Structure‐reactivity correlation in the aminolysis of 4‐fluorophenyl acetate in aqueous medium

The reaction of the title substrate with a series of amines of varying p K 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 (log k N against amine p K 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. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 366–373, 2002