Enhanced reactivity in the ammonolysis of phenyl thiolacetates in aqueous medium

Abstract

The ammonolysis of several substituted phenyl thiolacetates is kinetically studied in aqueous medium, 18°C, ionic strength 0.1 M (KCl). By following the leaving groups spectrophotometrically (λ max = 260-410 nm), under excess free ammonia, pseudo-first-order rate coefficients (k obs) are obtained. The plots of (k obs - k H) against free ammonia concentration are linear at constant pH. The macroscopic nucleophilic substitution rate coefficients (k N) are obtained as the slopes of these plots and found to be pH-independent for all the thiolesters. The Brönsted-type plot (log k N against pK a of leaving groups) and the Hammett plot (log k N against a values of substituents) obtained for the title reactions of thiolesters are linear with slope values of β lg = -0.34 and ρ = 0.74 respectively. From the magnitude of these values, experimental data, the kinetic law, and the analysis of products, it is deduced that the ammonolysis of thiolesters proceeds through a simple bimolecular nucleophilic substitution pathway with a zwitterionic tetrahedral addition intermediate (T ±), whereby its formation is rate-determining (k l step). Comparison of this reaction of thiolesters with a similar reaction of analogue oxyesters shows a mechanistic difference. Further, for thiolesters there is a rate enhancement with larger k N values. The change in mechanism and enhanced reactivity observed by substitution of the oxygen atom by sulphur atom on the phenyl moiety are discussed in detail. © 2001 John Wiley & Sons, Inc. Int. J. Chem. Kinet.