Hydrazo-compounds, formation

Since the condensation of the hydroxylamine intermediate with its precursor nitrosobenzene is a key step in generating coupled intermediates the presence of DMSO in the reaction mixture increased hydrazo compound formation but did so at the expense of a significant decrease in reaction rate. 2> 3... 

A great variety of solvents has been used with success. Reactive solvents, such as acetic anhydride, will react with the amine as formed. Basic solvents cause the formation of azo, azoxy. and hydrazo compounds, paralleling chemical reductions (39,73). 

Two other theories as to the mechanism of the benzidine rearrangement have been advocated at various times. The first is the rc-complex mechanism first put forward and subsequently argued by Dewar (see ref. 1 pp 333-343). The theory is based on the heterolysis of the mono-protonated hydrazo compound to form a n-complex, i.e. the formation of a delocalised covalent it bond between the two rings which are held parallel to each other. The rings are free to rotate and product formation is thought of as occurring by formation of a localised a-bond between appropriate centres. Originally the mechanism was proposed for the one-proton catalysis but was later modified as in (18) to include two-protons, viz. 

Recombination of the ion radicals within the cage is thought of as forming the path to rearrangement whilst escape of the radicals and subsequent reaction with the hydrazo compound leads to the formation of disproportionation products often observed. The theory is mainly directed at the two-proton mechanism and does not accommodate well the one-proton mechanism, since this requires the formation of a cation and a neutral radical, viz. 

With concentrated mineral acids azobenzene gives red salts, as may be shown by pouring hydrochloric acid on it. Addition of hydrogen leads to the re-formation of the hydrazo-compound. Oxygen is added on and the azoxy-compound formed by the action of hydrogen peroxide or nitric acid. The synthesis of asymmetrical aromatic azo-compounds from nitroso-compounds and primary amines was discussed above. 

Pyridine reacts with sodium hydrazide in the presence of hydrazine to yield 2-hydrazinopyridine in the absence of free hydrazine a hydrazo compound is formed (Scheme 88) (64AG(E)342). A difference between hydrazination and amination is the formation of 1,4-adducts which cannot be rearomatized even on heating. This is reflected in the behaviour of quinoline, which gives only a 0.5% yield of a -hydrazino product, whereas 4-methylquino-line is hydrazinated in 76% yield (64AG(E)342). Acridine behaves differently with sodium hydrazide/hydrazine, 9,10-dihydroacridine is formed almost quantitatively, but reaction in the absence of hydrazine yields 9-aminoacridine (65%). An even higher yield of 9-amino-acridine is obtained when sodium Af.AC-dimethylhydrazide is used (Scheme 89). Good evidence for intermediacy of (151) comes from the isolation of (152) on hydrolysis of (151). 

The bimolecular reduction of nitro compounds is believed to involve reduction of some of the starting material to a nitroso compound and another portion to either a substituted hydroxylamine or an amine. These intermediates, in turn, condense to form the azo compound. The exact mechanism of the reaction requires critical study. On the one hand, reducing conditions are always on the alkaline side to prevent the benzidine rearrangement of an intermediate hydrazo compound under acidic conditions, yet it is difficult to visualize the formation of hydrazo compounds by the indicated condensation. As a practical matter, this method is of value only if symmetrically substituted azo compounds are desired. 

Since azo compounds may be prepared by the oxidation of hydrazo compounds via procedures similar to those used in oxidizing azo compounds to azoxy compounds, better definition of reaction conditions is required to control the formation of either type of compound. The existing literature rarely, if ever, indicates that the oxidation of a 1,2-disubstituted hydrazine could conceivably produce a mixture of azo and azoxy compounds. 

Nickel or mercury cathodes increase the amount of azoxy- or azo-body formed, but lead, zinc, copper, and tin favour the formation of azo- and hydrazo-compounds.2 Addition of copper powder to the electrolyte increases amine formation. 

