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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 σ 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 1 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.

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The kinetics of aminolysis of several substituted phenyl acetates by imidazole is studied in aqueous medium at 20 • C and an ionic strength of 0.1 M (KCl). By following the leaving groups spectrophotometrically (λ max = 272-401 nm), under excess free imidazole, pseudofirst-order rate constants (k obs ) are obtained. For the esters with good nucleofuges, the reaction follows clean second-order kinetics and the plots of (k obs − k H ) against free imidazole concentration are linear at constant pH. The macroscopic nucleophilic substitution rate constants (k N ) are obtained as the slopes of these plots and found to be pH independent. For the esters with poor nucleofuges, a rate dependence on more than first power of the free imidazole and a linear dependence of k 2 on free imidazole is observed. Accordingly, the microscopic rate constants for the assisted paths viz. k ga and k gb have been disseminated besides for simple bimolecular attack. The Brönsted-type plots and Hammett plots were constructed whose slope values are consistent with a stepwise mechanism through a bipolar tetrahedral addition intermediate whose formation or breakdown being rate determining for various paths. Comparison of this reaction of oxyesters with the earlier reported works on similar reaction of analogue thiolesters under identical reaction conditions showed remarkable mechanistic differences which are discussed in detail. The discussion is extended to include the details on previously studied ammonolysis of these two types of esters wherein thiolesters showed differed reactivity than that reported in the present study. C 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: [211][212][213][214][215][216][217][218][219][220][221] 2005

The reaction of the title substrate with a series of amines of varying pK a , viz. ammonia, ethanolamine, glycine, 1,2-diaminopropane, 1,3-diaminopropane, n-butylamine, piperidine, hydrazine, imidazole, and hydroxylamine is subjected to a kinetic study in aqueous medium, 25 • C, ionic strength 0.1 M (KCl). Pseudo-first-order rate coefficients (k obs ) are found throughout under amine excess at various pH values for each amine. For amines, excluding hydrazine, ammonia, and hydroxylamine the reaction follows clean second-order kinetics and the plots of (k obs − k H ) against free amine 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 hydrazine, ammonia, and hydroxylamine, a rate dependence on more than first power of the amine is observed, accordingly, the rate constants for the assisted paths have been disseminated for these amines besides k N . The Brönsted-type plot (logk N against amine pK a ) is linear with a slope value of β = 1.02. The magnitude of the slope value is consistent with a stepwise mechanism through a zwitterionic tetrahedral addition intermediate whose breakdown to products is rate-determining (k 2 step). A remarkable reactivity difference is observed among the diamines, the reason for which is discussed in detail.