Effects of non‐ionic and mixed cationic–non‐ionic micelles on the rate of alkaline hydrolysis of phthalimide

Abstract

Pseudo-first-order rate constants (k obs ) for the alkaline hydrolysis of phthalimide (PTH) show a monotonic decrease with the increase in [C 16 E 20 ] T (total concentration of Brij 58) at constant [CH 3 CN] and [NaOH]. This micellar effect is explained in terms of the pseudophase model of micelles. The rate of hydrolysis of PTH in C 16 E 20 micellar pseudophase appears to be negligible compared with that in the aqueous pseudophase. The values of k obs for C 12 E 23 (Brij 35) show a sharp decrease at very low values of [C 12 E 23 ] T followed by a very slow decrease with increase in [C 12 E 23 ] T at relatively higher values of the latter. The rate of hydrolysis becomes too slow to monitor at [C 12 E 23 ] T !0.04 M in the absence of cetyltrimethylammonium bromide (CTABr) and at [C 12 E 23 ] T !0.05 M in the presence of 0.006-0.02 M CTABr at 0.02 M NaOH whereas such characteristic behavior is kinetically absent with C 16 E 20 . The values of k obs , obtained at different [NIS] T (where NIS represents C 16 E 20 and C 12 E 23 ) in the presence of a constant amount of CTABr, follow the empirical relationship k obs = (k 0 kK[NIS] T )/(1 K[NIS] T ) where k and K are empirical parameters. The values of k are only slightly affected whereas the values of K decrease with increase in [CTABr] T for the mixed C 16 E 20 -CTABr micellar system. The rate of hydrolysis of PTH at !0.01 M C 12 E 23 and !0.01 M CTABr reveals the formation of phthalic anhydride whereas this was not observed in the mixed C 16 E 20 -CTABr micellar system under similar experimental conditions. a Conditions: [phthalimide] 0 = 2 Â 10 À4 M, [NaOH] = 0.02 M, 35°C, l = 300 nm and aqueous reaction mixture for each kinetic run contained 2% (v/v) CH 3 CN. b Y obs = k h w/k obs with 10 4 k h w = 24.6 s À1 . c Calculated from Eqn. (4) with (1 À c.m.c.K s ) = 0.99 and K s = 7.5 M À1 . d Error limits are standard deviations.

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 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]. The observed kinetic data (k obs vs. [C m E n ] T ), reported in earlier studies [9][10][11][12][13][14][15], failed to obey PM model at greater than the typical values of R t (=[C m E n ] T /[HO À ] T ). The plausible reason(s) for such observations remained unknown.…”

“…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 symbol [X] T (throughout the manuscript) represents total concentration of X. 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].…”

“…Near irreversible micellar partitioning of the reactants (of bimolecular reactions), differing greatly in terms of hydrophobicity, has been observed at larger than a typical value of [C 12 E 23 ] T /[HO À ] T (=R t ) in C 12 E 23 micellar-mediated alkaline hydrolysis of phthalimide (2) [9], phenyl benzoate (3) [10], and phenyl salicylate (4) [10], as shown in reaction Scheme 1. The symbol [X] T (throughout the manuscript) represents total concentration of X.…”

“…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].…”