Химия и химические технологии /6. Органическая химия

A. S. Gotsulya, prof. O. I. Panasenko, prof. Ye. G. Knysh, O. O. Mikolasyuk

Zaporizhzhya State Medical University, Ukraine

Spectral researches of 4-(2-methoxyphenyl)-5-R-2H-1,2,4-triazoles-3(4H)-thiol

UV-spectra were measured in solvents of different polarity: water, 95% ethanol, 0,1 M HCl, 1 M H₂SO₄, 0,1 M NaOH, hexane. Measurements were conducted, using instrument SPECORD 200-222U214 in quartz cuvettes with a thickness of the working layer 1 cm. In all cases were used solvents, which had analytical qualification "c. f. a" (clean for analysis).

Observed electronic spectra of test substances are characterized in all cases by three absorbing bands, which are characteristic to have from three to five peaks.

Actually heterocyclic compound of 1,2,4-triazoles structure according to the literature in aqueous solution has light absorption only at 187 nm. According with Platt, spectroscopic moment (m, cm ∙ mol/L) for the dipole moment of the transition, which is induced for absorbing band of 1,2,4-triazoles by methyl grouping, equals zero, and by thiol – equals +27, namely maximum for 5-methyl-2H-1,2,4-triazoles-3(4Н)-thiol fragment of research substance should be approximately 214 nm with an average value of the molar light–absorbing coefficient (ε), which does not exceed the quantity of 3300 l • mol^-1 • cm ^-1. 2-Methoxyphenyl radical only partially connected with 5-methyl-2H-1,2,4-triazoles-3(4H)-thione fragment.

According to J. Brand and T. Ehlimton facts, methoxybenzen characterized in non-polar solvents by two absorption maxima at 221 nm (π→π*-electron transfer), namely ¹L_а-band and at 271 nm (local excitation of the phenyl nucleus - ¹L_в-band). In aqueous solutions methoxybenzene’s spectral curve characterized by maxima at 217 and 269 nm. Researches that we conducted show that the first absorption band of 4-(2-methoxyphenyl)-5-methyl-2H-1,2,4-triazoles-3(4Н)-thiol situated in the range 220-232 nm , the second - 245-263 nm , and the third as the average value at the bend of observed absorption band ~ 280 nm. Herewith, value of the light absorption molar coefficient is an average or high from 3,55 (λ_max at 280 nm, water) to 4,43 (λ_max at 220 nm, 0,1 M NaOH). Thus, on the basis of conducted researches and scientific literature facts we can conclude that the first absorption band of 4-(2-methoxyphenyl)-5-methyl-2H-1,2,4-triazoles-3(4H)-thione connected with overlay transition of n→π* (1,2,4-triazole cycle) and π→π*-electronic transitions of the benzene series type. The second absorption band is a result of the local excitation of the phenyl nucleus electrons and according with the classification of N.V. Klevens and J. R. Platt corresponds to the secondary benzene band, namely ¹L_в. The third absorption band should to consider as p-π-conjugation of the whole molecule of 4-(2-methoxyphenyl)-5-methyl-2H-1,2,4-triazoles-3(4H)-thione.

