride reduces aromatic nitro compounds (e.g. 75, equation 49) into hydroxylamines in moderate to good yield. However, it has been mentioned that the reaction is sensitive to the grade and quality of zinc dust (equation 50) and aromatic amines have been obtained as major products in zinc reduction reactions. ... 

Oximes undergo hydrogenation to hydroxylamines and/or amines depending on reaction conditions. Platinum oxide is the most frequently used catalyst for selective hydrogenation of oximes to hydroxylamines. Reduction of chiral oxime 96 over palladium catalyst (equation 66) proceeds in high yield and stereoselectivity. High stereoselectivity was observed in catalytic hydrogenation of a-alkoxyoximes . 

Hydroxylamine hydrochloride is used for controlled reduction reactions. It is used in organic synthesis as an antioxidant for fatty acids and in photographic developer solutions. 

The mesomeric quinonemethides and 0-quinonemethides described above are somewhat more stable than the simple p-quinonemethides whose properties are already well known even from classical studies. The o-quinonemethides XX and XVII do not add on water even in solution in aqueous organic solvents their solution in dioxane/water is stable for months. They do not add on methanol or higher alcohols and react only slowly with phenols and organic acids. The addition of water is not catalyzed by mild alkalies the red color of the phenoxide ion (XVIII) prevails for weeks in soda solution. Addition of water occurs more rapidly in strongly alkaline solution. The addition of mineral acids and reduction by sodium borohydride are instantaneous. The addition of HC1 is rapid even at pH 4.0, the conditions used for determining the carbonyl content of lignin by the hydroxylamine hydrochloride reaction 13). 

The preparation of hydroxylamine by reduction of nitrous acid with HSOj is an excellent example of a reduction reaction that proceeds in shifts of two electrons by formation and breakage of covalent bonds. The probable course of this reaction is ... 

Catalytic reactions in electrochemistry — When the product of an electrochemical reduction reaction is regenerated by a chemical reoxidation, or when the product of an electrochemical oxidation is regenerated by a re-reduction, the regeneration reaction is called a catalytic reaction. For thermodynamic reasons the chemical oxidant (or the reductant) has to be electro-chemically irreversible in the potential range where the catalyst is electroactive. The reduction of Ti(IV) in the presence of hydroxylamine is an example for an oxidative regeneration [i, ii] ... 

Surprisingly P450s are also involved in a considerable number of reduction reactions including alkyl halides, azo compounds, N-oxides, hydroxylamines, and nitro groups. Further discussion of these and the many other, more unusual reactions attributed to P450s is beyond the scope of this chapter, and the reader is referred to the excellent review by Guengerich [6]. 

The HPO (hydroxylamine phosphate oxime) process, developed byDSMand licensed by Siamicarbon, totally eliminates the production of ammonium sulfate, both in the production of hydroxylamine itself and in die oxime production stage. It consists of the catalytic hydrogenation of the nitrate ions resulting from the oxidation of ammonia to hydroxylamine ions in a solution buffered by phosphates. The nitrate reduction reaction is as follows ... 

Several studies have shown that the more crystalline the Fe(III) and Mn(IV) oxides, the slower the rate of their reduction (reviewed by Lovley, 1991, 2004). In laboratory experiments, Ottow and Klopotek (1969, cited by Lovley, 1987) reported that the reduction capacity of select iron minerals was found to be in the order of FeP04 > Fe(OH)j > FeOOH > Fe203. Amorphous and poorly crystalline forms of Fe(III) and Mn(IV) oxides dominate wetlands that undergo frequent wet and dry cycles. These systems are dynamic, and repeated oxidation-reduction reactions involving iron and manganese will not allow time for stable crystalline forms of Fe(III) and Mn(IV) oxides to form in these systems. Strong relationships are observed between Fe(III) reduction rates and poorly crystallined forms of Fe(III) oxides (as determined by hydroxylamine extraction) in fresh and brackish water sediments (Lovley and Philips, 1987). 

B. Reduction The enzymology of reduction is not as well as characterized as for oxidation but, for example, reductive reactions can be catalyzed by cytochrome P-450 and P-450 reductase and soluble enzymes such as DT-diaphorase (EC 1.6.99.2) Many compounds including azo-and nitro-compounds, epoxides, heterocycles and halogenated hydrocarbons Sources of reducing equivalents for the reactions include NAD PH and NADH. Chemical groups modified include nitro, nitroso, tertiary amine oxide, hydroxylamine, azo, quinone, nitroso, alkylhalide... 

Already in the last century Hantzsch (155) observed that reaction of a-halo acids (127) with excess hydroxylamine leads to oximes of a-keto acids (31) as the result of an oxidation-reduction process (Scheme 28). This process, as a method of synthesis of (31), was later thoroughly studied by Barry and Hartung (156). The oxidation-reduction reaction is responsible for the fact that later attempts to obtain the title compounds (1) from (127) by this route were never efficient (81,100) although sometimes they were patented (157). 

Hydrazine is also used in conjunction with HNA to impede hydroxylamine oxidation by nitrous acid, always present in nitric acid media, which increases the HNA availability for the plutonium reduction. Both hydrazine and HNA interact with nitrous acid (HNO2), but the hydrazine oxidation is much faster. On the other hand, the use of HAN in conjunction with nitric acid introduces the possibility of an autocatalytic reaction resulting in the overpressurization of the system or explosion in a reprocessing facility as pointed out in Barney s report (Barney, 1998). The main function of HNA is to reduce plutonium from the tetravalent state to the trivalent state and, thus, separate the plutonium from the uranium, which is retained in the hexavalent oxidation state and, hence, in the organic phase. The reduction reaction by HNA is the result of two irreversible reactions (Equations 14.10 and 14.11) ... 

Oximes prepared from aldehydes or ketones by reaction with hydroxylamine can be reduced to primary amines in high yields. One can use various reagents for the reduction step, including Ni-H2 in methanol or L1A1H4 in ether. Using a reduction reaction, carry out the following transformations ... 

Dissolve 0-5 g. of the substance in 10 ml. of 50 per cent, alcohol, add 0-5 g. of solid ammonium chloride and about 0 -5 g. of zinc powder. Heat the mixture to boiling, and allow the ensuing chemical reaction to proceed for 5 minutes. Filter from the excess of zinc powder, and teat the filtrate with Tollen s reagent Section 111,70, (i). An immediate black or grey precipitate or a silver mirror indicates the presence of a hydroxyl-amine formed by reduction of the nitro compound. Alternatively, the filtrate may be warmed with Fehling s solution, when cuprous oxide will be precipitated if a hydroxylamine is present. Make certain that the original compound does not aflfect the reagent used. 

Electrolytic reductions generally caimot compete economically with chemical reductions of nitro compounds to amines, but they have been appHed in some specific reactions, such as the preparation of aminophenols (qv) from aromatic nitro compounds. For example, in the presence of sulfuric acid, cathodic reduction of aromatic nitro compounds with a free para-position leads to -aminophenol [123-30-8] hy rearrangement of the intermediate N-phenyl-hydroxylamine [100-65-2] (61). 

Neta.1 Ama.lga.ms. Alkali metal amalgams function in a manner similar to a mercury cathode in an electrochemical reaction (63). However, it is more difficult to control the reducing power of an amalgam. In the reduction of nitro compounds with an NH4(Hg) amalgam, a variety of products are possible. Aliphatic nitro compounds are reduced to the hydroxylamines, whereas aromatic nitro compounds can give amino, hydra2o, a2o, or a2oxy compounds. 

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