Kinetics and Mechanism of Nanoparticles-Catalyzed Piperidinolysis of Anionic Phenyl Salicylate

The values of the relative counterion (X) binding constant R X Br (=K X/K Br, where K X and K Br represent cetyltrimethylammonium bromide, CTABr, micellar binding constants of X v− (in non-spherical micelles), v = 1,2, and Br− (in spherical micelles)) are 58, 68, 127, and 125 for X v− = 1−, 12−, 2−, and 22−, respectively. The values of 15 mM CTABr/[Nav X] nanoparticles-catalyzed apparent second-order rate constants for piperidinolysis of ionized phenyl salicylate at 35°C are 0.417, 0.488, 0.926, and 0.891 M−1 s−1 for Nav X = Na1, Na2 1, Na2, and Na2 2, respectively. Almost entire catalytic effect of nanoparticles catalyst is due to the ability of nonreactive counterions, X v−, to expel reactive counterions, 3 −, from nanoparticles to the bulk water phase.


Introduction
Research on nanoparticles has now become a cutting-edge area of chemical research [1]. Mono-and bilayer surfactant aggregates are nanoparticles which have been known for their characteristic physicochemical properties for more than 100 years [2]. The effects of surfactant aggregates/nanoparticles of different structural features on reaction rates have been extensively studied for the past nearly six decades [3][4][5]. These studies reveal very complex mechanistic aspects of micellar/nanoparticles catalysis of reaction rates [4][5][6]. Effects of counterionic salts on ionic surfactant as well as biomolecular structural transitions have been under extensive study since 1887 when Hofmeister first reported specific salt effects on the salting-out proteins [7]. But the mechanistic aspects of these specific salt effects are not yet fully understood [8][9][10].
Effects of inert salts of moderately hydrophobic counterions, such as benzoate and substituted benzoate ions, on ionic surfactant micellar growth have become very important for various industrial applications [9][10][11]. However, mechanistic details of such inert salt effects on ionic micellar growth are almost nonexistent. Effects of inert counterionic salts on pseudo-first-order rate constants ( obs ) for the ionic surfactant nanoparticle-catalyzed semi-ionic bimolecular reactions, where ionic reactant is also a counterion, have been explained quantitatively by the use of pseudophase ion-exchange (PIE) model. But the use of PIE model involves mostly counterionic salts of highly and moderately hydrophilic counterions [12]. However, some inherent weaknesses of PIE model have been also realized [13,14]. The increase in [MX] (MX = 3-and 4-FBzNa with Bz − = C 6 H 4 CO 2 − ) has caused nonlinear increase in obs for piperidinolysis of anionic phenyl salicylate (PSa − ) at a constant [CTABr] T ≫ cmc where [CTABr] T and cmc represent total concentration of cetyltrimethylammonium bromide and critical micelle concentration of CTABr, respectively [15]. The magnitudes of the gradient of the plot of obs versus [MX] show continuous decrease with increasing [MX] [15]. The values of obs remained almost independent of [MX] within its range where the presence of 5 mM CTABr resulted in more than 10-fold increase in obs . Thus, 5 mM CTABr/[MX] nanoparticles act as catalyst because, in the absence of CTABr, the values of obs remained independent of [MX] within its range covered in the study [15]. More than 10-fold catalytic effects of CTABr/MX nanoparticles were not emphasized and discussed in the report [15]. The catalytic effects of CTABr/MX/H 2 O nanoparticles catalyst (MX = 4methoxy and 4-methyl salicylates) on obs for piperidinolysis 2 The Scientific World Journal 1H, X = OMe, Y = Z = H Figure 1: Molecular structures of compounds 1H, Na1, Na 2 1, 2H, Na2, Na 2 2, 3H, Na3, 4, Na5, and 6.
of PSa − have been studied in the present study. The results and their probable explanations are described in this paper.

Materials.
Reagent-grade 4-methoxysalicylic acid (1H), 4-methylsalicylic acid (2H), cetyltrimethylammonium bromide (CTABr), phenyl salicylate (3H), and piperidine (4) ( Figure 1) were commercial products of highest available purity. Other common chemicals used were also of reagent grade. The stock solutions of 0.50 M M V (=Na V 1 and Na V 2 with V = 1 and 2) were prepared by adding 0.52 and 1.25 M NaOH to the corresponding 0.50 M solutions of 1H or 2H. The stock solutions of 0.01 M 3H were prepared in acetonitrile. Throughout the text, the symbol [ ] T represents the total concentration of .

