Micellar effects upon reactions of the 2,2′,4,4′,4″-pentamethoxytrityl cation with nucleophiles

Abstract

Cationic micelles of cetyltrimethylammonium chloride and bromide (CTACl and CTABr) speed attack of water upon the 2,2',4,4',4'-pentamethoxytrityl cation by a factor of ca. 5. The first-order rate constant in water is 5.51 s-' at 25.0°C. Anionic micelles of sodium dodecyl sulfate (SDS) have little effect on this reaction, but they strongly inhibit attack of OH-. In water, second-order rate constants for attack of OH-, respectively, 235,177, and 2.8 x lo5 M-' s-'. Rate constants of reaction in CTACl go through maxima with increasing [surfactant] and analysis of the data shows that second-order rate constants at the micellar surface are similar to those in water. IntroductionAqueous ionic micelles speed bimolecular reactions of counterions by bringing substrate and ionic reagent together in the small volume of the micelles (1-6). The dependence of rate on [surfactant] can be treated quantitatively assuming that reactants are distributed between aqueous and micellar pseudophases, which are treated as distinct reaction media. Secondorder rate constants can be calculated for reaction in the micellar pseudophases, and for many bimolecular ionic reactions these second-order rate constants are similar to those in water. Thus the micellar rate enhancements are due largely to concentration of reactants in the micellar pseudophase.Reactions of dinitrohaloarenes with azide ion are major exceptions to this generalization, and second-order rate constants in cationic micelles are greater than those in water by factors of lo2-lo3 (7). We found similar large micellar effects for reactions of azide ion with hydrophobic N-alkyl-2-bromopyridinium ions. However, rate constants of deacylation and SN2 displacements by azide ion are similar in micellar and aqueous pseudophases (7), so that these unusual rate effects are not simply characteristic of azide ion as a nucleophile.Azide ion is very reactive towards carbocations, either preformed (8, 9) or generated in SN1 solvolyses (10, 11). The present investigation had the aim of comparing second-order rate constants for reaction of azide ion with a carbocation in water and micelles. Micellar effects upon nucleophilic addition to triarylmethyl dye cations have been well studied (12, 13), but we could not use such cations as Malachite Green or Crystal Violet because of the unfavorable equilibria with azide ion (14). The tri-p-anisylmethyl cation is also unsatisfactory, because it binds weakly to cationic micelles (13) and even in water the reaction is so fast that it can only be followed in very dilute azide

A novel probe for determination of electrical surface potential of surfactant micelles: N,N'‐di‐n‐octadecylrhodamine

“…This effect is well documented for crystal violet and some other triphenylcarbonium cations. 30,31 In the case of DODR in CTAB micelles, slow ROHformation in alkaline region cannot hinder the pK a a determinations, which are performed at low and medium pH.…”

The Kemp elimination in membrane mimetic reaction media. Probing catalytic properties of cationic vesicles formed from a double-tailed amphiphile and linear long-tailed alcohols or alkyl pyranosidesElectronic supplementary information (ESI) available: Further experimental details and figures. See http://www.rsc.org/suppdata/ob/b4/b403237c/

“…This means that although the parameters have different physical meaning they compensate each other to a large extent. [43][44][45][46][47][48] In order to get a better insight into the meaning of the fitted values of the parameters we calculated the error in these parameters by calculating χ 2 .…”

Effects of alcohol-modified micelles on deacylation and nucleophilic aromatic substitution by azide ion

“…Second-order rate constants, calculated from reactant concentrations in the micellar pseudophase, are often very similar to or smaller than those in water. 14 The treatment is based on the assumption that reaction takes place either in the aqueous or in the micellar pseudopha~e,~*~ and it seems to be satisfactory except when an ionic reagent, e.g. OH -, is present in high concentration.…”