Хімія та хімічні технології/ 6. Органічна хімія

Panasenko O. I., Buryak V. P., Samura T. O., Panasenko T. V., Gotsulya A. S., Timoshik U. V., Guzhva A. A., Vovnjanko O. I.

The investigation of UV-spectra of some thiazoles and benzothiazoles

Zaporozhye state Medical University

National University of the life and environmental sciences of Ukraine

Abstract. The UV-spectra of thiazoline-2-thione, 3-N-methyl-benzthiazoline-2-thione and 2-methyl-thiobenzthiazole in range of 2-thione, 3-N-methylbenzthiazoline-2-thione and 2-methyl-thiobenzthiazole in a range of solvent have been determined, solvent effects noted and assignments given for the main absorption bands. The spectra of thiazoline-2-thione and thiazolidine-2-thione are interpreted by their relationship to the spectra of dithiocarbamates, the intense long-wavelength UV-absorption band in the spectra being assigned to an intramolecular charge-transfer transition involving the thioamide grouping. The UV-spectra of benzthiazoline-2-thione and 3-N-methylbenzthiazoline-2-thione are related to the spectra of benzthiazole and thioamides. The very intense long-wavelength band in the spectra of these compounds is also assigned to an intra-molecular charge-transfer transition involving the thioamide grouping.

The most extensive study of the ultra-violet spectra of thioamides is that of Brand and Eglinton which has been reported in a thesis [5] and a series of papers [3, 8, 9] published within the last few years. However, there has been little systematic study of the spectra of heterocyclic thioamides despite the many papers which have reported the spectrum of benzthiazoline-2-thione (I) which is an industrially important accelerator for the vulcanization of unsaturated elastomers [2]. Although (I) is often referred to as 2-mercaptobenzthiazole (II), the very close similarity of the intense long-wavelength UV absorption band with that of the N-Me derivative (III), and its dissimilarity from that of the S-Me derivative (IV), has been taken as evidence in favor of structure (I).

I II III IV

More direct evidence that 2-mercapto-benzthiazole has structure (I) is that in dilute solution in carbon tetrachloride there is a free-NM band at 340 nm. Also [1] report that in the crystalline state compound (II) molecules exist in the thione form, and are arranged in hydrogen bonded helical chains. Browning in 1986 attempted a partial description of the origins of the absorption bands of benzthiazoline-2-thione and painted out the difficulty of giving a full interpretation due to the unavailability of the spectra of reference compounds.

Recently, in article was reported a study of thiazole (V), and benzothiazole (VI), and given assignments which show the derivation of the spectra of these compounds from thiophen and benzo-[β]-thiophen respectively. In the present work author describe the spectra and where possible give assignments for thiazolidine-2-thione (VII), thiazone (I), 3-N-methylbenzthiazoline-2-thione (II) and 2-methyl-thiobenzthiazole (IV):

V VI VII VIII

The thione compounds (I), (VII), and bands [4] in the solid state and solution. The stability at room temperature of the hydrogen bonded species in solution decreases in the order (VIII), (I), (VII) [4]. The spectra were measured with a «Specord 200-222U214» spectrophotometer using matched 10 mm and variable path length silica cell. Errors in measurements of absorption coefficients are less than 5 per cent wave numbers are given to ± 4 nm, but because of the broadness of some of the maxima it was felt that the assignment of one frequency value was unjustified and accordingly limits between which the maximum lies have been quoted. Limits have also quoted for shoulders and inflections.

The ultra-violet spectra of thiazoline-2-thione in solution in n-hexane there are two intense bands at 202 and 312 nm respectively. Between these bands there are two shoulders at 222-227 nm and 244-253 nm, of much lower intensity. On the long wavelength side of band there is a shoulder at 323-328 nm. The spectra in water and other hydroxylic solvent closely resemble those in the hydrocarbon solvents, apart from the disappearance of the shoulder in the region 222-227 nm and that on this band and a red shift of band (323-328 nm) in methanol and ethanol. In 2M hydrochloric acid the position of band 323-328 nm and shoulder 244-253 nm negligibly different from those in water. In dilute sodium hydroxide only one band, at 294-295 nm, is resolved. The spectrum in concentrate sulfuric acid has a band at 293 nm, a broad shoulder extending from 204-215 nm, and a peak at 192-194 nm. The ultra-violet spectra of thiazolidine-2-thione in solution in n-hexane there are two intense bands 278-279 nm and 202 nm respectively. Between these bands are a shoulder at 217-224 nm and a subsidiary maximum at 247 nm. The spectrum for a solution in methanol shows a blue-shift in band to 275 nm and a merging of the subsidiary maximum 254 nm with band 275 nm to give a prominent shoulder at 250-254 nm. In ethanol band 275 nm and shoulder 254-256 nm are in the same position as in methanol. In water band 270 nm shows a further blue shift, and band 207-211 nm is broadened with a red shift. In dilute aqueous hydrochloric acid, band 270 nm is broader than in water and is asymmetric on the short-wave length side although no inflection can be resolved near 245 nm. In aqueous sodium hydroxide there is one broad band at 241 nm. The spectrum in concentrate sulfuric acid is quite different from those in the other solvents and consists of the broad band with a main maximum at 240 nm and a subsidiary maximum at 263-265 nm. In the spectra in benzene and ethanol there is a low intensity maximum (E_max.=100), band 352 nm and 330 nm respectively. This band was not observable in the hydrocarbon solvents and water.

