Diazo hydroxide

Synthesis. Almost without exception, azo dyes ate made by diazotization of a primary aromatic amine followed by coupling of the resultant diazonium salt with an electron-rich nucleophile. The diazotization reaction is carried out by treating the primary aromatic amine with nitrous acid, normally generated in situ with hydrochloric acid and sodium nitrite. The nitrous acid nitrosates the amine to generate the N-nitroso compound, which tautomerizes to the diazo hydroxide. 

In principle, the intermediates obtained under these conditions could have the structure of a nitrosoamine such as 3.12, but they could also be one of the isomers, namely a (Z)- or (ii)-nitrosoimine (3.14 or 3.15, respectively), or a (Z)- or (ii)-diazo-hydroxide (3.16 or 3.17, respectively). Analogous isomers exist, of course, for other nitrosation products of heteroaromatic amines. 

The compound which couples adds itself to the double bond of the diazo-hydroxide either in virtue of (a) the mobile hydrogen atom or (h) the double bond ... 

The method of reaction of the diazobenzene hydroxide is extraordinarily similar to that of nitrous acid practically all substances which are capable of coupling also react with this acid. Comparison of the formulae shows that in the diazo-hydroxide the bivalent radicle... 

A free-radical mechanism has been suggested for the nitrosation of 1,2-phenylenediamine (22) by peroxynitrite PN/CO2. 1,2,3-Benzotriazole (26) was formed as a result of an intramolecular nucleophilic displacement on the diazo hydroxide (25) by the neighbouring amine group. The authors suggest that the mechanism involves an initial H-atom abstraction or one-electron oxidation from (22) by CO3 , followed by the reaction of the product (23) with NO. The inhibitory effects of azide support a free-radical mechanism of the reaction. 

Whereas base-induced decomposition of N-nitrosourethanes has been utilized (9) as a popular method of generating diazoalkanes, only limited investigations on base treatments of nitrosamides have been reported (10). The primary product in the base treatment is assumed, in analogy to better investigate nitrosourethane cases, to be diazo hydroxides V via attack of a base on the carbonyl group as in IV. A diazo hydroxide V has been related to the diazo ester III by a reaction with benzoyl chloride. 

In addition, the formation of myosmine indicates that the diazo- hydroxide intermediate undergoes cyclization followed by loss of HONO. 

The mechanism of the reaction probably involves the intermediate formation of the oovalent diazo-hydroxide from the diazonium salt the former decomposes... 

Phenyl Nitroso Amine.—The iso-diazotate differs in a noticeable way from the diazotate. Not only is it more stable, but on treatment with acids it does not yield diazonium salts, but an entirely different compound, viz., phenyl nitroso amine, CeHs—NH—NO. This compound has the same composition as the diazo hydroxide and the relationship means that the isomeric diazo hydroxide undergoes structural rearrangement into a new compound. This is the reason for the view that this same compound, phenyl nitroso amine, is an intermediate product in the formation of diazo. compounds as already referred to (p. 587). These relationships may be shown as follows ... 

The formation of the diazotate from the diazonium salt by means of alkalies is explained by the reverse of the first reaction. This makes clear the reaction by which the diazonium salts yield azo compounds (p. 569). The reaction takes place best in neutral or alkaline solution and under these conditions the diazonium salt would yield the diazo hydroxide compound which would then couple with the amine and form the amino azo compound. 

Thus the acceptance of the Bloomstrand-Strecker-Erlenmeyer formula for the diazonium base and salts, and of the Hantzsch modification of the Kekule formula for the isomeric diazotates and the acid diazo hydroxide, together with the tautomeric transformations which occur, makes possible the explanation of all the facts which we have considered in the preceding discussion. These may be stated again briefly for the sake of emphasis and review, (i) The reaction of diazotization (2) the diazonium base and salts, (3) the diazotates and acid diazo hydroxide, (4) the isomerism of the diazotates, (5) the diazonium sulphites and diazo sulphonic acidsj (6) the relationship of diazo compounds to nitroso amines (7) to hydrazines (8) the coupling with amino and hydroxy compounds to form amino azo and hydroxy azo compounds, (9) the reactions of decomposition. Most of these last reactions we have not considered, but will do so presently and we shall find that they all may be likewise satisfactorily explained. The tautomeric constitution of the diazo compounds, therefore, meets every test and is generally accepted. 

Diazo Esters.—Before taking up the reactions of decomposition of diazo compounds there is one other class of derivatives which should be mentioned. As an acid compound benzene diazo hydroxide yields esterSy not by the direct action of alcohols, but by the action of alkyl halides upon silver diazotate. These esters are stable compounds like... 

As the oxidation occurs in alkaline solution the diazonium salt is first converted into the diazo hydroxide or the diazotate and this is then oxidized. 

When diazonium salts react in the absence of free acid with amines and phenols, derivatives of azobenzene are formed. As these contain trivalent nitrogen atoms only, it is assumed that in neutral or alkaline solutions the diazonium hydroxides rearrange to diazo hydroxides,... 

In step-in, the aryl-imino-nitroso derivative takes up a proton in such a manner that the 0-atom of the nitroso moiety gets protonated to give rise to a product bearing a -Fve charge on the 0-atom. Further, loss of a proton from the N atom adjacent to the aryl nucleus helps in shifting the double bond between terminal O and N atoms to N and N atoms thereby generating the aromatic diazo hydroxide. 

In step-IV, the resulting aromatic diazo hydroxide retains a proton to give an intermediate wherein the terminal O atom bears a +ve charge. This intermediate encounters a prototropic shift, loses a mole of water and ultimately gives rise to the desired aromatic diazonium ion. 

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