Kosak studied the hydrogenation of o-nitroanisole in details over palladium- and platinum-based catalysts at 95°C and 2.07 MPa H2.94 The hydrogenation of o-nitroanisole over palladium catalysts was always accompanied by the formation of o,o -hydrazoanisole, the amount of which varied significantly with different samples of starting o-nitroanisole. Impurities such as o-chloronitrobenzene and o,o -dichloroazoxybenzene present in o-nitroanisole were found to increase formation of the hydrazo compound with decrease in the yield of o-anisidine, while o-chloroanisole had no effect on the composition of the product. The hydrazo compounds were resistant to further hydrogenation under the conditions. Platinum-based catalysts were insensitive to the impurities and always gave o-anisidine of high purity. The results are summarized in eq. 9.46. 

Scheme 9.9 Reaction pathways of aromatic nitro compounds leading to the formation of azoxy, azo, and hydrazo compounds.

Kenner and Knight point out that hydrazones are formed from free hydrazine bases, whereas hydrazine salts, such as phenylhydrazine acetate, yield osazones. It is emphasized that salt formation greatly favors such reactions as the production of amines from hydrazo compounds, a reaction which is represented as follows. The cation therefore... 

Introduction. Nitro compounds are insoluble in water. Primary nitro compounds, RCH2NO2, and secondary, RjCHNOj, are soluble in alkaline solutions, due to the formation of an oci-form by tautomerization, which reacts with bases to form salts. The most important reaction of nitro compounds is their reduction in acid media to amines. In neutral solutions the intermediate products, hydroxylamines, may be isolated. In alkaline media, azoxy, azo, and hydrazo compounds are formed. The object of this experiment is to illustrate some of these reactions. 

Under the reaction conditions used for the kinetic measurements, no formation of intermediates (e.g. nitroso, hydroxylamino or hydrazo compounds) could be observed. The composition of reaction mixtures was checked by thin layer and gas chromatography, monitored by UV-Vis spectroscopy and finally verified by GUMS, H- and 3Q.j,jMR-spectroscopy. 

In alkaline and neutral solutions condensation reactions between the nitroso and hydroxylamine compounds and between the hydroxylamine and nitro compounds occur. The result is the formation of azoxy compounds which may undergo further reduction to the azo and hydrazo compounds-... 

Several other examples of azo compound formation from reaction of aryl azides with anilines are known and support for a hydrazo intermediate (34) comes from the work of Huisgen and von Fraunberg. They showed that 34 could be converted to 35 in a separate experiment... 

Scheme 17.12 Probable reaction mechanism behind the formation of hydrazo compounds or aldimine hydrodimers, catalysed by cadmium-sulfide powder or platinised cadmium sulfide (CdS/Pt).

For the identification of simple aromatic hydrazo compounds paper chromatography can be used successfully on papers impregnated with dimethylformamide (cyclohexane as the mobile phase) or formamide (cyclohexane or benzene as the mobile phase). Hydrazo compounds can be spotted on the paper in the form of an alcoholic or ethereal solution in concentrations from 0.5 to 200 pg. Detection is carried out by spraying with p-dimethylaminobenzaldehyde (p. 349). Under the influence of the hydrochloric acid present in the reagent, a rearrangement to benzidines or semidines takes place, and they then react with the reagent, with the formation of orange spots (53). 

This reaction suggests a mechanism for the formation of 5-phenylhydrazinouracil from the action of phenylhydrazine on 5,5-dibromo-6-hydroxy-5,6-dihydrouracil which was observed by Levene namely, the formation of uracil-5-azobenzene (much as given above) followed by its reduction to the hydrazo compound by the excess of phenylhydrazine present. 

The most noteworthy reaction of azo-compounds is their behaviour on reduction. Prolonged reduction first saturates the azo group, giving the hydrazo derivative (C NH-NH C), and then breaks the NH NH linkage, with the formation of two primary amine molecules. If method (1) has been employed to prepare the azo-compound, these two primary amines will therefore be respectively (a) the original amine from which the diazonium salt was prepared, and (6) the amino derivative of the amine or phenol with which the diazonium salt was coupled. For example, amino-azobenzene on complete reduction gives one equivalent of aniline, and one of p-phenylene diamine, NHaCeH NH benzene-azo-2-naphthoI similarly gives one equivalent of aniline and one of... 

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