Of hydrazo compounds

Azo-compounds can be obtained by reduction of nitro-compounds, or by oxidation of hydrazo-compounds. They are usually prepared, however, by reacting a phenol or amine with a diazonium salt. The coupling usually takes place in the position para to the hydroxyl or amino group, but if this position is occupied it goes to the ortho position, e.g. 

It has already been known that the reaction of primary amines with alkaline hypobromite gives nitriles, and the reaction of hydrazo compounds with bromine affords azo compounds. Recently, we also found that the reaction of primary amines and hydrazo compounds with BTMA Br3 in aq. sodium hydroxide or in water gave corresponding nitriles and azo compounds in satisfactory yields, respectively (Fig. 27) (ref. 35). 

Dehydrogenation of hydrazo compounds with bromine, 32, 16 Dehydrohalogenation by sodium amide, 30, 72... 

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. 

The reaction seems to be surprisingly dependent on the amount of nitric acid used. A run with 78.6 g of hydrazo compound and a sixfold excess of nitric acid was quenched after 22 hr and afforded 100% conversion to the desired a/.o compound (NMR analysis). Another run with... 

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). 

All of these compounds are termed hydrazines although the symmetrical di-hydrazines retain the name of hydrazo compounds because of their relationship to the azo compounds. 

Copper oxide, CuO, is an oxidant for the conversion of alcohols into aldehydes or ketones [349] and for the transformation of hydrazo compounds into azo compounds [350]. 

The domain of oxidations with silver oxide includes the conversion of aldehydes into acids [63, 206, 362, 365, 366, 367 and of hydroxy aromatic compounds into quinones [171, 368, 369]. Less frequently, silver oxide is used for the oxidation of aldehyde and ketone hydrazones to diazo compounds [370, 371], of hydrazo compounds to azo compounds [372], and of hydroxylamines to nitroso compounds [373] or nitroxyls [374] and for the dehydrogenation of CH-NH bonds to -C=N- [375]. Similar results with silver carbonate are obtained in oxidations of alcohols to ketones [376] or acids [377] and of hydroxylamines to nitroso compounds [378]. 

A -Bromosuednimide is used in the dehydrogenation of hydrazo compounds to azo compounds [744] and in the oxidative degradation of a-hydroxy acids to aldehydes or ketones [745]. This reagent also oxidizes alkyl trimethylsilyl ethers to esters or ketones [744],... 

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