S_N2 reaction of a sulfonate ester in the presence of alkyltriphenylphosphonium bromides and mixed cationic‐cationic systems

Abstract

The effects of alkyltriphenylphosphonium bromides (C n TPB, n ¼ 10, 12, 14, 16) on the rates of S N 2 reactions of methyl 4-nitrobenzenesulfonate and bromide ion have been studied. Observed first-order rate constants are significantly higher than those found for other cationic surfactants for the same reaction. The results have been analyzed by the pseudophase model of micellar kinetics and show true micellar catalysis in the sense that second-order micellar rate constants are higher than the second-order rate constants in water. An attempt has also been made to investigate mixed cationic-cationic surfactant systems with respect to observed rates and pseudophase regression parameters. In addition, modeling of some cationic head groups has illustrated possible differences in head group charges and counterion interactions that may prove kinetically relevant.

Study of the reaction of methyl 4‐nitrobenzenesulfonate and Br− in water–glycerol cationic micellar solutions

“…This explains the dependence of K m on the surfactant nature. The increase in the equilibrium-binding constant of methyl 4nitrobenzenesulfonate molecules upon increasing the hydrophobic chain length was also observed in the case of aqueous alkyltriphenylphosphonium bromide micellar solutions [30]. In regard to the effect of glycerol on K m , water-glycerol mixtures has a lower static dielectric constant than water (addition of 50 wt% of GLY decreases the static-dielectric constant from 78.5 to 64.0, at 298 K [22]).…”

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

“…Decrease in the observed reaction rates after maxima is due to the dilution of catalysts in the Stern layer of micelles [39]. 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].…”

Characterization of a cationic phosphonium surfactant for micellar electrokinetic chromatography: Using the linear solvation energy relationships model

“…The source of these changes could be related to the differences in electronegativity, atomic radius, and bond length between nitrogen and phosphorus. These differences, or the change in the charge distribution reported in modeling studies [11], may result in differences in the amount, orientation, and penetration of water at the interfacial regions of the micelle, leading to the observed differences in chemical selectively and changes in the LSER results.…”

“…In contrast, the model of a quaternary phosphonium headgroup surfactant shows that the phosphorus center is positively charged, giving slightly negative substituents [11]. The change in charge distribution between ammonium and phosphonium is also believed to enhance the rate of S N 2 type reactions in micellar catalysis systems using the phosphonium surfactants [11]. Despite these chemical differences between nitrogen and phosphorus, ammonium and phosphonium hexadecyltributyl surfactants have similar CMCs of 0.27 and 0.26 mM for the ammonium [12] and phosphonium [13] surfactants, respectively.…”

“…The results of the modeling show that the positive charge of an ammonium headgroup is distributed over the substituent groups, giving the nitrogen atom a negative charge. In contrast, the model of a quaternary phosphonium headgroup surfactant shows that the phosphorus center is positively charged, giving slightly negative substituents [11]. The change in charge distribution between ammonium and phosphonium is also believed to enhance the rate of S N 2 type reactions in micellar catalysis systems using the phosphonium surfactants [11].…”