Nitrogen compounds diazo coupling

Nitrite can be deterrnined by reaction with sulfanilamide to form the diazo compound, which couples with /V-(1-naphthyl)ethylenediamine dihydrochloride to form an intensely colored red azo dye. Nitrate can be deterrnined in a similar manner after reduction to nitrite. Suitable reducing agents are cadmium filings or hydrazine. This method is useful at a nitrogen concentration of 10 -lO " M. 

Fig. 7-2. Potential energy E as a function of the reaction coordinate for reactions of the P-nitrogen of arenediazonium ions with nucleophiles yielding (Z)- and (is)-azo compounds, a) Reactant-like transition states (e. g., reaction with OH) b) product-like transition states (e. g., diazo coupling reaction with phenoxide ions product = cyclohexadienone-type o-complex (see Sec. 12.8).

The pH used for diazo coupling of amines is very important in determining the nature of the products. Under near-neutral conditions the diazonium ion may attack the nitrogen of the arenamine rather than a ring carbon. In this event a diazoamino compound, a triazene, —N=N—N—, is formed ... 

Azo or diazene compounds possess the —N=N— grouping. Aliphatic azo compounds of the type R—N=N—H appear to be highly unstable and decompose to R—H and nitrogen. Derivatives of the type R—N=N—R are much more stable and can be prepared as mentioned above by oxidation of the corresponding hydrazines. Aromatic azo compounds are available in considerable profusion from diazo coupling reactions (Section 23-IOC) and are of commercial importance as dyes and coloring materials. 

The nitrogen atoms are retained in the product. This electrophilic aromatic substitution reaction is called diazo coupling, because in the product, two aromatic rings are coupled by the azo, or —N=N—, group. Para coupling is preferred, as in eq. 11.34, but if the para position is blocked by another substituent, ortho coupling can occur. All azo compounds are colored, and many are used commercially as dyes for cloth and in (film-based) color photography ... 

Oxidative Couplings of Heterocyclic Hydrazones. This method has opened the way to the preparation of azo derivatives of diazo compounds unobtainable by other means, ie, heterocycHc compounds ia which the diazotizable amino group is conjugated with the heterocycHc nitrogen atom as ia 2- and 4-amiQopyridine, compounds which do not normally yield stable diazonium salts (38). The reaction occurs as illustrated by equation 7 for the iateraction of (A/-methylcarbostyryl)hydrazone [28219-37-6] and dimethyl aniline the overall process is oxidation. 

Since 1910 the method of Bart has been modified by a number of investigators, Bart, himself, being the first to improve the reaction. He found that coupling of aryldiazonium compounds with alkali arsenites is catalyzed by copper salts and by silver or copper powder. In a later patent the use of metallic catalysts, copper, nickel, or cobalt, as well as their salts is said to facilitate the removal of diazo nitrogen at low temperatures and to obviate the formation of by-products. Though many have since observed that the coupling reaction is speeded by the use of the above catalysts, no systematic study has been made to determine the effect of such catalysts on the final yield. 

Strictly speaking, therefore, it covers also the well-known rearrangement of diazonium salts to diazoates, e.g., the formation of diazocyanides from diazo-nium cyanides.316 According to the element with which the terminal nitrogen atom forms a new covalent bond in the coupling process, the product is called a C-, O-, N-, or S-azo compound. This Section is concerned only with C-azo compounds. 

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