Aminolysis of Carboxylic Acid Esters in Direct, Bicontinual, and Inverse Microemulsions Based on Cetyltrimethylammonium Bromide

Abstract

The reactivity of primary alkylamines in cleavage of p-nitrophenyl esters of carboxylic acids in microemulsions of different structures based on cetyltrimethylammonium bromide was studied. The aminolysis rate considerably increases in going from inverse to direct microemulsions, mainly owing to the concentration of the reactants in the boundary layer.Detergent microemulsions are macroscopically uniform, thermodynamically stable, self-organizing dispersions with aqueous and hydrocarbon (oil) phases, in which the phase boundary is occupied by molecules of micelle-forming surfactants and cosurfactants. Variation of the water-to-oil volume ratio significantly alters the structure and properties of the microemulsions, with the phase inversion and rearrangement of the phase boundary [1]. Both inverse, bicontinual, and direct microemulsions can be formed, with the system as a whole remaining macrohomogeneous and optically transparent and preserving a high solubilizing capacity toward both polar and nonpolar substances. Unique solubilizing properties of microemulsions, along with exceedingly developed phase boundary surface, provide efficient contact between hydrophilic and hydrophobic reagents and allow successful use of these systems as nanoreactors for various chemical processes. With such systems, it is possible to control not only the reaction rates but even the reaction mechanisms [235].This study continues our previous investigations concerning the catalytic properties of microemulsions in nucleophilic substitutions [538]. To reveal how the structure of the reaction medium affects the rate of ester cleavage, we studied the reactions of p-nitrophenyl esters of carboxylic acids with primary aliphatic amines in microemulsions consisting of water, cetyltrimethylammonium bromide, n-butanol, and n-hexane. The content of the surfactant and cosurfactant in the microemulsions was 9.42 wt % each (molar ratio 1 : 5), which made it possible to avoid phase separation when the content of the two other components, water and hexane, was varied in a wide range. For kinetic experiments, we used the microemulsions with the water volume fraction (j) of 0.77, 0.30, and 0.14, which corresponds to the direct, bicontinual, and inverse structures, respectively [8,9].In molecular aqueous solutions, the cleavage of carboxylic acid esters in the presence of primary amines mainly occurs along two pathways: aminolysis (major reaction) and base hydrolysis (side reaction):R`NH 2 R and R`= C n H 2n+1 .The kinetic data characterizing the reaction of p-nitrophenyl acetate I and p-nitrophenyl laurate II with primary amines in microemulsions of various types are given in Table 1. It is seen from these results that, at equal concentrations of amines (C am ), the observed rate constants (k obs ) only slightly depend on the hydrophobicity of the amine. At the same time, the rate constants depend on the structure of the microemulsions, increasing in going from the inverse to direct microemulsion. In all the microemulsions studied, acetate I is cleaved by...

Acylation of piperidine and morpholine with phenyl benzoates in the mixtures water-2-propanol and water-dioxane

“…Nowadays, as one of such models, the reaction of amines of different types with esters in solutions or microemulsions is considered [4][5][6][7]. In the industrial synthesis of amides, water-organic media are widely used [8,9], which allows to substantially vary the rate of the target reaction and the simultaneously occurring hydrolysis of the acylation agent [7].…”

“…They show that at 298 K and at the molar fraction of water in the mixture x 1 < 0.77 the contribution of hydrolysis into the rates of the reactions under consideration is small, so the rate constant for reaction (1) was calculated as the arithmetic mean of k values determined from Eq. (5). With the increase of temperature and concentration of water in the solution the contribution of hydrolysis is increased resulting in substantial variations of k. In this case, the value of k was calculated by Eq.…”