Enhanced reactivity in the ammonolysis of phenyl thiolacetates in aqueous medium

✍ Scribed by D. Rajarathnam; J. Babu; P. Ananthakrishna Nadar

• Publisher: John Wiley and Sons
• Year: 2001
• Tongue: English
• Weight: 153 KB
• Volume: 34
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.10014

✦ Synopsis

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 p__K__~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. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 18–26, 2002