Micellar Catalysis reaction mechanism

Inhibition may be incorporated into the mechanism of micellar catalysis in the same way it is handled in enzyme kinetics. Representing the inhibitor by /, we can revise Reaction (G) as follows ... 

If the radicals R and/or R are hydrophobic, the ester will be solubilized by the micelles and the hydrolysis will occur at the surface of the micelle. When the micelles are formed by a cationic surfactant, the local concentration of OH- ions close to the micellar surface will be substantially larger than the bulk concentration. Thus, by using a micellar system, one has been able to assemble the reactants in some spatial region which makes the reaction a more probable event. In enzymatic catalysis this mechanism has been termed the proximity effect283. ... 

Substrate-catalyst interaction is also essential for micellar catalysis, the principles of which have long been established and consistently described in detail [63-66]. The main feature of micellar catalysis is the ability of reacting species to concentrate inside micelles, which leads to a considerable acceleration of the reaction. The same principle may apply for polymer systems. An interesting way to concentrate the substrate inside polymer catalysts is the use of cross-linked amphiphilic polymer latexes [67-69]. Liu et al. [67] synthesized a histidine-containing resin which was active in hydrolysis of p-nitrophenyl acetate (NPA). The kinetics curve of NPA decomposition in the presence of the resin was of Michaelis-Menten type, indicating that the catalytic act was accompanied by sorption of the substrate. However, no discussion of the possible sorption mechanisms (i.e., sorption by the interfaces or by the core of the resin beads) was presented. 

In a series of papers [109,110,112,113] this catalytic action has been explained in the context of the mechanism of micellar catalysis and has been attributed to the reactants concentrating in the foam adsorption layers to a local change in the pH to the effect of reactant molecules orientation in the adsorption layer and to the surface charge of the transition state (intermediate complex). Compared to micellar catalysis the higher efficiency of film catalysis in a foam has been attributed to the structural features of the surface layers in the foam (the type of adsorption films) that facilitate formation of the reaction transition states of the reaction. However, no special studies that would have unambiguously confirmed these assumptions were undertaken. 

Most of the work concerned with micellar catalysis of nucleophilic substitution refers to reactions of the Aac2 and SN2 types and will not be reviewed here. To date only a few systems have been examined in which a micellar medium affects the partitioning of solvolytic reactions between unimolecular and bimolecular mechanisms. The effects of cationic (hexadecyltrimethylammonium bromide = CTAB) and anionic (sodium lauryl sulfate = NaLS) micelles on competitive SN1 and SN2 reactions of a-phenylallyl butanoate 193) have been investigated189. The rate of formation of the phenylallyl cation 194) is retarded by both surfactants probably as a consequence of the decreased polarity of the micellar pseudo phase. The bimolec-... 

Interest in the kinetics and mechanism of organic reactions occurring in the presence of micelles has been prompted by recognized analogies between enzymatic and micellar catalysis, and between the structures of proteins and micelles. The subject of micellar effects on the rates of organic reactions has been reviewed by Fendlerand Fendler. ... 

Work in the area of micellar catalysis in both aqueous and nonaqueous solvent systems is certain to continue to grow in importance as a tool for better understanding the chemistry and mechanics of enzymatic catalysis, as a probe for studying the mechanistic aspects of many reactions, and as a route to improved yields in reactions of academic interest. Of more practical significance, however, may be the expanding use of micellar catalysis in industrial applications as a method for obtaining maximum production with minimum input of time, energy, and materials. 

From the mechanism of micellar catalysis outlined in Scheme 6 the ratio kjky, gives the difference between the free energy of activation in the micellar phase and in the bulk aqueous phase. For bimolecular reactions an apparent rate enhancement of 10 to 10 can result from the higher concentration of reactants in the smaller volume of micelles given by RT In VJ where and are the respective volumes of micelle and aqueous phases. This acceleration can occur even if the rate constants within the two phases are identical. To observe this maximum rate enhancement resulting from a simple concentration effect, the free energy of transfer of the reactant from the aqueous to the micellar phase must be more than enough to offset the loss of entropy from its restriction to a smaller volume within the micelle. 

The subject of micellar catalysis and inhibition of reactions can be divided into the types of reaction occurring, e.g. base-catalysed and acid-catalysed hydrolyses, oxidation, etc., or in terms of mechanisms, e.g. juxtaposition of reactive groups in micelles, attraction of counterions to an oppositely charged micellar surface, protection by solubilization within non-ionic micelles, etc. It is not possible to adhere rigidly to either scheme but we will attempt here to consider, in turn, hydrolysis, oxidation in aqueous micelles, reactions in inverse micelles, reactions involving drugs and miscellaneous reactions of interest. Bunton s summary of the topic in his recent review of the subject is worth repeating here [12] ... 

The coexistence of hydrophilic and hydrophobic nano-domains separated in space, with a local order and fluidity typical of liquids, confer to supramolecular surfactant structures remarkable properties, which are advantageous in applications involving molecular confinement within nanoscopic regions and reactivity in micro-heterogeneous media. Micelle-mediated reactions constitute the basis of the so-called micellar catalysis [62, 116], admicellar catalysis [117] or admicellar polymerisation [118] in which reaction mechanisms may be controlled at a molecular level to save energy and raw materials, as well as to avoid lengthy post-reaction purification and analytical steps. 

General Mechanisms of Micellar Catalysis Kinetic Models for M i ce 11 e-Catalyzed Reactions... 

Micelles are prevalent in naturally occurring and biological catalytic reactions. However, it is only in recent decades that scientists have developed kinetic models clarifying how micelle-mediated catalysis works at a molecular level. Written by a leading expert in the field, Micellar Catalysis is an in-depth examination of how micelles affect reaction mechanisms and reaction rates in organic and inorganic reactions. 

Catalysis, micellar, in organic reactions kinetic and mechanistic implications, 8, 271 Catalysis, phase-transfer by quaternary ammonium salts, 1, 267 Cation radicals in solution, formation, properties and reactions of, 13, 155 Cation radicals, organic, in solution, kinetics and mechanisms of reaction of, 20, 55... 

Eor substrate 1 the kinetics of hydrolysis is measured in the SDS/PEI-30 system at different temperatures, and then the activation parameters for the reaction in different pseudo phases are calculated. For micellar pseudo-phase the enthalpy of activation AH = 55.2 kJ mol and entropy of activation AS = -121.7 J mol K are calculated, which are in agreement with the bimolecular mechanism of the reaction. These values differ little from the activation parameters obtained for the reaction in the aqneons pseudophase (AHo = 56.6 kJ mol, ASS = -100.3 J mol K ). This probably provides evidence that the mechanism of catalysis remains unchanged when the process is transferred from water to aggregates, i.e. the general basic catalysis occnrs in both cases. 

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