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

Abstract

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...

The dependence of micellar rate effects upon reaction mechanism

“…In summary, for hydrolyses of both these acid chlorides the transition state is anionoid with strongly electron-withdrawing substituents, e.g., NO 2 , but becomes cationoid when strongly electron-donating substituents, e.g., OMe, are present [61,62]. However, there is no reason to believe that acyl or sulfonyl cations are reaction intermediates in aqueous media.…”

“…This behavior is understandable in view of the extensive involvement of water in these reactions and to this extent, micelles behave like electrolyte-rich aqueous-organic mixtures where the inhibition generally increases with decreasing water content, to extents depending on the mechanism [7, [56][57][58]. Extents of micellar inhibition do not follow any simple pattern, except that for reaction of a fully micellar-bound substrate reaction is faster in a cationic than in an anionic micelle when water is involved covalently, as in an S N 2 reaction, but the situation is the opposite for S N 1-like reactions where water is not involved covalently in the rate limiting step but solvates the leaving anion and reacts very rapidly with the intermediate carbocation [59][60][61]:…”

“…CH 3 O [61,62]. Fernando and Fromherz had used micellar effects on acid-base equilibria [11] to estimate micellar surface potentials, and the relationship between log(k + / k − ) and the absolute difference in the estimated surface potentials of CTACl and SDS micelles gives the apparent fractional charges at the reaction centers.…”

Effects of Nonionic Micelles on Dephosphorylation and Aromatic Nucleophilic Substitution

“…This mechanistic distinction is similar to that used to rationalize the different micellar effects upon decarboxylations, which are strongly accelerated by cationic micelles (36), and watercatalyzed deacylations which, with few exceptions, are micellar inhibited (35,37,38).…”

Micellar‐improved synthesis of bis‐quaternary ammonium salts by the epichlorohydrin route

“…In this connection, our primary impulse to improve the preparation of bis-quaternary ammonium salts by the epichlorohydrin route arose from the unique features of the micellar systems for alteration of rates and/or paths of reaction (15)(16)(17)(18). Our earliest results were in agreement with this expectation (19).…”