Micellar effect

These have formed the subject of a general review and of an issue of Berichte der Bunsengesellschaft fiir Physikalische Chemie specifically inorganic applications have also been briefly reviewed in a paper which attempts a correlation using the Poisson-Boltzmann equation applied to a parallel-rod cell model. 

In these general reviews organic systems, and both redox and substitution reactions of inorganic species, are covered. 

Chagas, M. Tubino, and E. J. S. Vichi, Inorg. Chim. Acta Lett., 1978, 28, L137. 188 S. Raman, J. Inorg. Nucl. Chem., 1978, 40, 1073. 

Treindl and S. Drojakovd, Collect. Czech. Chem. Commun., 1978, 43, 1561. 

There has been much activity in the area of micellar and polyelectrolyte effects on formation of nickel(ii) complexes, especially of pada (7) and murexide (8). An earlier investigation of the effects of sodium dodecyl sulphate on the reaction with pada has been extended. The use of relaxation techniques 

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Figure 4 indicates the effect of the CTAB concentration on the rate constant of the complexes of 29 and 32. The CMC of CTAB is near 1 x 10 3 M. Below CMC, the rates cannot be measured because of insolubility of the ligands. Although unmeasured, the rates of the 29 and 32 complexes must be greatly enhanced in the presence of CTAB micelles up to CMC, but further increase of the micelle concentration above CMC cause a rate decrease. This type of micellar effect can be seen in many micellar reactions 27). Hence, it should be noted that the rate constants in Table 3 would be several times larger if they are measured by using a lower concentration of CTAB than 5 x 10-3 M. 

The apparent difference seems to be due to the difference in the binding constants of the complexes to micelles which is much larger in the lipophilic 38c than in the hydrophilic 38b complex27 . A somewhat different, but not an unusual micellar effect is observed in the case of the non-ionic surfactant Triton X-100 as shown in... 

Bunton, C. A., Romsted, L. S. Micellar Effects upon Deacylation, in The Chemistry of Acid Derivatives (Patai, S. Ed.), part 2, Chapt. 17, John Wiley, New York 1979... 

The micellar effect on the endo/exo diastereoselectivity of the reaction has also been investigated. The endo/exo ratio of the reaction of cyclopentadiene with methyl acrylate is affected little (compared to water) by the use of SDS and CTAB [73b], while a large enhancement was observed in SDS solution when n-butyl acrylate was the dienophile used [74]. The ratio of endo/exo products in the reaction of 1 with 113c is not affected by CTAB, SDS and C12E7 [72a]. 

Micellar effects can play an important part in aqueous organometallic reactions. Surface active diphosphines have been synthesized and sparingly soluble solutes like decene may well benefit through miceller effects. 

A survey of micellar effects on chemical and photochemical reactions 282 Quantitative treatment of micelle-assisted bimolecular reactions 295 References 299 Notes added in proof 309... 

The general principles which govern the effects of normal, aqueous, micelles upon reaction rates and equilibria are considered first, and then we discuss some specific reactions and the relation of micellar effects to mechanism. We also briefly consider some non-micellar species generated by amphiphiles which can also mediate reactivity. 

Micellar effects upon reaction rates and equilibria have generally been discussed in terms of a pseudophase model, and this approach will be followed here. 

Fig. 1 Micellar effects upon reaction of p-nitrophenyl diphenyl phosphate with benzimidazolide ion (solid points) open points are for reaction in the absence of benzimidazole , 10 4 M benzimidazole, pH 10.7 , 1.2 x 10-4 M benzimidazole, pH 11 O, pH 10.7, and 11 respectively. The solid lines are theoretical. (Reprinted by permission of the American Chemical Society)...

Micellar effects upon reactions of organic nucleophiles... 

The original ion-exchange treatment was developed for competition between reactive and inert monoanions, but Chaimovich, Quina and their coworkers have extended it to competition between mono and dianions (Cuccovia et al., 1982a Abuin el al., 1983a). The ion-exchange constant for exchange between thiosulfate dianion and bromide monoanion is not dimensionless as in (7) but depends on salt concentration, and the formalism was developed for analysing micellar effects upon reaction of dianionic nucleophiles, e.g. thiosulfate ion. 

