Mechanistic change in catalyzed reactions

Sometimes, a catalyzed reaction follows a different reaction mechanism compared to that of its uncatalyzed counterpart, and by changing the reaction mechanism, a catalyst changes the overall activation energy or overall energy barrier of the reaction through predominant effects on either GS or TS. 

Detailed kinetic stndies on the rates of hydrolysis , aminolysis , and thiolysis of 9-aminoacridine and its A-substituted derivatives revealed that the rate of thiolysis is mnch faster than those of aminolysis and hydrolysis under similar experimental conditions. This difference in the reactivity of thiols and amines as well as HO/H2O toward 9-aminoacridine and its N-substituted derivatives is ascribed partly to the fact that HO/H2O, RO/ROH, and amines are hard, whereas thiols are soft nncleophiles, and as the reactive centers in 9-aminoacridine and its Af-substituted derivatives are soft electrophilic in nature, a soft nucleophile should react much faster than a hard nucleophile of comparable basicity. 

In a typical kinetic run at 37°C, the UV-visible spectra of the reaction mixture containing 0.2-Af ME buffer of pH 8.8 and 4.2 x Kh M 9-ANA were scanned at different reaction time ranging from 90 sec to 19 h. The appearance and disappearance of the intermediate product, 9-acridinyl 2-hydroxyethy thioether (IP), was confirmed owing to increase in the absorbance valne of a characteristic absorption peak at 363 nm because of IP in the initial phase of the reaction (reaction time, t ranges from 90 sec to 44 min) followed by the decrease in its absorbance value in the final phase of the reaction (t ranges from 44 min to 19 h). The UV-visible spectrum of the reaction mixture scanned at t = 44 min was similar to that of 9-acridinyl n-propyl thioether. o The UV-visible spectrum of the reaction mixture scanned at t = 19 h was similar to a typical absorption spectrum 

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