Effect of colloidal self‐assemblies on the basic hydrolysis of 2‐(4‐bromophenoxy)quinoxaline

Abstract

In the presence of cationic surfactants (C16H33NR3Cl; R = Me, n‐Pr, n‐Bu), the shape of rate versus surfactant concentration profiles for the basic hydrolysis of 2‐(4‐bromophenoxy)quinoxaline depends on substrate concentration. At low substrate concentration there is a single rate maximum and with a 10‐fold substrate concentration increase a double rate maximum is observed. The first rate maximum is ascribed to reaction occurring in premicellar aggregates and the second to reaction in micelles. At low substrate concentration the effect of surfactant head group size was examined. Second‐order rate constants in the micellar pseudophase increase with increasing head group size. Copyright © 2003 John Wiley & Sons, Ltd.

The effect of the nature of the head group on the micellar effects of functional/cationic co-micelles in acyl transfer reactions

“…In the case of the cyclization of the 2-(3-halogenopropyl)hydroxyphenolate ions [12], which can serve (if the molecularity of the process is disregarded) as the mechanistic equivalent of S N 2 reactions the rate constants in the micelles are somewhat higher than in water and increase with increase in the size of the quaternary ammonium head group. The same pattern is observed for the spontaneous decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion [1], the reaction of OH -, Cl -, and Br -ions with methylnaphthalene-2-sulfonate [2], and the alkaline hydrolysis of 2-(4-bromophenoxy)quinoxaline [13] and substituted nitrobenzenesulfonates [14] in the presence of a series of cetyltrialkylammonium SFAs; the k 2 m value here varies in the order Me 3 N + < Et 3 + N < n-Pr 3 N + < n-Bu 3 N + . Here the SFAs containing quinuclidine and morpholine groups give values for the micellar effects almost identical with CTAB in spite of the large volume of the head group in them compared with trimethylammonium [1, 2].…”

“…The thermodynamic kinetic parameters, calculated in terms of Eqs. (5) and (6) and presented in Table 1, indicate that the rates of the reactions in water and in the micellar pseudophase are close for the S N 2 reactions investigated in the present work, as also for many ion-molecular reactions [1, 2,8,13].…”

“…Data on the reactivity of the anionic nucleophiles in S N 2 and E2 reactions in the micellar pseudophase of cationic SFAs [1-3, [11][12][13][14] indicate that bulky alkyl substituents are probably capable of screening the quaternary nitrogen atom of the head group from the counterions (both from the counterion of the detergent and from the anionic nucleophile). The water content in the region where the chemical reaction occurs is reduced as a result of the decrease in the charge density at the surface of the micelle on account of screening.…”

“…This is probably due to the fact that the physicochemical characteristics of the micellar surface of the cationic SFAs, containing both trialkylammonium [1, 2] and pyridinium [11] head groups, hardly depend at all on the length of the hydrocarbon "tail." Nevertheless, variation of the head group of the SFA can affect the rate of the process significantly in the case of monomolecular reactions, which are extremely sensitive to the properties of the medium [1, [12][13][14].…”

Hydrolysis of carboxylate and phosphate esters using monopyridinium oximes in cationic micellar media

“…The rate enhancement in the phosponium-and pyridinium-based cationic surfactants is greater than in alkyl ammonium head groups due to their bulky head group [22,40]. The effect would correspond to a general decrease in the polarity of the head group region that would therefore increase the rate of the nucleophilic substitution [16,41]. Another possible explanation is the steric effects by the phenyl rings in the head group region that would activate the bound substrate toward substitution [33].…”