Kinetic Coupled with UV Spectral Evidence for Near-Irreversible Nonionic Micellar Binding of N-Benzylphthalimide under the Typical Reaction Conditions:  An Observation Against a Major Assumption of the Pseudophase Micellar Model

Abstract

Pseudo-first-order rate constants (k(obs)) for alkaline hydrolysis of N-benzylphthalimide (1) show a nonlinear decrease with the increase in [C(m)E(n)]T (total concentration of Brij 58, m = 16, n = 20 and Brij 56, m = 16, n = 10) at constant [CH(3)CN] and [NaOH]. These nonionic micellar effects, within the certain typical reaction conditions, have been explained in terms of the pseudophase micellar (PM) model. The values of micellar binding constants (KS) of 1 are 1.04 x 10(3) M(-1) (at 1.0 x 10(-3) M NaOH) and 1.08 x 10(3) M(-1) (at 2.0 x 10(-3) M NaOH) for C(16)E(20) as well as 600 M(-1) (at 7.6 x 10(-4) M NaOH) and 670 M(-1) (at 1.0 x 10(-3) M NaOH) for C(16)E(10) micelles. The pseudo-first-order rate constants (kM) for hydrolysis of 1 in C(16)E(20) micellar pseudophase are approximately 90-fold smaller than those (kW) in water phase. The values of kM for hydrolysis of 1 in C(16)E(10) micelles are almost zero. Kinetic coupled with UV spectral data reveals significant irreversible nonionic micellar binding of 1 molecules in the micellar environment of nearly zero hydroxide ion concentration at >or=0.14 M C(16)E(20) and 1.0 x 10(-3) M NaOH while such observations could not be detected at <or=0.17 M C(16)E(20) and 2.0 x 10(-3) M NaOH. Significantly, such irreversible C(16)E(10) micellar binding of 1 molecules could be detected at 8.8 x 10(-2) M C(16)E(10) and 1.0 x 10(-3) M NaOH as well as at >or=3 x 10(-3) M C(16)E(10) and 7.6 x 10(-4) M NaOH, while the rate of hydrolysis of 1 is completely ceased at >or=0.05 M C(16)E(10) and 7.6 x 10(-4) M NaOH. The rate of hydrolysis of 1 at 5.0 x 10(-2) and 8.8 x 10(-2) M C(16)E(10) and 1.0 x 10(-3) M NaOH reveals the formation of presumably phthalic anhydride, whereas such observation was not observed in the C(16)E(20) micellar system under similar experimental conditions.

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

“…Recently, similar near irreversible micellar trapping of N-benzylphthalimide (5) was obtained in the alkaline hydrolysis of 5 in the presence of C 16 E 10 and C 16 E 20 micelles [12]. But the values of R t were 88 and $5 at respective 1.0 and 0.76 mM NaOH for C 16 E 10 as well as 140 at 1.0 mM NaOH for C 16 E 20 [12]. In all these studies [9][10][11][12], the observed data, obtained at R < R t , have been explained in terms of PM model.…”

“…But the values of R t were 88 and $5 at respective 1.0 and 0.76 mM NaOH for C 16 E 10 as well as 140 at 1.0 mM NaOH for C 16 E 20 [12]. In all these studies [9][10][11][12], the observed data, obtained at R < R t , have been explained in terms of PM model. The unusual near irreversible trapping of moderately hydrophobic reactants by C m E n micelles at R P R t cannot be attributed to the usual shielding effect of micelles [12].…”

“…But such unusual observations were not obtained in the presence of C 16 E 20 micelles under essentially similar conditions [9,11]. Recently, similar near irreversible micellar trapping of N-benzylphthalimide (5) was obtained in the alkaline hydrolysis of 5 in the presence of C 16 E 10 and C 16 E 20 micelles [12]. But the values of R t were 88 and $5 at respective 1.0 and 0.76 mM NaOH for C 16 E 10 as well as 140 at 1.0 mM NaOH for C 16 E 20 [12].…”

“…In all these studies [9][10][11][12], the observed data, obtained at R < R t , have been explained in terms of PM model. The unusual near irreversible trapping of moderately hydrophobic reactants by C m E n micelles at R P R t cannot be attributed to the usual shielding effect of micelles [12]. The near irreversible micellar trapping of anionic N-hydroxyphthalimide (6) could not be observed in the alkaline hydrolysis of 6 in the presence of C 16 E 20 (with R 6 8.5) and C 18 E 20 (with R 6 5.0) at 0.02 M NaOH [13].…”

“…But the presence of C 12 E 23 micelles caused significant near irreversible trapping of 6 at R P 5.0 and 0.02 M NaOH [14]. The origin of near irreversible micellar trapping of one of the reactants under the typical reaction conditions was not explored in all these studies [9][10][11][12][13][14][15]. It has been observed that an increase in the total concentration of cetyltrimethylammonium bromide ([CTABr] T ) from 0.0 to 0.03 M has no effect on the apparent pK a of the conjugate acid of 4 in the presence of 0.02 M C 12 E 23 [15], which is considered to be unusual in view of our understanding (through pseudophase ion exchange model of micelle) of the occurrence of ion exchange of HO À /Br À at CTABr micellar surface [16][17][18].…”