Concerted Mechanisms of the Reactions of Phenyl and 4-Nitrophenyl Chlorothionoformates with Substituted Phenoxide Ions.

The title reactions are subjected to a kinetic study in 3% (v/v) dioxane in water, 25.0 degrees C, ionic strength 0.2 M (KCl). By following the reactions spectrophotometrically, pseudo-first-order rate coefficients (k(obsd)) are found under an excess of the nucleophile. Plots of k(obsd) vs phenoxide anion concentration at constant pH are linear, with the slope (k(N)) independent of pH. The Brönsted-type plots (log k(N) vs pK(a) of the phenols) are linear with slopes beta = 0.55 and 0.47 for the reactions of the phenyl and the 4-nitrophenyl derivatives, respectively. These Brönsted slopes are in agreement with the ones found in the concerted reactions of the same nucleophiles with reactive phenyl esters and acetic anhydride in water. In contrast to the concerted mechanism of the title reactions that of the same substrates with secondary alicyclic amines is stepwise, which means that substitution of an amino moiety in a tetrahedral intermediate with a phenoxy group by another phenoxy group destabilizes the intermediate to the point that it no longer exists.