THE ACID DISSOCIATION-CONSTANT OF TRIPHENYLETHENETHIOL, A SIMPLE THIOENOL, AND THAT OF ITS OXYGEN-ENOL ANALOG

The acidity constant, pQ(a)(E) = 8.49, for the stable thioenol, triphe nylethenethiol, was determined by spectrophotometric titration, and th at, pQ(a)(E) = 11.37, for its unstable oxygen analog, triphenylethenol , was determined by analysis of its ketonization rate using enol gener ated flash photolytically from triphenylvinyl bromide. (These acidity constants are concentration quotients applicable at 0.10 M ionic stren gth in 50 vol.% aqueous methanol.) This appears to be the first determ ination of the acid strength of a simple thioenol. Triphenylethenethio l was unreactive in dilute acid or base, but in concentrated perchlori c acid solutions it was slowly transformed into diphenylacetophenone a nd 2,3-diphenylbenzo[b]thiophene; the mechanisms of these reactions ar e believed to involve rapid equilibrium protonation of the enol on the beta-carbon followed by rate-determining capture of the cation so for med, either externally by solvent or internally by one of the beta-phe nyl groups of the ion.