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) dioxa ne in water, 25.0 degrees C, ionic strength 0.2 M (KC1). By following the reactions spectrophotometrically, pseudo-first-order rate coeffici ents (k(obsd)) are found under an excess of the nucleophile. Plots of k(obsd) vs phenoxide anion concentration at constant pH are linear, wi th the slope (k(N)) independent of pH. The Bronsted-type plots (log k( N) vs pK(a) of the phenols) are linear with slopes beta = 0.55 and 0.4 7 for the reactions of the phenyl and the 4-nitrophenyl derivatives, r espectively. These Bronsted slopes are in agreement with the ones foun d in the concerted reactions of the same nucleophiles with reactive ph enyl esters and acetic anhydride in water. In contrast to the concerte d mechanism of the title reactions that of the same substrates with se condary 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.