Hydrazo compounds rearrangement

Similar experiments on the analysis of the products from the three isomeric hy-drazonaphthalenes30 and substituted hydrazobenzenes31 confirm the intramolecu-larity of the reaction. In addition, products resulting from attack on the solvent of fragments from a hydrazo-compound during rearrangement have never been detected30. 

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. 

The similarity of the rearrangement of aromatic hydrazo-compounds to the exchange reactions with which they were compared above becomes more marked in those cases where the -positions of the two benzene nuclei are occupied. Then, as a rule, a diphenyl base is not produced, but the radicle which separates moves so that its nitrogen atom takes up the o-position with respect to the other nitrogen atom derivatives of o-aminodiphenylamine are thus produced, e.g. 

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. 

Further complications of the reduction of aromatic nitro compounds are the possibility of complete reduction to aromatic amines (which may condense with nitroso compounds to give the desired azo compounds), reduction of azo compounds to the corresponding hydrazo compounds, followed by a benzidine (or semidine) rearrangement. It is clear, therefore, that the level of reducing agent used and other reaction conditions are quite critical. 

Acid-catalyzed rearrangement of aromatic hydrazo compounds leads to diaminobiaryl compounds (see Section 23-1OD). 

Reaction LI. (a) Action of Acids on the non-para substituted Hydrazo Compounds. (A., 270, 330 287, 97 B 26, 681, 688, 99.)—When hydrazobenzene is treated with mineral acids, an intermolecular rearrangement to benzidine (pj-pg-diaminodiphenyl) takes place. 

Good agitation is essential, and a solvent may be used in some cases. This, however, is not always necessary if the agitation is efficient. The compounds are isolated by dissolving out the zinc with ice-cold hydrochloric acid. The hydrazo compounds, when heated with mineral acids, undergo a rearrangement (benzidine conversion). 

Reductions of aromatic nitro compounds often proceed to generate mixtures of nitroso and hydroxyl-amine products which then condense to form azoxy and, eventually, azo compounds. This bimolecular reduction is practical only for the generation of symmetrically substituted azo compounds. The situation can be further complicated if the reduction continues such that aromatic amines are formed the amines may then condense with the intermediate nitroso compounds to generate hydrazo compounds which can then undergo a benzidine rearrangement. 

Benzidine Dyes.— The importance of hydrazo compounds in connection with dyes is not on their own account for, as has been stated, they are colorless compounds but because they are easily oxidized to azo compounds which are dye compounds and because of the above rearrangement into compounds like benzidine which yield dyes known as benzidine dyes (p. 787). 

Derivation The methyl ether of o-nitrophenol is reduced by zinc dust and caustic soda to the hydrazo compound, which is then rearranged with hydrochloric acid. 

Some examples of a benzidine-type of rearrangement are known for N-arylamines which, to some extent, may be considered as analogues of hydrazo compounds. Thus, 1-anilinoimidazole (279), on heating in concentrated hydroxhloric acid, is transformed to 4(5)-p-aminophenyl derivative (280) (70ZC289). 

A number of patents have been issued on the catalytic reduction of nitro compounds in the presence of alkali to give hydrazo compounds. These are useful intermediates for the benzidine series by acid rearrangement. 

When nitrobenzene or its homologues are treated in alkaline solution in the presence of finely divided zinc or iron, they can be reduced step by step to the hydrazo stage. Such hydrazo compounds— hydrazobenzene, hy-drazotoluene, hydrazoanisole—may be converted readily to benzidine, tolidine, and dianisidine by intramolecular rearrangement in cold concentrated hydrochloric or sulfuric acid. 

Unsymmetrical azo- or azoxy-compoimds on reduction with tin and hydrochloric add produce a mixture of two amines, while hydrazo-compounds may yield amine(s) formed by normal reduction of the hydrazo-link and a diamine produced by a benzidine rearrangement. Nitro-samines, on reduction with tin and acid, give the secondary amines from which they were derived.)... 

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 is the first example of a direct coupling of a diazo compound in a / position of the thiophene nucleus. The Beckmann rearrangement with the ketoxime of 2-thienylketone was found to give poor yields of the aceto-2-thiopheneamide. Furthermore, the Schmidt reaction using hydrazoic and sulphuric acids was investigated and did give rise to aceto-2-thiopheneamide but probably as an eutectic with the isomeric 2-thenoylmethylamide. 

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