The quantitative treatment of micellar rate effects upon spontaneous reactions is simple in that the overall effect can be accounted for in terms of distribution of the substrate between water and the micelles and the first-order rate constants in each pseudophase (Scheme 2). The micelles behave as a submicroscopic solvent and to a large extent their effects can be related to known kinetic solvent effects upon spontaneous reactions. It will be convenient first to consider unimolecular reactions and to relate micellar effects to mechanism. 

In the discussions of micellar effects thus far there has been essentially no discussion of the possible effect of micellar charge upon reactivity in the micellar pseudophase. This is an interesting point because in most of the original discussions of micellar rate effects it was assumed that rate constants in micelles were affected by the presence of polar or ionic head groups. It is impracticable to seek an answer to this question for spontaneous reactions of anionic substrates because they bind weakly if at all to anionic micelles (p. 245). The problem can be examined for spontaneous unimolecular and water-catalysed reactions of non-ionic substrates in cationic and anionic micelles, and there appears to be a significant relation between reaction mechanism and the effect of micellar charge upon the rate of the spontaneous hydrolysis of micellar-bound substrates. 

For reaction of benzoyl chlorides in water, reactivity follows the electronreleasing ability of p-substituents, and there are striking differences in the micellar effects (Table 7). With electron-withdrawing substituents k+/k > 1, but its value progressively decreases with increasing electron release from a p-substituent. Thus an increase in electron release changes k+/k from values characteristic of a carbonyl addition reaction to values characteristic of SN reactions at saturated carbon. This classification also... 

Another problem is that different workers make their calculations of second-order rate constants in the micelle in different ways. For example, the surface potential of a micelle may be specifically included in the calculation in order to estimate ion binding, but there is a circularity in this argument because surface potentials are often estimated from micellar effects upon indicator acid-base equilibria which themselves depend upon ion-binding to micelles (Fernandez and Fromherz, 1977). 

It is more difficult to interpret micellar effects upon reactions of azide ion. The behavior is normal , in the sense that k /kw 1, for deacylation, an Sn2 reaction, and addition to a carbocation (Table 4) (Cuenca, 1985). But the micellar reaction is much faster for nucleophilic aromatic substitution. Values of k /kw depend upon the substrate and are slightly larger when both N 3 and an inert counterion are present, but the trends are the same. We have no explanation for these results, although there seems to be a relation between the anomalous behavior of the azide ion in micellar reactions of aromatic substrates and its nucleophilicity in water and similar polar, hydroxylic solvents. Azide is a very powerful nucleophile towards carboca-tions, based on Ritchie s N+ scale, but in water it is much less reactive towards 2,4-dinitrohalobenzenes than predicted, whereas the reactivity of other nucleophiles fits the N+ scale (Ritchie and Sawada, 1977). Therefore the large values of k /kw may reflect the fact that azide ion is unusually unreactive in aromatic nucleophilic substitution in water, rather than that it is abnormally reactive in micelles. 

Discussion of acid-base equilibria may seem out of place in a chapter devoted to reactivity, but micellar effects upon indicator equilibria played a key role in the development of ideas regarding micellar effects upon reactivity, so a brief discussion is in order (Hartley, 1948, Fendler and Fendler, 1975). 

Hartley showed that micellar effects upon acid-base indicator equilibria could be related to the ability of anionic micelles to attract, and cationic micelles to repel, hydrogen ions. More recently attempts have been made to quantify these ideas in terms of the behavior of a micelle as a submicroscopic solvent, together with an effect due to its surface potential (Fernandez and Fromherz, 1977). 

It is convenient to define the basicity constant, K]of the indicator, BH, in the micellar pseudophase as a dimensionless quantity in terms of the mole ratio of micellar bound OH- (p. 225) using (21). The quantity [OHm] (or m oh) can be calculated from the ion-exchange relation and the experimental data fitted, usually by computer simulation, following the approach discussed for treatment of rate constants (p. 229). This treatment fits micellar effects upon the deprotonation of benzimidazole for a variety of CTAX surfactants (X = Cl, Br, N03) over a range of concentrations of NaOH and of added salts (Bunton et al., 1982a). A similar, but less general approach, was also applied to deprotonation of phenols and oximes (Bunton et al., 1980c). 

