Ammonium-azonium tautomerism

Orange 3 or methyl orange has not the same behaviour as the previous dyes no azo-hydrazone tautomerism is possible. Nevertheless, in strong acid medium, an ammonium-azonium equilibrium occurs, as shown in Fig. 5. 

A widely applied discipline of chemometrics is pattern recognition, which involves the classification and identification of samples. Its purpose is to develop a semiquantitative model that can be applied to the identification of unknown sample patterns. To assure the best reliability, pattern recognition requires the applications of a minimum of two analytical methods. The ammonium-azonium tautomerism in /V, /V-d i a I ky I a minoazo dyes used as indicators requires three techniques UV-Vis, IR, and Raman spectroscopy.223 A study of the matrix composition concerning the ratio of the compounds is necessary as well. The application of pattern recognition analysis in geochemical techniques not only assures better reliability but is also quite useful in addressing real-world environmental problems.224... 

It is well known that azobenzenes containing amino or dialkylamino groups undergo a pronounced color change in solution or in polymer film under protonation [45]. This phenomenon, called halochromism, is the basis of their use as optical sensors and pH indicators [46].The relative effect of the color changes in such dyes is associated with the existing ammonium-azonium (bt a) tautomerism and the tautomeric ratio, which depends, as usual, on the solvent, temperature, and existence of additional substituents in the protonated molecule. 

The process of biprotonation can be treated in a similar way [47], leading to the observation that the biammonium band (8b) at 320 nm decreases slightly, while the ammonium-azonium band (8a) at 520 nm increases with increase in water content The calculated tautomeric constants for the equilibrium 8b tr 8a (0.53 and 0.88 for 10% and 30% water) show that the ammonium-azonium form, which is strongly polar and more acidic, is favored by the increase of water in the solvent composition. As a final result, omitting chemical details, within several steps this compHcated four-component tautomeric equilibrium has been analyzed using band decomposition techniques. 

The introduction of a p-amino-substituent enhances the basicity of azobenzene very considerably. The value of p/iTa changes from —2-90 (Yeh and Jaffe, 1959a) to -i-2 82 (Bascombe and Bell, 1959). The controversy over the site of protonation of p-aminoazobenzene has been resolved in favour of a tautomeric mixture of the ammonium [48] and azonium [49] forms on the basis of an analysis of... 

The monoprotonation of p-[(/>-dimethylamino)phenyI]-azo-benzenesulphonic acid (methyl orange) also gives a mixture of the azonium ion and the ammonium ion, in which the former predominates according to Reeves (1966). Bolton et al. (1973). however, using linear free energy relationships, conclude that the ammonium form is dominant, and question the general acceptance of the tautomeric model for the protonation of p-aminoazobenzene and its derivatives. 

The tautomerism of />-amino- and p-dimethylamino-azobenzene cations has also been studied recently in nitromethane and acetonitrile (Korolev and Titova, 1971), and it has been shown that azonium forms are much more favoured in these solvents than in aqueous acid. The results are shown in Table 3. The estimates are based on the assumption that the extinction coefficient of the ammonium ion [48] may be teiken to be equal to that of p-tri-methylammoniumazobenzene and that of the azonium ion to be equal to that of the conjugate acid of 4-amino-3,5-di-t-butylazo-benzene (Yeh and Jaffe, 1959c), which is assumed to exist only in... 

In theory, azo dyes can undergo tautomerism azo/hydrazone for hydroxyazo dyes azo/imino for aminoazo dyes, and azonium/ammonium for protonated azo dyes. A more detailed account of azo dye tautomerism can he found elsewhere. 

Figure 2.7b confirms this expectation. Here, as one example, the spectra of 7 obtained from DMSO/water mixtures with diHerent water contents (10 and 30%) are compared. It is obvious that the fraction of the ammonium form 7b, absorbing at 420 nm, dominates, and decreases with increasing water content. The opposite is observed for the azonium form. These two spectra show different positions in the tautomeric equilibrium and can be processed using the algorithm given in Section 2.3.1. The resulting individual spectra are also shown in Figure 2.7b. The estimated content of 7a varies from 7% (in 10% water) to 14% (30% water), giving a tautomeric constant from 0.07 to 0.16.

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