Kinetic evidence for the occurrence of general acid-base catalysis in N-methylhydroxylaminolysis of N-ethoxycarbonylphthalimide (NCPH)

Kinetic evidence for the occurrence of kinetically detectable intermediates in the cleavage of N‐ethoxycarbonylphthalimide under N‐methylhydroxylamine buffers

“…The value of k ga is associated with high standard deviation and hence it is not very reliable. The respective values of k gb (=54 M −2 s −1 ) and k ga (=4.2 M −2 s −1 ) are nearly 9‐fold larger and 25‐fold smaller than the corresponding values of k gb (=6.2 M −2 s −1 ) and k ga (=111 M −2 s −1 ) for acetate buffer 19. These results are conceivable for the reason that the acetic acid (p K a 4.68) is a stronger acid than CH 3 NH 2 OH + (p K a 6.24) by 1.56 pK units.…”

“…Kinetic data on hydroxylaminolysis of NCPH have revealed the occurrence of a stepwise mechanism 23. The occurrence of intermolecular general acid–base catalysis in the reaction of CH 3 NHOH with NCPH has been explained in terms of a stepwise mechanism 19. Significantly lower nucleophilic reactivity of CH 3 NHOH (p K a 6.24) compared with HNHOH (p K a 5.97) toward NCPH and NHPH is difficult to explain in terms of steric hindrance.…”

“…The details of the kinetic procedure were same as described elsewhere 19. The observed data for a few typical kinetic runs are shown as the plots of observed absorbance at 300 nm ( A obs ) versus reaction time ( t ) in Fig.…”

“…Since, in the present reacting system, k 1 obs ≫ k 2 obs and k 3 obs , the values of k 1 obs and E NC were calculated from Eq. (3) using the observed data points in the initial phase of the reaction through a technique described elsewhere 19. …”

“…Under typical reaction conditions, the reactions of N ‐hydroxyphthalimide with hydroxylamine 17 and N ‐methylhydroxylamine 18 produced cyclic stable intermediates formed presumably by the intramolecular nucleophilic attack of hydroxyl moiety at the most electrophilic carbonyl carbon of the respective aminolysis immediate products, o ‐( N ‐hydroxycarbamoyl)benzohydroxamic acid and o ‐( N ‐methyl‐ N ‐hydroxycarbamoyl)benzohydroxamic acid. In a study aimed at diagnosing the occurrence of intermolecular general acid and general base catalysis in N‐methylhydroxylaminolysis of N ‐ethoxycarbonylphthalimide (NCPH), it has been observed that the plots of observed absorbance ( A obs ) versus reaction time ( t ) contained both minima and maxima 19. The occurrence of maxima in these plots that revealed the conversion of immediate N‐methylhydroxylaminolysis product to another stable intermediate product was not kinetically investigated.…”