Micelles exert large rate effects upon organic reactions and can in principle discriminate between different reactions, depending upon their charge type or molecularity. There are a number of examples of this type of discrimination in the literature, and they are easily explained in terms of the generally accepted models of kinetic micellar effects. 

Most studies of micellar effects upon rates or products of organic reactions have been made with very low concentrations of reactants, and this small scale of work is not very encouraging for the synthetic organic chemist. An additional disadvantage is that surfactants complicate product separation by extraction or distillation, and to date most studies in this general area have been exploratory and have been aimed at solving these problems. 

A survey of micellar effects on chemical and photochemical reactions... 

This section gives tabulated examples of recent work on micellar effects upon chemical and photochemical reactions. In general the examples given in this section do not duplicate material covered elsewhere in the chapter for example micellar effects on some photochemical reactions and reactivity in reversed micelles are listed here although they are neglected in the body of the text. For many ionic reactions in aqueous micelles only overall rate effects have been reported, in many cases because the evidence did not permit estimation of the parameters which describe distribution of reactants between aqueous and micellar pseudophases. These reactions are, nevertheless, of considerable chemical importance, and they are briefly described here. 

Substituted benzamides + OH - CTABr + OH -. Possible micellar effects on mechanism Broxton et al., 1981... 

N-Methylphenothiazine + iron or molybdenum complex Micellar effects upon rates and equilibria of electron transfer Minero et al., 1983... 

CTABr, aq.dioxan. Micellar effects consis- Menger et al., 1981 tent with an aqueous micellar surface... 

The use of a lipophilic zinc(II) macrocycle complex, 1-hexadecyl-1,4,7,10-tetraazacyclododecane, to catalyze hydrolysis of lipophilic esters, both phosphate and carboxy (425), links this Section to the previous Section. Here, and in studies of the catalysis of hydrolysis of 4-nitrophenyl acetate by the Zn2+ and Co2+ complexes of tris(4,5-di-n-propyl-2 -imidazolyl)phosphine (426) and of a phosphate triester, a phos-phonate diester, and O-isopropyl methylfluorophosphonate (Sarin) by [Cu(A(A(A/,-trimethyl-A/,-tetradecylethylenediamine)l (427), various micellar effects have been brought into play. Catalysis of carboxylic ester hydrolysis is more effectively catalyzed by A"-methylimidazole-functionalized gold nanoparticles than by micellar catalysis (428). Other reports on mechanisms of metal-assisted carboxy ester hydrolyses deal with copper(II) (429), zinc(II) (430,431), and palladium(II) (432). 

From time to time various models have been developed to explain the micellar effect on the rate of reaction. Each model has its own strength and weakness. 

The extent of solubilization of the substrate to the micelle can be related with the association constant or the binding constant. Some of the important models developed to explain the micellar effect are described briefly as follows ... 

The protonation of the triplet jtjt state of 3-bromonitrobenzene is shown to be responsible for the acid-catalysed promotion of halogen exchange which follows a S y23Ar mechanism26 (equation 23). Cationic micellar effects on the nucleophilic aromatic substitution of nitroaryl ethers by bromide and hydroxide ions have also been studied27. The quantum efficiency is dependent on the chain length of the micelle. The involvement of counter ion exchanges at the surface of ionic micelles is proposed to influence the composition of the Stem-layer. 

In conclusion it can be said, that micellar effects offer useful possibilities to tune the reactivity and separation characteristics of aqueous/organic biphasic hydroformylations. Nevertheless, the added sensitivity of the systems to small changes in process variables and the added cost of surfactants and/or specially synthetized ligands have to be justified by high added value products or on grounds of process cost savings. Whether this will happen on industrial scale (perhaps in the hydroformylation of higher olefins) remains to be seen. 

For the anode process at comparable conditions, the yield of l,2-dimethoxy-2-nitrobenzene depends distinctly on the electrical natnre of a micelle. Namely, the yields are equal to 30, 40, and 70% for the positively, negatively, and nentrally charged micelles, respectively. The observed micellar effect corroborates the mechanism that inclndes 1,4-dimethoxybenzene cation-radical and nitrogen dioxide radical as reacting species. 

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