ON THE USE OF THE PYRIDINE–PIPERIDINE REACTION FOR DETECTING THE QUINOID STRUCTURE OF CERTAIN DYES

V. A. Malevannyi, Yu. L. Lel’chuk, K. I. Banyukina

(Presented by the scientific seminar of the Department of Analytical Chemistry)

In studying the structure of metal complexes with organic dyes of analytical significance, a method is needed that makes it possible, when necessary, to clarify the structure of the dye under study. Such a method is especially necessary in determining the functional-analytical groups of these compounds for one or another ion in the search for new analytical reagents.

The fundamental importance of accurately establishing the structure of the reagent being studied is evident from the following example. Many authors [1–3] consider that aluminon has a quinoid structure. In this case, the functional-analytical group for aluminum should be regarded as the phenolic hydroxyl located in the o-position to the carbonyl group. However, some investigators [4, 5] use Dilthey’s formula, with a coordinatively unsaturated central carbon atom. If this formula is considered correct, then it must be admitted that two phenolic hydroxyls, situated in the o-position to one another, act as the functional-analytical group. Unfortunately, in the literature one often encounters the situation in which authors give different structural formulas for one and the same compound.

Dilthey and Wizinger [6] noted that the pyridine complexes of certain triphenylmethane dyes are decolorized under the action of piperidine. Kul’tberg and Mustafin [7] used this fact to develop a procedure for determining piperidine in pyridine. Later Mustafin [8] showed that intensification or weakening of the color of pyridine solutions of dyes upon addition of piperidine is evidence of the triphenylmethane structure of the dye being studied. However, Molot [5] asserts that weakening of the color of a pyridine solution of a dye under the action of piperidine indicates the absence of a quinoid structure in the dye under study. This contradiction prompted us to carry out a study to clarify the nature of the pyridine–piperidine reaction and the range of its applicability.

Experimental Part

The pyridine–piperidine reaction was carried out as follows. A weighed portion of the dye under study was dissolved in dry reagent-grade pyridine. Then the spectrophotometric characteristic of the so—

of the solution on an SF-10 spectrophotometer in the wavelength range 400–750 mµ at a temperature of \(18 \pm 1^\circ\)C. Pyridine served as the compensation liquid. To each such solution three drops of piperidine were added, and after thorough mixing the spectrographic curve of the new solution was recorded under the same conditions.

The optical characteristics obtained for the pyridine and piperidine complexes of the compounds studied are given below. The following designations are used in the table: \(C\) is the concentration of the dye in pyridine; \(\lambda_{\max}\) is the wavelength at which the complex under study has the maximum optical density. \(D_1\) is the optical density of the pyridine complex at the \(\lambda_{\max}\) of this complex; \(D_2\) is the optical density of the piperidine complex at the \(\lambda_{\max}\) of the pyridine complex. Measurement of \(D_2\) under these conditions was dictated by the fact that complex formation of piperidine with the dye was in most cases accompanied by a bathochromic or hypsochromic effect.

Compounds belonging to various classes were selected for study. Compounds 1–18 belong to the class of arylmethane dyes, 19–20 to oxazine dyes, 21 to thiazine compounds, 22–24 to azo dyes, 25–26 to the oxyflavone group, 27–31 to the class of anthraquinone derivatives, 32 to phthalocyanines, 33 to triazine dyes, and 33–36 to the class of quinones.

Discussion of Results

The presence of a bathochromic or hypsochromic effect indicates that reactions leading to structural changes in the dye molecule occur in the given system [9]. On the basis of the change in the maximum of light absorption, all the compounds we studied may be divided into three groups. The first group includes 5, 7, 9, 10, 17, 18, 26–30, 35. All of them react with piperidine, giving a distinct bathochromic effect. The second group includes compounds 1–4, 6, 13, 15, 16, 19, 25, 34. They react with piperidine with a clearly pronounced hypsochromic effect. The third group consists of compounds 3, 11, 12, 14, 20–23, 31–33, 36, which do not change the position of the maximum of light absorption under the action of piperidine. However, for most compounds the optical density of the solution changes appreciably. Evidently, in this case as well, the interaction of the corresponding dyes with piperidine is accompanied by a process of complex formation, but the complexes formed differ from the pyridine complexes only in the value of the molar extinction coefficient.

