Nucleophilic aromatic micellar

Nucleophilic aromatic photosubstitution reactions in aqueous solutions and in micellar media has been investigated extensively.42... 

We can also describe the differences between these reaction types in terms of Pearson s hard-soft description (Pearson, 1966 Pearson and Songstad, 1967). Cationic micellar head groups interact best with soft bases, e.g. relatively large anions of low charge density such as bromide or arenesulfo-nate, or anionic transition states such as those for nucleophilic aromatic substitution. They interact less readily with hard bases, e.g. high charge density anions such as OH ", or anionic transition states for deacylation. 

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. 

There has been a useful review of phase-transfer catalysis in nucleophilic aromatic substimtion. A comparison has been reported of the reactions with nucleophiles of l-chloro-2,4-dinitrobenzene (substimtion) and 4-nitrophenyl diphenyl phosphate (dephosphorylation) in neutral micelles of dodecyl (10) and (23) polyoxyethylene glycol. In the substimtion reaction considerable amounts of ether may be formed by reaction with alkoxide ions at the micellar surface. Differences in reactivity of the two substrates are probably due to differences in their location in the micellar structures. ... 

The effects of micelles on the rates of nucleophilic aromatic substitution reactions (equations 42a-42e) follow a similar pattern. The rate constant for reaction (42a) was unaffected by the presence of micellar sodium dodecyl sulfate, even though 2,4-dinitrofiuorobenzene partitions strongly in favor of the micelles and the free base of glycineamide is not appreciably solubilized. On the other hand, the rate constant for the reaction with glycylglycine (equation 42b) decreased by a factor of 3-6 in micellar sodium dodecyl sulfate solution and increased by a factor of ca. 15 in the presence of CTAB (Herries et al., 1964). Since the rate retardation in NaLS solutions was observed to parallel the partitioning of 2,4-dinitrofluorobenzene in favor of the micellar phase and the... 

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. 

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

Small rate enhancements occur for aromatic nucleophilic substitution as compared to deacylation or dephosphorylation. These effects may be due to favorable interactions between cationic micellar head groups and the transition states whose structures are akin to those of Meisenheimer complexes. 

The use of organic microheterogeneous systems for developing new or improving known kinetic-based determinations is progressing well. Many micellar kinetic determinations based on uncatalyzed reactions have involved aromatic nucleophilic substitutions, e.g., reactions between l-fluoro-2,2-dinitrobenzene and primary and secondary amines, phenolic compounds, and thiols. These reactions... 

It has been suggested that micellar catalysis could be exploited in analytical chemistry to increase the rate of derivative formation prior to spectroscopic measurement of the product [223, 224]. This has been attempted in the assay of amino acids and peptides following reaction with l-fluoro-2,4-dinitrobenzene (this undergoes aromatic nucleophilic substitution by amines to give arylated amines) [225]. For this reaction some amines require up to 20 min. In the presence of cetrimonium bromide, catalysis was achieved, although absorbances some 10% higher were obtained in the presence of surfactant. 

It is evident from Equation 3.4 through Equation 3.8 that, under typical experimental conditions in which one reactant or both reactants are completely micellized by micelles of nonionic or ionic surfactants with counterions as inert ions, rate constants should decrease with the increase in [SurfJx (total concentration of micelle-forming surfactant) at a constant [R]t- There seems to be no report in the literature on the occurrence of cross-interface reactions involving nonionic or ionic micelles with inert counterions. However, crossinterface reactions with reaction schemes similar to the one represented by Scheme 3.5 were reported for the first time in the aromatic hydroxide ion nucleophilic substitution reactions in the presence of p-CgH,70C6H4CH2NMe3+ HO" where counterion HO" was also a reactant. The proposition that crossinterface reactions had occurred in this study was based on the linear increase observed in with increase in [D ] even when there was strong evidence that the aromatic substrate was fully micellar bound. These observations were explained by the following kinetic equation ... 

German), R., Savelli, G., Romeo, T., Spreti, N., Cerichelli, G., Bunton, C.A. Micellar head group size and reactivity in aromatic nucleophilic substitution. Langmuir 1993, 9(1), 55-60. 

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