Reactions in micelles

Surfactants have also been of interest for their ability to support reactions in normally inhospitable environments. Reactions such as hydrolysis, aminolysis, solvolysis, and, in inorganic chemistry, of aquation of complex ions, may be retarded, accelerated, or differently sensitive to catalysts relative to the behavior in ordinary solutions (see Refs. 205 and 206 for reviews). The acid-base chemistry in micellar solutions has been investigated by Drummond and co-workers [207]. A useful model has been the pseudophase model [206-209] in which reactants are either in solution or solubilized in micelles and partition between the two as though two distinct phases were involved. In inverse micelles in nonpolar media, water is concentrated in the micellar core and reactions in the micelle may be greatly accelerated [206, 210]. The confining environment of a solubilized reactant may lead to stereochemical consequences as in photodimerization reactions in micelles [211] or vesicles [212] or in the generation of radical pairs [213]. 

Much of the impetus for the study of reactions in micelles is that they model, to a limited extent, reactions in biological assemblies. Synthetic vesicles and cyclodextrins are other model reaction media and the term Biomimetic Chemistry has been coined to describe this general area of study. Work in this area is reviewed in recent publications (Kunitake and Shinkai, 1980 Fendler, 1982). 

The problem of slow turnover has been noted. Initial reaction of a nucleophilic group, e.g. imidazole or oximate, in a functional micelle, with a carboxylic or phosphoric ester, for example, gives an acylated or phosphory-lated imidazole or oxime, and these derivatives hydrolyze slowly to regenerate the nucleophile. Kunitake and Shinkai (1980) discuss a number of reactions in micelles which contain both nucleophilic and basic groups which are potentially capable of acting as bifunctional reagents (Tonellato, 1979, Kunitake and Shinkai, 1980 Bunton, 1984)... 

Comparison of rate constants of reactions in micelles and in non-micellar assemblies... 

Pronounced differences have been observed for the rates of chemical reactions in micelles as compared to pure water. For example, the solvolysis of the l-methylheptyl sulfonate, 5, in dilute water solution proceeds 70 times slower... 

Tachiya M. (1987) Stochastic and diffusion models of reactions in micelles and vesicles. In Kinetics of Nonhomo eneous Processes. A Practical Introduction for Chemists, Biologists, Physicists, and Material Scientists, (ed.) Freeman GR, Wiley, New York, pp. 575-650. 

Kinetic Analysis of Electron-transfer Reactions in Micelles... 

Quantitative approaches to describing reactions in micelles differ markedly from treatments of reactions in homogeneous solution primarily because discrete statistical distributions of reactants among the micelles must be used in place of conventional concentrations [74], Further, the kinetic approach for bimolecular reactions will depend on how the reactants partition between micelles and bulk solution, and where they are located within the microphase region. Distinct microphase environments have been sensed by NMR spectrometry for hydrophobic molecules such as pyrene, cyclohexane and isopropylbenzene, which are thought to lie within a hydrophobic core , and less hydrophobic molecules such as nitrobenzene and N,N-dimethylaniline, which are preferentially located at the micelle-water interface [75]. Despite these complexities, relatively simple kinetic equations for electron-transfer reactions can be derived for cases where both donors and acceptors are uniformly distributed inside the micelle or on its surface. 

Reactions in micelles and in the double layers of vesicles are analogous to natural membrane systems and in special cases are comparable with the function of an enzyme (cf. Section 3.2.1). In contrast to macromolecular enzymes, amphiphile aggregates with linked catalytic centers have a less rigid supramolecular structure. The preorganization of an enzyme is highly selective and as a consequence enzyme catalysis is much more effective than micellar catalysis, a type of artifical mimic [6]. 

On the basis of the above arguments, it appears clear that reactions in micelles can be accelerated by realizing high local concentrations of reactants. Obviously, for opposite reasons reactions can be retarded. This happens, for instance, when only one of the reacting species is transferred into the aggregate. The basic requisite for the occurrence of a reaction, the encounter of the reactants is prevented in this case. Aspects of reactivity in organized assemblies have been reviewed [4-6, 26-29], and this topic continues to attract the interest of several research groups. The analysis of reactivity has also led to quantitative treatments... 

With the exception of a few examples, bimolecular reactions in micelles are largely controlled by the local concentration (and pH) realized at the micellar pseudophase. The data reported in Table 2 [28, 31] give a comparison of the second-order rate constants measured for a series of functional derivatives in aqueous and micellar pseudophases. The ratios of the two rate constants (taking into account concentration and deprotonation effects in micelles) are all close to unity, confirming the above assertion. Finally, although the quantification of rate accelerations has been done mainly with micellar aggregates, the reactivity in vesicles appears to follow basically the same rules with minor differences due to the different lipophilicity and/or order of the membrane [37]. 

As mentioned above, surfactant micelles in water as medium create a special environment from high to low polarity. According to Brown et al. [9] the rate enhancement of organic reactions in micelles can be a combination of the following effects ... 

Nucleophilic reactions in micelles with water as reagent have been investigated as models of enzymic reactions. The enhancement of the reaction rate as well as the stereoselectivity of the reactions was studied. Typical substrates were activated esters of amino acids [14], carboxylic acids [15], and phosphoric acid [16], and typical catalysts were surface-active peptides with histidine as active component. 

Thus, rate constants can be extracted for both spontaneous and bi-molecular, nonsolvolytic reactions in micelles, and when these rate constants are compared with rate constants in water, the factors that control micellar rate enhancements can be identified. Micelles can exert a medium effect on reaction rate because the polarities of their surfaces appear to be lower than that of water (32, 33), and micelles could also, in principle, reduce the nucleophilicity or basicity of water. They could also affect the reactivity of nucleophilic anions, and this aspect of the problem will be considered first. 

Bunton, C. A. Reactions in micelles and similar self-organized aggregates, in The Chemistry of Enzyme Action (ed. Page, M. I.) p. 461. Elsevier, Amsterdam 1984... 

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