Thioenol tautomerization

Thioketo compounds that can undergo enolization exist as thioketo-thioenol tautomeric systems such systems show S—H stretching absorption. The thioenol tautomer of ethyl thiobenzoylacetate,... 

Chemically, the grouping R-CS-N- as been referred to as thioamide, thionamide, thiocarbamide, or if R is N, as it is in thiouracil, PTU, and MMI, it is called a thioureylene. This structure may exist in either the thioketo or thioenol tautomeric forms. 

After thioformaldehyde, the thiocarbonyl compound that has received most attention is thioacetone. Unlike thioformaldehyde, it can be kept in monomeric form, but only if kept below - 50° C (30). It is a red oil that freezes at about — 55° C and boils at about 70° C. If pure it polymerizes to a white solid in a few hours at — 78° C or quite rapidly at room temperature. It also exists as a thioenol tautomer, which is stable at temperatures below — 50° C but which tautomerizes at higher temperatures to give an equilibrium mixture of thioketo and thioenol forms. 

Spectra of these tautomers were obtained on samples separated by gas chromatography from thioketo-thioenol mixtures. Trithioacetone pyrolysate was passed through a DC 200 silicone oil on Gas Chrom Z packed column. As the appropriate sample eluted from the column, it was condensed on a sodium chloride plate cooled to —100° C and its spectrum immediately determined. The very low temperature employed prevented loss of thioenol by tautomerization and of thioketo by polymerization. 

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