Nucleophilicity micellar effects

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. 

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. 

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. 

Because of the relative simplicity of carboxylic ester hydrolysis, in general, and that of base catalyzed ester hydrolysis, in particular, these reactions have served well as model systems in investigations of micellar effects on reaction rates and activation parameters. In addition, the prevalence in biological systems of carboxylic ester hydrolyses catalyzed by nucleophiles and by enzymes renders the investigation of micelle-catalyzed ester hydrolyses of obvious importance. 

No comparable micellar effects have been observed in 1,2-alkyl shifts which proceed with inversion of configuration above and below the cmc195 A slight increase in racemization above the cmc may be attributed to delayed nucleophilic capture of the carbocations in the less aqueous micellar environment. 

Aliphatic and aromatic nucleophilic substitution reactions are also subject to micellar effects, with results consistent with those in other reactions. In the reaction of alkyl halides with CN and S Oj in aqueous media, sodium dodecyl sulfate micelles decreased the second-order rate constants and dodecyltrimethylammonium bromide increased them (Winters, 1965 Bunton, 1968). The reactivity of methyl bromide in the cationic micellar phase was 30 to 50 times that in the bulk phase and was negligible in the anionic micellar phase a nonionic surfactant did not significantly affect the rate constant for n-pentyl bromide with S2O3-. Micellar effects on nucleophilic aromatic substitution reactions follow similar patterns. The reaction of 2, 4-dinitrochlorobenzene or 2, 4-dinitrofluorobenzene with hydroxide ion in aqueous media is catalyzed by cationic surfactants and retarded by sodium dodecyl sulfate (Bunton, 1968, 1969). Cetyltrimethylammonium bromide micelles increased the reactivity of dinitrofluorobenzene 59 times, whereas sodium dodecyl sulfate decreased it by a factor of 2.5 for dinitrochlorobenzene, the figures are 82 and 13 times, respectively. A POE nonionic surfactant had no effect. 

In Chapter 29, Bunton presents a brief review of micellar effects on nucleophilicity, and he describes recent work of his own in this area. A major contribution of Bunton s has been his development of a quantitative model for calculating nucleophile concentration in the pseudophase of the micelle thus, calculation of rate constants in the micelle is possible. Using this model, Bunton finds that the reaction rates in micelles are very similar to those in water. Thus, micellar accelerations result from reactant concentration. Bunton notes that this conclusion also applies to microemulsions, vesicles, and inverse micelles. A second important contribution of this chapter is a summary of the large amount of experimental work on the contrasting effects of cationic and anionic micelles on reactions of anionic and neutral nucleophiles and on hydrolyses. 

Table I. Micellar Effects on Reactivity of Organic Nucleophiles...

The data in Tables I and II, together with extensive additional evidence, allow several generalizations to be made about micellar effects upon bi-molecular reactions (5). First, overall rate constants follow the distribution of both reactants between water and micelles. Second, second-order rate constants for reactions of nonionic nucleophiles are lower in micelles than in water. Third, second-order rate constants for reactions of anionic nucleophiles are similar in water and micelles except for some reactions of azide ion (37). 

L. Ya. Zakharova, F. G. Valeeva, L. A. Kudryavtseva, Yu. F. Zuev, Factors determining the micellar effect in nucleophilic substitution reactions, Russ. J. Phys. Chem., 2000, 74, 1825-1829. 

Cationic and anionic micellar effects upon competitive 5, ], 5 2, and E2 reactions have been interpreted in terms of enhanced nucleophilicity of hydroxide ion and reduced electrophilic and nucleophilic properties of water . 

Bunton, C.A., Robinson, L. Micellar effects upon nucleophilic aromatic and aliphatic substitution. J. Am. Chem. Soc. 1968, 90(22), 5972-5979. 

Bnnton, C.A., Moffatt, J.R. Micellar effects upon substitutions by nucleophilic anions. J. Phys. Chem. 1988, 92(10), 2896-2902. 

Bunton, C.A., Huang, S.K. Micellar effects upon the reaction of the tri-jp-anisyl carbonium ion with nucleophiles. J. Org. Chem. 1972, 57(11), 1790-1793. 

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