β-Thioketones. Part 6. Electronic Absorption Spectra of Aromatic β-Thioketones. A Study of Enol-Enethiol Tautomerism

Article abstract of DOI:10.1039/P29800001768

Aromatic β-thioketones exist in solution as mixture of rapidly interconverting Z-enol and Z-enethiol tautomers.The electronic absorption spectra exhibit in general four absorption bands in the u.v.-visible region at ca. 265 (ArC=O ?,?^*; ArC=C ?,?^*), 330 (ArC=S ?,?^*; O=CC=CS ?,?^*; C=O n,?^*), 415 (OC=CC=CS ?,Π^*), and 520 nm (C=S n,Π^*), respectively.The β-thioxoketones are converted by sodium hydroxide into the corresponding anions.CNDO/B Calculations predict that the negative charge in the β-thioxoketonates is delocalized over the OCCCS system, suggesting simultaneously sickles or W shaped conformations.Two characteristic absorption bands found for the β-thioxoketonates at ca. 275 and 400 nm are assigned to ?,?^* transitions involving the Ar-C<*>C<*>C-Ar' and S<*>C<*>C<*>C<*>O chromophores, respectively.The enol-enethiol tautomeric equilibrium has been studied by means of low temperature spectroscopy.At room temperature equilibrium constants (K₂₉₃) of 3-5 have been found corresponding to a 4:1 enol-enethiol concentration ratio.The reaction entropy (ΔS_r) has been found to be negative for the enethiol->enol conversion,reflecting the intramolecular O-H...S hydrogen bonds to be considerably stronger than the corresponding O...H-S hydrogen bond.Variation in ΔS_r and K₂₉₃ as functions of substitution in the aryl group next to oxygen are discussed.

Full text of DOI:10.1039/P29800001768

View Article Online / Journal Homepage / Table of Contents for this issue

View Article Online

View Article Online

View Article Online

View Article Online

View Article Online