Determination of an Ion Exchange Constant by the Use of a Kinetic Probe: A New Semiempirical Kinetic Approach Involving the Effects of 3-F- and 4-F-Substituted Benzoates on the Rate of Piperidinolysis of Anionic Phenyl Salicylate in Aqueous Cationic Micelles

Abstract

Pseudo-first-order rate constants (k(obs)) for the nucleophilic substitution reaction of piperidine (Pip) with ionized phenyl salicylate (PS(-)), obtained at a constant [Pip](T) (= 0.1 M), [PS(-)](T) (= 2 x 10(-4) M), [CTABr](T) (cetyltrimethylammonium bromide), < or = 0.06 M NaOH, and a varying concentration of MX (= 3-FC(6)H(4)CO(2)Na, 3-FBzNa and 4-FC(6)H(4)CO(2)Na, 4-FBzNa), follow the kinetic relationship k(obs) = (k(0) + thetaK(X/S)[MX])/(1 + K(X/S)[MX]) which is derived by the use of the pseudophase micellar (PM) model coupled with an empirical equation. The empirical equation explains the effects of [MX] on CTABr micellar binding constant (K(S)) of PS(-) that occur through X(-)/PS(-) ion exchange. Empirical constants theta and K(X/S) give the parameters F(X/S) and K(X/S), respectively. The magnitude of F(X/S) gives the measure of the fraction of micellized PS(-) transferred to the aqueous phase by the limiting concentration of X(-) through X(-)/PS(-) ion exchange. The values of F(X/S) and K(X/S) have been used to determine the usual thermodynamic ion exchange constant (K(X)(Y)) for ion exchange process X(-)/Y(-) on the CTABr micellar surface. The values of K(X)(Br) (where Br = Y) have been calculated for X = 3-FBzNa and 4-FBzNa. The mean values of K(X)(Br) are 12.8 +/- 0.9 and 13.4 +/- 0.6 for X(-) = 3-FBz(-) and 4-FBz(-), respectively. Nearly 3-fold-larger values of K(X)(Br) for X = 3-FBz(-) and 4-FBz(-) than those for X = Bz(-), 2-ClBz(-), 2-CH(3)Bz(-), and the 2,6-dichlorobenzoate ion (2,6-Cl(2)Bz(-)) are attributed to the presence of wormlike micelles in the presence of > 50 mM 3-FBz(-) and 4-FBz(-) in the [CTABr](T) range of 5-15 mM. Rheological properties such as shear thinning behavior of plots of shear viscosity versus the shear rate at a constant [3-FBz(-)] or [4-FBz(-)] as well as shear viscosity (at a constant shear rate) maxima as a function of the concentrations of 3-FBz(-) and 4-FBz(-) support the conclusion, derived from the values of K(X)(Br), for the probable presence of wormlike/viscoelastic micellar solutions under the conditions of the present study.

Kinetic and rheological measurements of the effects of inert 2-, 3- and 4-bromobenzoate ions on the cationic micellar-mediated rate of piperidinolysis of ionized phenyl salicylate

“…The mechanistic details of CTABr micellar-mediated piperidinolysis of PS À have been described elsewhere [27,29]. The observations (a-e) reveal a brief reaction scheme for the cleavage of PS À , under the experimental conditions of the present study, as shown recently elsewhere [19].…”

“…More than 2-fold stronger CTA + micellar affinity of 3-CH 3 Bz À and 4-CH 3 Bz À than that of 2-CH 3 Bz À explains quantitatively the rheological observations obtained previously [6]. The values of K Br X for X = 3-and 4-FBz À and the effects of [MX] on CTABr micellar growth have been reported recently [19]. The values of K Br X for X = 2-BrBz À , 3-BrBz À and 4-BrBz À are not available from literature.…”

“…The absolute magnitudes of these negative deviations increase with the decrease in [MX] as well as increase in [CTABr] T . Such observations, in the related studies [18,19], have been attributed to the effects of the occurrence of independent ion exchange processes X À /HO À and X À /Br. The occurrence of these ion exchange processes (X À /HO À and X À /Br À ) is expected to decrease the effective concentration of [MX] (½MX ef S ) affecting the ion exchange process X À /S À (where S À = PS À ).…”

“…The rheological study was carried out using Brookfield R/S + rheometer with double gap coaxial cylinder (CC-DG) and the experimental details are the same as described earlier [19]. Table 1, at ½MX op 0 = 0.…”

Quantitative Correlation between Counterion (X) Affinity to Cationic Micelles and X-Induced Micellar Growth for X = 2,4-; 2,5-; 2,6- and 3,4-Dichlorobenzoate Ions

“…The rate of disappearance of PSa , as a function of reaction time t , was monitored at 350 nm using Shimadzu double beam UV-vis spectrophotometer Model UV-1650 . The details of the kinetic procedure and product characterization have been described elsewhere 27 . The absorbance values A ob at different t were found to fit to Eq.…”