In some cases (water, 0,1 M HCl, 1 M H₂SO₄) absorption spectra of 4-(2-methoxyphenyl)-5-phenyl-2H-1,2,4-triazoles-3(4H)-thione have more intense maximums at slight their hypsochromic displacement. This can be explained by the fact that in this case, in the process of electronic transitions dipole moment of the solute molecule decreases, and therefore in the excited state test compound situated in a tense cell of solvent, polar molecules of which are oriented in such way that they can not effectively stabilize the excited state. Therefore, in polar solvents, the energy of the basic state is lower than the energy of the excited state and this contributes to their hypsochromic displacement of electronic bands maximums of 4-(2-methoxy-phenyl)-5-phenyl-2H-1,2,4-triazoles-3(4H)-thione absorption. In addition, increasing the intensity of the observed absorption spectra can be explained also from the position of the spectroscopic moment (m). Increasing the intensity of the bands caused mainly by the inductive effect of phenyl substituent in position of 5-th cycles of the 1,2,4-triazoles molecule or if the contribution of the inductive effect and coupling effect are parallel. It should also to compare the UV-spectra of 4-(2-methoxyphenyl)-5-methyl-2H-1,2,4-triazoles-3(4H)-thione and of 4-(2-methoxy-phenyl)-5-phenyl-2H-1,2,4-triazoles-3(4H)-thione in hexane, because they are characterized by the presence of only one fine-structure absorption band within 250-270 nm with four maximums respectively at 250 , 256, 263, 270 nm. This band for both compounds is certainly "benzoic" by N.V. Klevens and J. R. Platt classification is a typical ¹L_B-band, namely it is caused by π→π*-transitions of benzene cycle.

In terms of the 4-(2-methoxyphenyl)-5-methyl-2H-1,2,4-triazoles-3(4H)-thione and of 4-(2-methoxyphenyl)-5-phenyl-2H-1,2,4-triazoles-3(4H)-thione structure it was interesting to study the presence of thion-thiol tautomerism in solvents of different polarity. It is known, tautomerism is dynamic isomerism, as it forms are easily turn on each another in solution. Electronic spectroscopy is successfully applied for studying the process of equilibrium establishing in cases when the when the chromophore group undergoes isomerization, namely occurs migration of the double bond. Scientific literature indicates that a simple thionеs are very unstable and so, studying of the thione-thiol tautomerism in contrast to keto-enol, amino-imine and azo-hydrazine is accompanied by certain complications appearance. In alcoholic solution 4-(2-methoxyphenyl)-5-methyl-2H-1,2,4-triazoles-3(4H)-thione and 4-(2-methoxyphenyl)-5-phenyl-2H-1,2,4-triazoles-3(4H)-thione are mainly exist in thion form, and higher band intensity at 263 nm of 4-(2-methoxyphenyl)-5-phenyl-2H-1,2,4-triazoles-3(4H)-thione is explained by the positive contribution of benzoic fragment into this molecule. The nature of the basic absorption band of 2.9 and 2.10 compounds with maximums at 252 nm (4-(2-methoxyphenyl)-5-methyl-2H-1,2,4-triazoles-3(4H)-thione) and 250 nm (4-(2-methoxyphenyl)-5-phenyl-2H-1,2,4-triazoles-3(4H)-thione) shows that in aqueous solution studying compounds are mainly represented in the thiol form, which can be explained by the strong coupling of phenyl radicals which impart to 1,2,4-triazoles "aromatic" character. At comparing the solutions 4-(2-methoxyphenyl)-5-methyl-2H-1,2,4-triazoles-3(4H)-thione and of 4-(2-methoxyphenyl)-5-phenyl-2H-1,2,4-triazoles-3(4H)-thione in acid we can concluded that in 0,1 M HCl studied by us compounds exist predominantly in the thiol form because an acidic environment makes it difficult of hydrogen ion transition from the -SH group up to N₂-atom. In 1 M H₂SO₄ solution concentration of thiol forms of the studied compounds significantly exceeds the same in 0,1 M hydrochloric acid solution and this may explain by the likely hydrogen ion addition to N₂- atom of 1,2,4-triazole cycle. In alkaline solution (0,1 M NaOH) thione-thiol equilibrium displaced toward the thiol form, because in the molecules of both compounds hydrogen ion in thiol grouping replaced by sodium cation, namely, occurs a partial consolidation of this form of tautomerism. According to the generally accepted classification hexane is inert aprotic solvent and therefore by the nature of the 4-(2-methoxyphenyl)-5-methyl-2H-1,2,4-triazoles-3(4H)-thion and of 4-(2-methoxy-phenyl)-5-phenyl-2H-1,2,4-triazoles-3(4H)-thion compounds UV-spectra in this case they exist in the balanced proportion of their thion and thiol forms.