Galloblue DF belongs to leuco bases and does not react with piperidine. In all probability this is connected with the fact that, upon formation of the leuco base, the quinoid ring is reduced to a phenolic one.

The data obtained show that one and the same effect is characteristic of dyes of different classes, which differ sharply in their structure. This indicates that identification of dyes by the shift of the maximum of light absorption in the course of their complex formation with piperidine is practically impossible.

According to the change in the magnitude of the optical density under the action of piperidine, all the compounds studied fall into two groups. The first group includes compounds 5–13, 17, 23–26, 28, 29, 32–36. All of them, upon interaction with piperidine, increase the intensity of their color. It should be especially noted that this hyperchromic effect is also characteristic of quinones (compounds 34–36).

Table 1

Optical characteristics of the complexes studied

This fact makes it possible to assert that the presence of a hyperchromic effect indicates the existence of a quinoid ring in the molecule of the dye under study. But, in addition to triphenylmethane and anthraquinone-

dyes having a quinoid grouping; the hyperchromic effect is also characteristic of azo dyes, oxyflavones, phthalocyanine and triazine dyes.

Under the conditions in which the reaction is carried out, azo dyes undergo azo–hydrazone tautomerism [10]. In a weakly alkaline medium an azo dye often passes into the quinonehydrazone form, which, in all probability, also enters into interaction with piperidine.

Oxyflavones contain a chromone ring, which is an analogue of a quinoid ring. The structure of tetrazaporphin is present in phthalocyanine dyes. Titan Yellow contains a benzothiazole ring [11]. Thus, the hyperchromic effect observed in the interaction of the pyridine complex of a dye with piperidine indicates the presence in the molecule of the compound under study of a quinoid, chromone, tetrazaporphin structure or a benzo-thiazole ring.

The second group comprises compounds 1–4, 14–16, 18, 19, 21, 22, 27, 30, 31. All of them, under the action of piperidine, decrease the intensity of their color. According to Molot [5], a hypochromic effect indicates the absence of a quinoid structure. Our data refute this assertion. Such an effect was given by compounds among the triphenylmethane, oxazine, azo, and anthraquinone dyes. Gallocyanine, which contains a quinoid ring, under the action of piperidine lowers the optical density of the solution by 0.82.

The hypochromic effect is also characteristic of compounds containing p-quinoneimine and phenothiazine groupings (oxazine derivatives of aminotriarylmethane and thiazine dyes). Thus, the pyridine–piperidine reaction proposed by Mustafin for detecting the triphenylmethane structure of dyes makes it possible to detect certain other structures as well.

Conclusions

The hyperchromic and hypochromic effects observed in the interaction of pyridine solutions of triphenylmethane dyes with piperidine are proof of the presence in the molecule of the compound under study of quinoid or n-quinoneimine structures. These same effects are also characteristic of dyes containing chromone, tetrazaporphin, and phenothiazine groupings.

The presence of a hyperchromic or hypochromic effect in the interaction of pyridine solutions of azo dyes with piperidine may also serve as confirmation of the azo–hydrazone tautomerism of these compounds.

Literature

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3. G. Schiemann. Die Cemie der naturlichen und kunstluchen organischen farbstoffe, 76, Leipzig, 1936.

4. R. Wizinger. Introduction to the Chemistry of Coloring Substances on the Basis of Coordination Theory. ONTI, Leningrad, 1936.

5. L. A. Molot, L. M. Kul’berg. Journal of Analytical Chemistry, XI, 2, 198 (1956).

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