The apparent dipole moments and molar volumes of 1,3-benzothiazole and some derivatives, in nonpolar solvents

Abstract

The apparent molecular polarizations at infinite dilution, P_2∞, molecular refractions, R_D, dipole moments, µ₂, polarizabilities, α, and partial molar volumes, V ⁰_2,m, of 1,3-benzothiazole and some derivatives at position two or three, have been determined from relative permittivity, density and refractive index measurements in n-hexane or 1,4-dioxane solutions at 25 °C.

The R_D and V ⁰_2,m values correlate well for all substances studied, independently of the solvent used. The P_2∞ values appear to be slightly dependent on the solvent showing the presence of specific interactions as hydrogen bond.

The apparent relative permittivity, ε₂′, and the refraction index extrapolated at the IR region, n_IR, have been estimated for all solutes. In the case of 1,3-benzothiazole and its 2-methyl derivative, these results have been compared with the relative permittivities measured at 25 °C at low and high frequencies, respectively, by the Cole–Cole dynamic method, showing a good agreement.

The apparent dipole moments and their changes with the corresponding substituent group have been determined by three different methods. From the measurement of dipole moment it has been unambiguously established that the tautomeric equilibrium between 2-mercapto-1,3-benzothiazole and 2-thioxo-3H-1,3-benzothiazole is strongly displaced to the latter. Changes in the apparent molar solution volumes and apparent molar polarizations of the solutes in the considered solvents are qualitatively discussed.

Comparison between pK_a values reported in the literature and the corresponding µ values for free base benzothiazole derivatives have been carried out as well as the appropriate qualitative discussion.