First Evidence of Simultaneous Different Kinetic Behaviors at the Interface and the Continuous Medium of w/o Microemulsions

Abstract

A study was carried out on the butylaminolysis reaction of 4-nitrophenyl caprate in AOT/chlorobenzene/water microemulsions, with the observed rate constant, kobs, showing both first- and second-order dependence on butylamine concentration. The first-order term in [BuNH2] is due to the reaction occurring at the interface of the microemulsion while the second-order term is due to the reaction in the continuous medium. The different kinetic behavior is accounted for by the mechanism by which the reaction proceeds: at the interface of the microemulsion, the rate-determining step is the formation of the addition intermediate, T+/-, whereas in the continuous medium the slow step is the base-catalyzed decomposition of this intermediate. The application of the pseudophase formalism allows the observed kinetic behavior to be explained and to obtain the rate constants at the interface, ki2=0.13 M-1 s-1, and in the continuous medium, ko2KT=2.46x10(-2) M-2 s-1. These values indicate that the reaction rate decreases approximately 23 times upon going from the aqueous medium to the interface of the microemulsion, whereas the rate constant in the continuous medium is consistent with that obtained in pure chlorobenzene, ko2KT=2.09x10(-2) M-2 s-1.

Microemulsions as microreactors in physical organic chemistry

“…In an attempt to shift the reaction to the continuous medium, the hydrophobic character or the ester was increased by replacing NPA with 4-nitrophenyl caprate (NPC), and the polarity of the continuous medium was increased by replacing isooctane with chlorobenzene. The results obtained [26] by …”

Influence of n-alkyl acids on the percolative phenomena in AOT-based microemulsions

“…In keeping with this ever expanding range of applications, there is an increasing interest in studying the details of chemical [1], photochemical [20], and enzymocatalytic [1,21,22] processes in microemulsions. In particular, microemulsions-as phase transfer catalysis systems-are able to enhance reactions between nonhydrosoluble organic substrates and hydrosoluble reagents and, as a result, the kinetics of numerous reactions in microemulsions have been studied [2,[23][24][25][26][27][28][29][30]. Electric conductivity measurements constitute a very useful technique for obtaining information about micellar interactions [31][32][33][34][35][36].…”

Chlorination of aromatic compounds in micellar media: Regioselectivity

“…Hence, it is anticipated that the attack of a strong electrophile such as chlorine at para position of phenol would be suppressed by the non-polar micellar core, while the polar aqueous phase would provide a more favorable locale for the attack of chlorine at the ortho position. Similar effects of orientation on selectivity improvement [24][25][26][27][28][29][30][31][32] and rate enhancement [33][34][35][36][37][38][39][40] in micellar and microemulsion media were also reported.…”