Kinetic
Measurements. The rate of CTABr/Na V nanoparticles-catalyzed nucleophilic substitution reaction of 4 with Na3 was studied spectrophotometrically at 35 ∘ C by monitoring the disappearance of Na3 at 365 or 370 nm. The products of the reaction of 4 with Na3 are sodium Npiperidinyl salicylate (Na5) and phenol (6) ( Figure 1). The details of the kinetic procedure and product characterization have been described elsewhere [16]. Absorbance values ( ob ) at different reaction time ( ) were found to fit to (1) for ∼8 half-lives of the reactions. In (1), [ 0 ] represents the initial concentration of 3H, ap is the apparent molar absorptivity of the mixture, obs is the pseudo-first-order rate constant, and ∞ = obs at = ∞. Throughout the study, the initial concentrations of 3H or Na3 were kept constant at 0.2 mM. The choice of this specific concentration was governed by the need to keep it sufficiently low so that it is less than the other salicylate counterions but high enough to measure the absorption spectrophotometrically.   Table 2. The dotted line is drawn through the predicted data points assuming the presence of WM at [Na1]  [ and Na V 2. The shape of the plot of Figure 3 is similar to that of Figure 2 when [Na V 2] ≤ ∼20 mM. The increase in [Na V 2] at ∼20 mM Na V 2 reveals a mild increase followed by a decrease and then increase again in the values of obs (Figure 3). Similar break in the plot (not shown) of obs versus [Na V 2] was also obtained at [NaOH]/[2H] = 2.5. These observations may be attributed to the change in the structure of Na V /CTABr nanoparticles to some higher interfacial curvature structures such as curved bilayer structures at ∼20 mM Na V 2 [17]. The absence and presence of break in the monotonic plot of respective Figures 2 and 3 are indirectly supported by the following observations. The values of ap , obtained for piperidinolysis of 3 − at 10 mM NaOH, 100 mM Pip, 0.2 mM 3H, 35 ∘ C, and 370 nm, increase nonlinearly from 1750 to 4350 M −1 cm −1 with the increase in CH 3 CN content from 2 to 92% v/v in mixed aqueous solvent ( Table 1). The values of ap , obtained for piperidinolysis of 3 − at 30 mM NaOH, 100 mM Pip, 0.2 mM 3H, 35 ∘ C, 370 nm, and different values of [Na V ], for Na V 1 and Na V 2 (V = 1, 2), are also summarized in Table 1. It is evident from Table 1  within its range ∼70-300 mM. These observations simply demonstrate that Na V -induced CTABr/Na V nanoparticles structural transition, within [Na V ] range of 50-300 mM, is not the same for Na V 1 and Na V 2 (V = 1, 2).

