No abstract

No abstract

The transformation of 5-diazoimidazoles containing a carboxamide, ethoxycarbonyl, or nitro group at position 4 into diazonium salts was studied by UV and IR spectroscopy. It was shown by means of model compounds and chemical transformations that nitrosamines are involved in the transformation.Study of the acid-base transformations of certain diazo compounds of the azole series into the corresponding diazonium salts has made it possible to obtain the pK a values of these compounds [1]. For 5-diazoimidazole-4-carboxamide (Ia) it was shown that at pH 1, 2.5, and 7 it exists in the form of the diazonium salt, diazo compound, and diazohydroxide respectively [2].The present communication is devoted to a study of the specific characteristics of the acid-base equilibrium in 5-diazoimidazole derivatives at acidities H 0 -3.38 to -0.02 and in the range of pH 0-10.We studied the IR and UV spectra of the amide (Ia), ethyl 5-diazoimidazole-4-carboxylate (Ib), and 5-diazo-4-nitroimidazole (Ic) under various conditions. 4-Ethoxycarbonylimidazole-5-diazonium tetrafluoroborate (II) was synthesized from the imidazole (Ib) as model compound, and its spectra were recorded. The data are given in Table 1.As seen from Table 1, in strongly acidic media (H o -1.9-0) the diazoimidazoles (Ia, b) change into the corresponding diazonium salts (IIIa, b). This is demonstrated by the shift of the frequency for the stretching vibrations of the diazo group in the IR spectra by 40-60 era-1 toward larger values. [They approximate to the frequency for the vibrations of the diazonium group in the model compound (II).] In the UV spectra, recorded under the same conditions, a hypsochromic shift of the longwave absorption maximum by 30 nm is observed. At pH 1 the IR spectrum of the ester (Ib) contains bands for the vibrations of both the diazo and the diazonium groups.Thus, the presented data indicate that compound (Ib) in 1-5 N hydrochloric acid exists in the form of the diazonium salt (IIIb). At pH 1 there is an equilibrium between the salt (IIIb) and the diazo compound (Ib). At pH 5-9.2 only compound (Ib) is present in the solution.The existence of ranges of pH in which 5--diazoimidazoles and the corresponding diazonium salts exist in the solution makes it possible to determine the pK a values of the diazonium salts as NH acids. For this purpose the UV spectra of compounds (Ia, b) in 5-0.01 N hydrochloric acid and also of 5-diazo-4-nitroimidazole (Ic) in 50, 20, 10, 5, and 2.5% sulfuric acid (H 0 -3.38, -1.01, -0.31, -0.02, and 0 respectively) and in water were studied. Here it was unexpectedly found that the spectral curves of solutions of compounds (Ia, b) did not pass through an isobestic point (Figs. 1, 2). A similar pattern was observed for solutions of diazonitroimidazole (Ic) (Fig. 3). This could be explained by the stow establishment of an equilibrium between the two tautomeric forms (A and B) of the diazonium salts (III) and also by the participation of the diazo compound (I) in an equilibrium. However, only one band for the stretching vibra...

No abstract

Reaction of 5-diazo-4-nitroimidazole with concentrated hydrochloric acid leads to 4-chloro-5-diazoimidazole. The same product is formed together with a small quantity of 5-chloro-4-nitroimidazole Previous investigations of the acid--base properties of 5-diazo-4-nitroimidazole (Ia) showed that in 50 and 20% solutions of sulfuric acid (H0 -3.38; -1.01 respectively) this compound exists predominantly in the form of 4-nitroimidazole-5-diazonium sulfate (IIa) (X = HSO4-). At pH 0 an equilibrium between compounds (la), (Ha), and 4-nitro-5-nitrosaminoimidazole (III) was observed. At pH 1-5 diazo compound (Ia) and nitrosamine (III) were present in solution [1]. It is known that heterocyclic nitrosamines are converted in 1-10% hydrochloric acid into the corresponding halo derivatives. An increase in acid concentration leads to a reduction of their yield [2]. Decomposition of aromatic diazonium salts in the absence of catalyst by an S~I mechanism also leads to a haloarene [3].The aim of the present work was to study the reactions of compound (Ia) in concentrated, 1 N, and 0.1 N hydrochloric acid.On solution in concentrated hydrochloric acid (H0 -3.95) diazoimidazole (Ia) is converted practically instantaneously into another diazo derivative of the imidazole series giving a yellow coloration on chromatograms when treated with m-phenylenediamine. 2-Nitrobenzenediazonium chloride reacts with concentrated hydrochloric acid forming 2-chlorobenzenediazonium chloride [4]. By analogy with this we propose that replacement of the nitro group by chlorine occurs in the salt (lib) X = CI-, formed initially from imidazole (Ia), with the formation of 4-chloro-5-diazoimidazole (Ib). To confirm this structure azo couplings with B-naphthol and dimethylamine, characteristic of diazo compounds, were carried out.As a result individual products were obtained and characterized by elemental analysis, PMR and IR spectroscopy data as 4-chloro-5-(3,3-dimethyl-l-triazeno)-imidazole (IV) and 4-chloro-5-(2-hydroxy-1-naphthalene-diazo)imidazole (V). There were bands in the IR spectra of these compounds for the stretching vibrations of N=N and C C! bonds but the bands characteristic of nitro group absorption were absent. In the PMR spectra signals for the imidazole ring protons were observed, in the case of azo compound (V) a multiplet signal for the naphthalene ring protons was also present, and in the spectrum of triazene (IV) there was a singlet for the methyl group protons.Refluxing imidazole (Ia) in 1 N hydrochloric acid gave diazo compound (Ib) together with an insignificant quantity of 5-chioro-4-nitroimidazole (VI) identical (TLC, mp, IR spectrum, and PMR data) with a sample of known structure synthesized by the procedure of [5]. Refluxing of compound (la) in 0.1 N hydrochloric acid for 6 h or leaving the solution at room temperature for a month (until disappearance of the compound (Ia) from the reaction mixture according to TLC) leads mainly to chloronitroimidazole (VI). We also confirmed the presence of a small