Concerted mechanisms of the reactions of ethyl S-aryl thiocarbonates with substituted phenoxide ions

The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl O-ethyl thiolcarbonates with substituted phenoxide ions are subjected to a kinetic study in water, 25.0 degrees C, ionic strength 0.2 M (KCl). By fol lowing the reactions spectrophotometrically, pseudo-first-order rate coeffi cients (k(obsd)) are found under excess of the nucleophile. Plots of k(obsd ) VS phenoxide anion concentration at constant pH are linear, with the slop e (k(N)) independent of pH. The Bronsted-type plots (log kN vs pK(a) of the phenols) are linear with slopes beta = 0.92, 0.77, and 0.61 for the reacti ons of the 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl deriv atives, respectively. For these reactions, a concerted mechanism is propose d since the slope values are similar to those found in the concerted phenol ysis of aryl acetates; the slope magnitudes are not consistent with a stepw ise mechanism where the formation of a tetrahedral intermediate is rate lim iting. Our results are in line with the finding that the reactions of the 2 ,4-dinitrophenyl and 2,4,6-trinitrophenyl derivatives with secondary alicyc lic amines in water are concerted. In contrast, the reactions of the same s ubstrates with pyridines are stepwise, which means that substitution of a p yridine moiety in a tetrahedral intermediate with a benzenethio group by a phenoxy group destabilizes the intermediate.