Micellar Charge Effects Upon Spontaneous Nucleophilic Additions and Substitutions

Bunton, Clifford A.; Mhala, Marutirao M.; Moffatt, John R.

doi:10.1007/978-1-4615-7981-6_6

Cited by 1 publication

(7 citation statements)

References 40 publications

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“…Zwitterionic, sulfobetaine, micelles behave very much like cationic micelles in their effect on spontaneous hydrolyses at acyl, alkyl, and sulfonyl centers ,,9a, consistent with similarities in their charge asymmetries in the interfacial region.…”

Section: Resultsmentioning

confidence: 58%

“…Anionic micelles of SDS inhibit reaction more than cationic and sulfobetaine micelles, showing that micellar charge, as well as polarity, is important. Consistently S N 1 1d,,, reactions are strongly micellar-inhibited, but inhibition by SDS is less than that by cationic or sulfobetaine micelles, although kinetic solvent effects are qualitatively similar to those on spontaneous S N 2 reactions . Sulfobetaine micelles are formally uncharged, but they behave like cationic micelles in their effects on rates of hydrolyses and other spontaneous reactions. ,, …”

mentioning

confidence: 73%

“…The effects depend on the transfer of substrate from water to micelles, the reaction mechanism, and properties of the interfacial region, e.g., local charge, and polarity and water content, which are lower than in water. Spontaneous reactions that are accelerated by decreases in solvent polarity, e.g., anionic decarboxylation and dephosphorylation, are micellar-accelerated, and most spontaneous hydrolyses that are accelerated by increases in solvent polarity are micellar-inhibited. 1d,f,g,− Comparisons of kinetic solvent and micellar effects on spontaneous hydrolyses provide evidence on the structures of interfacial regions, especially with regard to changes in the headgroups. This kinetic evidence complements physical evidence and that from the use of spectral, or other, probes. , The polarities of the interfacial regions can be compared with those of bulk solvents in terms of E T (30) or effective dielectric constant, which agrees with independent evidence that these regions are partially depleted in water …”

mentioning

confidence: 99%

“…Spontaneous hydrolyses of carboxylic anhydrides, diaryl carbonates, and acyltriazoles are micellar-inhibited, but, as for S N 2 hydrolysis, inhibition is greater with anionic than with cationic, or sulfobetaine, micelles. 1d,− ,,9a It appears that reactions in which there is extensive nucleophilic intervention by water in formation of the transition state are most inhibited by anionic micelles, probably because of development of negative charge in the organic residue.…”

mentioning

confidence: 99%

“…The situation for hydrolyses of acyl chlorides is complex, because cationic micelles accelerate hydrolyses when substrates contain strongly electron-withdrawing substituents but otherwise micelles inhibit reactions. Hydrolyses of aryl chloroformates and acyl chlorides are affected similarly by aqueous micelles. 1d,,,9a A systematic study of micellar and solvent effects upon these spontaneous hydrolyses should provide evidence on both reaction mechanism and the nature of the interfacial region as a reaction medium.…”

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confidence: 99%

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Micellar Charge Effects upon Hydrolyses of Substituted Benzoyl Chlorides. Their Relation to Mechanism

et al. 2000

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Micellar rate effects on hydrolyses of substituted benzoyl chlorides, 1, depend on headgroup charge and electron donation or withdrawal by substituents. Micellized sodium dodecyl sulfate, SDS, inhibits hydrolyses, and first-order rate constants in the micellar pseudophase, k′M, decrease, relative to those in water, k′W, over a range of ca. 10, but in cetyl trimethylammonium chloride, CTACl, k′M/k′W > 1 for hydrolyses of 1,3,5-(NO2)2 and 1, 4-NO2, and decreases steeply with electron-donating substituents in the following sequence: 1,4-Cl ≈ 4-Br > 4-H > 4-Me > 4-OMe, over a range of >10 3 . Cetyl trimethylammonium bromide and mesylate behave like CTACl. Fits to the Hammett equation give F ≈ 1 in SDS and F ≈ 4 in CTACl. Anionic micelles have higher interfacial polarities than cationic micelles, but micellar and solvent effects do not correspond because over a range of solvents, H2O to H2O-MeCN, 1:1 w/w plots of log k′W against σ go through minima with positive F for 1, 3,5-(NO2)2, and 4-NO2 and negative for the other substrates. The micellar effects correspond to differing extents of bond-making and -breaking in the transition state. Values of k + /k -(rate constants in CTACl and SDS) decrease strongly with increasing electron donation by substituents. Micellar rate effects in hydrolyses of benzyl bromide and 4-methoxybenzyl chloride are similar to those with the benzoyl chlorides. Although data were analyzed by a pseudophase treatment, application of transition state theory and reported micellar surface potentials allows estimation of local charge at the reaction center for hydrolyses of the benzoyl chlorides.There is extensive evidence on the ability of aqueous micelles and other association colloids to influence reaction rates and equilibria, and concentration, or depletion, of reactants in the interfacial region have major effects on rates of bimolecular reactions. 1 However, this region differs from water as a reaction medium, 2-4 which can affect rates of spontaneous reaction. The effects depend on the transfer of substrate from water to micelles, the reaction mechanism, and properties of the interfacial region, e.g., local charge, and polarity and water content, which are lower than in water. Spontaneous reactions that are accelerated by decreases in solvent polarity, e.g., anionic decarboxylation and dephosphorylation, are micellar-accelerated, and most spontaneous hydrolyses that are accelerated by increases in solvent polarity are micellar-inhibited. 1d,f,g,4-6 Comparisons of kinetic solvent and micellar effects on spontaneous hydrolyses provide evidence on the structures of interfacial regions, especially with regard to changes in the headgroups. This kinetic evidence complements physical evidence and that from the use of spectral, or other, probes. 2,3 The polarities of the interfacial regions can be compared with those of bulk solvents in terms of E T (30) or effective dielectric constant, which agrees with independent evidence that these regions are partially depleted in water. 3 Hydrolysis of met...

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