Effects of [NaBr] on the Rate of Intramolecular General Base-Assisted Hydrolysis of N-(2′-Hydroxyphenyl)phthalimide in the Presence of Cationic Micelles: Kinetic Evidence for the Probable Micellar Structural Transition

Abstract

Pseudofirst-order rate constants for aqueous cleavage of N-(2'-hydroxyphenyl)phthalimide (1), obtained at 0.001 M NaOH, 2 x 10(-4) M 1, 2% v/v CH(3)CN, and 30 degrees C, show a nonmonotonic decrease with the increase in the total concentration of cetyltrimethylammonium bromide ([CTABr](T)) within its range >/=9 x 10(-5)-<or= 0.17 M. Similar observations have been obtained in the presence of the constant concentration of NaBr at <or=0.02 M. The values of k(obs) become independent of [CTABr](T) at >or=0.04 M CTABr and within a [NaBr] range of 0.0-0.005 M. These observations, in view of the pseudophase (PP) model of the micelle, reveal the presence of presumably spherical micelles at <or=3 x 10(-4) M CTABr in the presence of a constant concentration of NaBr within its range of 0.0-0.01 M. The average value of the CTABr micellar binding constant (K(S)) of ionized 1 (i.e., 1(-)), under these conditions, is (1.88 +/- 0.62) x 10(3) M(-1). The increase in [CTABr](T) at >or=4 x 10(-4) M causes a micellar structural transition from most likely spherical to cylindrical, which is evident from the increase in K(S) values from 3.46 x 10(3) to 11.4 x 10(3) M(-1) with the increase in [CTABr](T) from 4 x 10(-4) to approximately 1 x 10(-3) M in the absence of NaBr. The values of k(obs) at different [NaBr] and at a constant [CTABr](T) follow a kinetic relationship derived from an empirical equation coupled with a PP model of micelle. This relationship gives the value of a kinetic parameter, F(X/S), which represents the fraction of micellized S(-) (S(-) = 1(-)) transferred to the aqueous phase by the limiting concentration of X(-) (X(-) = Br(-)) through ion exchange X(-)/S(-). The value of F(Br/1) is 0.65 +/- 0.12.

Proximity effect on the general base catalysed hydrolysis of amide linkage: The role of cationic surfactant, CTABr

“…Our observation is similar to that of Khan et al 18 Thus the observed inhibitory effect of CTABr (see figure 5) on the rate of hydrolysis of the dianion of I, (I-2H) 2− is consistent with the absence of the OH − dependent path, the reaction being mediated by the general base catalytic effect of the phenoxide groups favourably disposed around the reaction centre and directing a molecule of water to the amide C=O centre. Figure 5 further depicts that there is a catalytic effect of CTABr at <0.001 mol dm −3 .…”

Kinetic Evidence for Near Irreversible Nonionic Micellar Entrapment ofN-(2′-Methoxyphenyl)phthalimide (1) under the Typical Alkaline Reaction Conditions

“…The observed data failed to obey the pseudophase micellar (PM) model at >1.4 mM C 12 E 23 because the values of micellar binding constant of 1 ( K S ) increase significantly (~10 3 -fold) with the increase in [C 12 E 23 ] T from 1.4 to 50 mM at 1.0 mM NaOH (Tables 1 and 3). Similar but not identical observations have been obtained in CTABr-(cetyltrimethylammonium bromide-) mediated pH-independent hydrolysis of N -(2-hydroxyphenyl)phthalimide [17]. The scenario exhibited by these observations supports the multicompartmental model of micelle [2, 18, 19] and it may best be represented by Scheme 1, where n 1 1 M molecules are in equilibrium with n 1 W molecules and equilibrium or micellar binding constant K S 1 at R ≤ 2 and [NaOH] = 1.0 mM.…”

Effects of nonionic micelles on the rate of alkaline hydrolysis of N-(2′-methoxyphenyl)phthalimide (1): Kinetic and rheometric evidence for a transition from spherical to rodlike micelles under the typical reaction conditions

“…It appears that no single technique can fully characterize a complex fluid system, and hence the best approach to rule out shear artifacts is to combine more than one technique. 1 H NMR [3] and rheometric [7,34] measurements supported the cationic micelle-catalyzed kinetic data for sphere-to-rodlike micellar structural transition with increasing values of the total concentrations of cationic surfactants. But the studies [4][5][6]8] provided only the micelle-catalyzed kinetic data as the evidence for sphere-to-rodlike micellar structural transition.…”

“…The value(s) of either k M or K S or both k M and K S may be expected to be sensitive to the micellar structural changes [7,8,34]. Thus, the observed kinetic data may not be expected to fit to Eq.…”

“…The detection of a micellar structural transition by kinetic measurements appears to be rather recent [3][4][5][6][7][8]34]. It appears that no single technique can fully characterize a complex fluid system, and hence the best approach to rule out shear artifacts is to combine more than one technique.…”