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 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 25degreesC, 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 kN The Bronsted-type plot (logk(N) against amine pK(a)) is linear with a slope value of beta = 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. (C) 2002 Wiley Periodicals Inc.