Evaluation of Methylthio and Phenylthio Group Effects in the Solvolyses of 2-Chloro-2-(methylthio)acetone and 2-Chloro-2-(phenylthio)acetophenone Using Extended Grunwald-Winstein Equations

摘要

The specific solvolysis rates of 2-chloro-2-(methylthio)acetone (3) at 25.0 oC, and 2-chloro-2-(4-methoxyphenylthio)- acetophenone (4) at 50.0 oC have been measured in more than 34 aqueous solvent mixtures including aqueous 2,2,2-trifluoroethanol (TFE), for 4, and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), and in TFE-ethanol. The kinetic solvent isotope effect in methanol and solvolysis rate ratios, at identical ionizing power in 40 (v/v)% ethanol/water compared with 97 (w/w)% TFE/water have been determined. Additional data were also obtained for solvolyses of other substrates related to this work. The investigation of structural effects, caused by the existence of neighboring sulfur-containing and carbonyl (C=O) groups, on the solvolytic reactivity and/or mechanism and the possibility for the stabilization of the solvolytic TS by the developing mesomeric cation, due to the role of the PhS-group, have been attempted using various Grunwald-Winstein (GW) parameter treatments. Regardless of -COPh or -COCH3 neighboring groups as neighboring group, the solvolytic reactions for both 3 and 4 were evaluated as reactions primarily influenced by the sulfur atom of the ?SR group as the activator to make the positive polar TS. In particular, the solvolytic reactions of 4, having an electron donating p-OCH3 substituent, were indicative of the possibility of an ionization pathway with development of a mesomeric cation, stabilised by the sulfur atom and its attached group.