2- Naphthol reaction with diazonium salts

An interesting reaction of diazonium salts with 2-naphthol-l-sulfonic acid was observed in the pH range 10-15. Instead of the usual diazodesulfonation reaction... 

Naphthols undergo the usual reactions of phenols. Coupling with diazonium salts is particularly important in dye manufacture (see Sec. 23.17) the orientation of this substitution is discussed in the following section. 

The azo compounds are important chromophores because of extended electronic delocalisation between the two aromatic rings via the azo bond. The darkness of the dye is enhanced by extensive delocalisation combined with several sulfonic acid groups which function as auxochromes. An example is provided by Naphthol Blue Black B (10), prepared from 8-amino-l-hydroxynaphthalene-3,6-disulfonic acid (H-acid) (11) by coupling it in the 7-position with diazotised p-nitroaniline in acidic solution and subsequently coupling in the 2-position with diazotised aniline in alkaline solution (Scheme 3). The H-acid (11) is a very versatile component in dye manufacture because it can couple with diazonium salts in either the 2-position or 7-position depending on the pH of the reaction medium, as indicated in Scheme 3. 

Aromatic amines can sometimes be chromatographed successfuly after their conversion to derivatives. For example, isomeric toluidines and aniline, which are poorly separated as salts, can be clearly separated after their conversion to bromo derivatives (105) a series of primary amines can be separated after conversion to arylazo-2-naphthole (p. 342), and for the separation of isomers the products of the reaction of amines with diazonium salts (p. 324) can also be employed. They can be chromatographed in a formamide/hexane system. Thin-layer chromatography of free bases can be carried out on nonadhering layers of alumina (106, 107), or on silica gel G layers using the mobile phase cyclohexane — carbon tetrachloride-ethyl ecetate (10 70 20) ... 

The most important color reaction of diazonium salts is their coupling with a suitable passive component, i.e., with a phenol in alkaline medium, or with an aromatic amine in acid medium (see p. 341). Among phenols, resorcinol and 2-naphthol or R-salt are suitable passive components among amines, 1-naphthylamine or N-l-naphthylethylenediamine are suitable. 

A diazonium salt is a weak electrophile, and thus reacts only with highly electron-rich species such as amino and hydroxy compounds. Even hydroxy compounds must be ionized for reaction to occur. Consequendy, hydroxy compounds such as phenols and naphthols are coupled in an alkaline medium (pH > of phenol or naphthol typically pH 7—11), whereas aromatic amines such as N,N diaLkylamines are coupled in a slightly acid medium, typically pH 1—5. This provides optimum stabiUty for the dia2onium salt (stable in acid) without deactivating the nucleophile (protonation of the amine). 

Morken and co-worker (57) recently reported using a visual colorimetric assay to evaluate a variety of catalyst systems for allylic alkylation. This method uses the reaction of naphthol with Fast Red diazonium salt as a method for determination of catalyst activity. Reaction of the naphthyl allyl carbonate (222) with palladium gives the naphthoxide (223) after loss of C02. The naphthoxide then deprotonates... 

A sample of the reaction mixture should not become turbid when sodium acetate solution is added. But if now a few drops of a solution of an aniline salt are poured in, there is precipitated yellow diazoaminobenzene, which can be re-dissolved with concentrated hydrochloric acid after a few small pieces of ice have been added. Further, when a few particles of j8-naphthol or of R-acid are dissolved in a small excess of 2 A-sodium hydroxide solution and a sample of the diazo-solution is added, an intense red colour is produced. The development of this colour, which results from coupling , is an infallible test for a diazonium salt, and hence also for the corresponding primary aromatic amine. 

The completeness of coupling may be checked by the R-salt test as follows A few drops of the clear supernatant liquid is added to a small quantity of a solution of R-acid (jS-naphthol-3,6-disulfonic acid) in an excess of 2 TV sodium hydroxide. An intense red color is produced if unreacted diazonium salts are present. Obviously this test is satisfactory only if R-acid couples more rapidly with the diazonium salt than the coupling agent involved in the reactions and if the change to a red color is not obscured by other colors present in the reaction system.]... 

A similar template-directed approach was employed (Figure 31) by Anderson et al. [48] to self-assemble azo-dye [2]rotaxanes. The azobenzene diazonium salt 98 is bound by a-cyclodextrin 99 and also by / -cyclodextrin 100 in H20 with pseudorotaxane geometries. Reaction of the terminal diazonium functions with / -naphthol 101 affords the purple [2]rotaxanes 102 and 103 in yields of 12 and 15%, respectively. These two [2]rotaxanes were characterized by electrospray mass spectrometry which revealed peaks corresponding to the loss of all four sodium cations in both cases. By irradiating some of the protons of the dumbbellshaped components of these [2]rotaxanes, enhancement of the resonances of some of the cyclodextrin protons could be observed as a result of an intercomponent... 

Morken and Lavastre used the formation of a colored side product to identify catalysts for the allylation of /i-dicarbonyl compounds [8]. The researchers employed 1-naphthyl allyl carbonate 5 as an allyl source and the diazonium salt of fast red as an indicator. Formation of the active 7z>allyl complex furnishes C02 and 1-naphthoxide which deprotonates the 1,3-dicarbonyl compounds which can, in turn, react with the 71-allyl metal complex. 1-Naphthol is the only species in the reaction mixture that can react with the diazonium salt 6 to generate the bright red azo dye fast red. Thus the red color is indicative of successful formation of the active re-allyl complex (Figure 5.4.3). 

In 1896 Knorr and Stolz succeeded in diazotizing 4-amino-1-phenyl-2,3-dimethylpyrazole and so obtained the first diazonium salt with a pyrazole nucleus.26 Much later, Morgan and Reilly diazotized 4-amino-3,5-dimethylpyrazole and studied the reactions of the resultant diazonium salt (49).27,28 In aqueous solution it coupled with /3-naphthol and the crystalline diazonium chloride was quite stable. However, these workers did not isolate the free diazo compound. The diazo compounds can be readily obtained by treating the... 

Reaction with phenols and naphthols are usually carried out in the pH range 8-11, when the coupling species is the phenoxide ion. A cold, acidic solution of the diazonium salt is added to an alkaline solution of the phenol, when a fast electrophilic aromatic substitution occurs at the 4-position (Scheme 8.25). If this position is already occupied, attack occurs at the 2-position. 2-Naphthol couples at the 1-position. 

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