The UV-spectra of solutions of benzthiazoline-2-thione and 3-N-methyl-benzthiazoline-2-thione have three main bands A, B and C of closely comparable extinction coefficients, which occupy the wavenumber regions 196-204 nm, 222-224 nm and 323-333 nm respectively. Including subsidiary maxima and inflections there ore ten spectral features, not all of which ore observable in every solvent. Band has a maximum occurring in the region 323-333 nm, and in hydrocarbon solvents also has two shoulders on either side of the maximum. The series of four inflections lying between 250 and 299 nm are not observable in n-hexane solution whereas in aqueous solution, they are clearly visible, being resolved as a small peak. Band B consists of peak with a maximum lying between 229 and 235 nm with shoulder on the lower frequency side. For the spectrum of 3-N-methyl-benzthiazoline-2-thione is resolved as a peak in hydrocarbon and methanol solution, whereas for the spectrum of benzthiazoline-2-thione in the same solvents is a broad shoulder. Shoulder at 247 nm is not resolved for either compound in water or aqueous hydrochloric acid. Band C consists of the single peak which occurs at 197 nm for each compound in n-hexane and 204-205 nm in water. The extinction coefficients of bands A, B, C in water are respectively 24700, 14000 and 20200 for benzthiazoline-2-thoine and 23500, 13850 and 15350 for 3-N-methyl-benzthiazoline-2-thione. The spectrum of 3-N-methyl-benzthiazoline-2-thione is solution in dilute alkali resembles very closely the spectrum in water and dilute hydrochloric acid, whereas the spectrum of ionized benzthiazoline-2-thione in dilute alkali is different from that in water, in that band a has a maximum at 312 nm and an extinction coefficient of 20500. The spectra of these two compounds protonated in concentrate sulfuric acid are very similar but differ considerably from the unionized species. The UV-spectrum of solutions of 2-methyl-thiobenzthiazole has three main bands D, E, F which occupy the regions 198-200 nm, 223-228 nm and 278-302 nm respectively. The extinction coefficient of E and F are almost identical whereas that of D is about 60 percent lower. Including subsidiary maxima and inflections, there are seven spectral features. Band D has the main maximum at 278-280 nm and two subsidiary maxima in the region 288-291 nm and 299-302 nm respectively. The two features are observable in both hydrocarbon and aqueous solutions. Band E consists of peak which occurs in the region 198-200 nm. The extinction coefficients for bands D, E and F in water are respectively 13100, 18800 and 18600. The spectrum in dilute aqueous alkali is essentially the same as that in water. In dilute hydrochloric acid the spectrum is altered considerably.

The extinction coefficient of band D is increased to 17750 and subsidiary maxima disappear leaving a brood maximum at 307-308 nm. The extinction coefficient of band E is diminished to 10600 leaving the peak and shoulder in the 238-256 nm region appearing more prominent. Band F was not completely observable because of solvent out-off. The spectrum in concentrate sulfuric acid closely resembles that in 2M hydrochloric acid. Solute-solvent hydrogen bonding and not to differences in dielectric constant and refractive index of the solvents since there is a very wide range.

Results. UV-spectra of thiazoline-2-thione and thiazolidine-2-thione have four and five main spectral features respectively. There systems are similar to the thioamides except that the -NH-C=S system forms part of the thiazoline ring. The UV-spectra of both thiazoline-2-thione and the saturated thiazolidine-2-thione are readily interpreted in terms of the four bands, that in to be present in nearly all thioamides. The positions of these bands and their relationship to the four main bands characteristic of thioamides are show in our investigation by reference to the ultra-violet spectra of the methyldithiocarbamates.

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