Discussion
The experimental data ( obs versus [Na V ]) exhibited by Figures 2 and 3 (at [Na V 2] < ∼21 mM) were found to fit to empirical equation: op 0 were calculated using an iterative technique as described elsewhere [15]. These values of op 0 obtained by the graphical technique [5]. As described in detail elsewhere [15,18], the value of [Na V ] op 0 represents the optimum value of [Na V ] required for the occurrence of ion exchange processes − /OH − and − /Br − . Equation (2), with replacement of cat by / where is an empirical constant, has been found to explain quantitatively similar observed data ( obs versus [Na V ]), for different Na V [5]. The nonlinear least-squares technique was used to calculate cat and / from (2) Table 2. The statistical reliability of the observed data fit to (2) is evident from the standard deviations associated with the calculated values of cat and / as well as from the solid line plots of Figures 2 and 3 which were drawn through the least-squares calculated data points.
It has been described in detail elsewhere [5,15,18] that the nonlinear increase in obs with the increase of [Na V ] at a constant [CTABr] T is due to the transfer of micellized 3 − (i.e., 3 − with subscript indicating micellar pseudophase) to aqueous phase (i.e., 3 − with subscript indicating bulk water phase) through the occurrence of ion exchange process V− /3 − . This is due to the reason that the value of obs is more than 10-fold larger in the bulk water phase than that in the micellar pseudophase as evident from the listed values of MX and 0 in Table 2. The occurrence of ion exchange V− /3 − in the related reaction systems [5] has been found to decrease the CTABr micellar binding constant ( ) of 3 − with the increasing [Na V ] through an empirical relationship: where 0 = at [Na V ] = 0 and / represents an empirical constant whose magnitude is the measure of the ability of counterion V− to expel another counterion − from the cationic micellar pseudophase to the bulk aqueous phase through the occurrence of ion exchange process V− / − at the cationic micellar surface. It can be easily shown that the reaction mechanism for nucleophilic reaction of 4 with 3 − , expressed in terms of pseudophase micellar (PM) model and (3), can lead to (2) [18] with cat and / expressed by (4) and (5), respectively. As shown in the following equation, MX = obs [Na V ] obs [Na V ] / is an MX = obs obtained within [Na V ] range where obs values are independent of [Na V ] in the absence of CTABr and / is an empirical constant whose magnitude should vary in the range >0.0 to ≤1.0 [18]. The following equation  Table 2 and these calculated values of / for Na1, Na 2 1, Na2, and Na 2 2 are also listed in Table 2. The value of / was calculated from (5) with the reported value of 0 (=7 × 10 3 M −1 [5,15]). The calculated values of / for Na V with V = 1,2 and = 1, 2 are shown in Table 2. It has been concluded elsewhere [5,18] [21] and WM for V = sodium salicylate [22], sodium 3-, 4-, and 5-methyl salicylate [23], and Na V 1, Na V 2 where V = 1, 2. These observations cannot be explained in terms of Hammett substituent constants ( , 4-OMe ). These observations reveal that the shapes and sizes of the aqueous CTABr/ V nanoparticles depend apparently upon the magnitudes of Br . The magnitude of Br is apparently governed by the combined effects of steric requirements and hydrophilic and hydrophobic interactions of counterion − with cationic headgroup. Hydrophilic interaction includes ion-ion, ion-dipole, dipoledipole, and inter-and intramolecular hydrogen-bonding interactions.
The values of cat versus Br (Table 2) reveal a linear relationship with intercept = 0 and slope = (7.20 ± 0.07) × 10 −3 M −1 s −1 . This observation implies that almost entire catalytic effect of CTABr/Na V nanoparticles catalyst is due to the ability of nonreactive counterions V− to expel the reactive counterions 3 − from CTABr/Na V nanoparticles to the bulk water phase. The values of cat and Br for Na are not significantly different from the corresponding values for Na 2 for = 1 and 2 ( Table 2). These results reveal that energetically favorable electrostatic interaction is apparently insignificant compared with hydrophobic interaction between counterions, V− , and aqueous cationic interface of CTABr/Na V nanoparticles. Perhaps, this is the first quantitative explanation of the earlier qualitative experimental observation that sodium salicylate and salicylic acid are equally effective in driving the micellar structural transition SM-to-WM at a constant temperature [23]. The aqueous structure of CTABr/Na V nanoparticles remains WM at 35 ∘ C, ≤15 mM CTABr and 12 mM ≤ [Na V ] ≤ ∼22 mM for Na V = Na V 1 and Na V 2 (V = 1, 2). But the values of cat are ∼2-fold larger for Na V 2 than those for Na V 1 ( Table 2). Thus, it is apparent that a quantitative correlation between cat and Br is better than that between cat and the aqueous structures of CTABr/Na V nanoparticles where rheologically assigned structures remain the same (WM) for both Na V 1 and Na V 2 at <22 mM Na V .

Conclusions
The linear plot of cat versus Br with essentially zero intercept reveals indirectly that the catalytic efficiency of CTABr/Na V /H 2 O nanoparticles catalyst is almost entirely due to the ability of nonreactive counterions, V− , to expel reactive counterions, 3 − from nanoparticles to the bulk water phase. Binding affinity of counterions, − and 2− , with CTABr/Na V /H 2 O nanoparticles (measured by the magnitude of Br ) remains nearly unchanged for = 1 and 2. The polarity of the CTABr/Na V /H 2 O nanoparticles-bound 3 − is not the same for V− = 1 V− and 2 V− , V = 1, 2, within [Na V ] range of ∼70-300 mM.