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Kinetics and mechanism of large rate enhancement in the alkaline hydrolysis of N′-morpholino-N-(2′-methoxyphenyl)phthalamide
Sim, Yoke-Leng,Ariffin, Azhar,Khan, M. Niyaz
, p. 3730 - 3737 (2008)
(Chemical Equation Presented) The apparent second-order rate constant (kOH) for hydroxide-ion-catalyzed conversion of 1 to N-(2′-methoxyphenyl)phthalamate (4) is ～103-fold larger than kOH for alkaline hydrolysis of N-morpholinobenzamide (2). These results are explained in terms of the reaction scheme 1→ k1obs3→k2obs4 where 3 represents N-(2′- methoxyphenyl)phthalimide and the values of k2obs/k1obs vary from 6.0 × 102 to 17 × 102 within [NaOH] range of 5.0 × 10-3 to 2.0 M. Pseudo-first-order rate constants (kobs) for alkaline hydrolysis of 1 decrease from 21.7 × 10-3 to 15.6 × 10-3 s-1 with an increase in ionic strength (by NaCl) from 0.5 to 2.5 M at 0.5 M NaOH and 35°C. The values of kobs, obtained for alkaline hydrolysis of 2 within [NaOH] range 1.0 × 10-2 to 2.0 M at 35°C, follow the relationship kobs = kOH[HO-] + kOH′[HO -]2 with least-squares calculated values of kOH and kOH′ as (6.38 ± 0.15) × 10-5 and (4.59 ± 0.09) × 10-5 M-2 s-1, respectively. A few kinetic runs for aqueous cleavage of 1, N′-morpholino- N-(2′-methoxyphenyl)-5-nitrophthalamide (5) and N′-morpholino-N- (2′-methoxyphenyl)-4-nitrophthalamide (6) at 35°C and 0.05 M NaOH as well as 0.05 M NaOD reveal the solvent deuterium kinetic isotope effect (= kobsH2O/kobsD2O) as 1.6 for 1, 1.9 for 5, and 1.8 for 6. Product characterization study on the cleavage of 5, 6, and N-(2′-methoxyphenyl)-4-nitrophthalimide (7) at 0.5 M NaOD in D 2O solvent shows the imide-intermediate mechanism as the exclusive mechanism.
