The 3-Thioxo-1,2-Dithiol-4-yl Group: A Versatile One Endowed Also With a –R Electronic Effect

Abstract

The pK_a values in an aqueous solution of some 4-aminodithiolethiones, 4-aminodithiolones, 5-phenyl-4-hydroxydithiolethione, and 5-phenyl-4-hydroxydithiolone were determined by UV-VIS spectrophotometry at 298 K. These compounds exhibited unexpectedly very low pK_a values in the range of 0–2 for amino derivatives and in the range of 7 for hydroxy derivatives. Electronic structure calculations by ab initio and DFT methods together with an X-ray measurement indicated that these results may be attributed to a markedly negative π charge on the carbon 5 together with a positive π charge on the basic substituent of the carbon 4. As a result, the dithiole nucleus exerts in this case a strong ─R effect. Although they were very low, pK _a values of 4-aminodithiolethiones remained still higher than those found with 5-aminodithiolethiones, which were also determined in this work for comparison. The latter were not unexpectedly in the range -3 < pH < −2.

Keywords:

• 1,2-dithiole-3-thione
• acidity constant
• ab initio Hartree-Fock
• electronic structure
• density functional theory

Acknowledgments

The authors would like to thank Dr. Eric Furet for helpful discussions on the theoretical calculations part of this work. We thank also Mr. G. Bouer for his assistance in preparing the English manuscript.

Notes

^a The 95% confidence limits on the average value are given by –log(K_a ± 2.447 σ_n−1/n^1/2) according to a student's table.

^bpK_a determined in methanol-water 2% (v/v) at 298 K.

^cpK_a determined in ethanol-water 5% (v/v) at 298 K.

^dpK_a determined in methanol-water 1% (v/v) at 298 K.