Different Kinetic Behaviors for Unimolecular and ... - ACS Publications

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J. Phys. Chem. B 2009, 113, 8828–8834

Different Kinetic Behaviors for Unimolecular and Bimolecular Ester Hydrolysis Reactions in Strongly Acidic Microemulsions E. Fernańdez, L. Garcıá-Rıó,* M. Meńdez-Pe´rez, and P. Rodrı´guez-Dafonte Departament of Physical Chemistry, Faculty of Chemistry, UniVersity of Santiago, 15782 Santiago, Spain ReceiVed: January 16, 2009; ReVised Manuscript ReceiVed: May 4, 2009

Replacing the counterion in sodium bis(2-ethylhexyl)sulfosuccinate with H+ allows strongly acidic microemulsions to be obtained. These systems are the only known colloidal medium in which it is possible to reach local concentrations of acid, expressed as Hammett acidity function (H0), lower than H0 ) -0.2, which corresponds to a concentration of acid above 1 M in aqueous solution. In the present work, there has been analyzed the influence of this type of microemulsion on the acid hydrolysis of two esters derived from picolinic acid: 4-nitrophenylpicolinate (NPP) and 2,4-dinitrophenylpicolinate (DNPP). The reaction rate for NPP and DNPP increases up to 16 times on increasing the size of the aqueous nanocore of the microemulsion, which supposes an experimental behavior opposed to the one observed for the hydrolysis of nitrophenylacetate (NPA). The key to this differentiated behavior of NPP and DNPP resides in the fact that the rate-determining step for the acid hydrolysis mechanism is the water addition to the protonated ester. The reaction rate increases on increasing the nucleophilicity of water; that is, on increasing W (W ) [H2O]/[surfactant]). Therefore, the acid hydrolysis of esters in strongly acidic microemulsions presents an A2 mechanism when reactivity increases with W, and an A1 mechanism if it decreases with W. SCHEME 1

Introduction Water in oil microemulsions are composed of a polar solvent sequestered by a surfactant in a nonpolar solvent. The interaction of the surfactant polar headgroups with the polar solvent can result in the formation of a well-defined solvent pool.1 In water in oil (w/o) microemulsions, solvent pools are formed by water and act as nanobeakers in which chemical reactions can be carried out.2,3 The most used surfactant for the preparation of microemulsions is the sodium salt of bis(2-ethylhexyl)sulfosuccinate (NaOT or AOT). In the present work, we have modified the surfactant NaOT, substituting the Na+ ion with H+ (Scheme 1). The new surfactant (HOT) offers a range of stability similar to the NaOT one. The main characteristic of these new systems is the fact that the concentrations of acid achieved in the aqueous nanodroplet are superior to the ones which could be obtained through the addition of acid to a NaOT microemulsion. This type of microemulsions could be designated as strongly acidic microemulsions. The acidity function, H0, of the aqueous core of these microemulsions decreases with W (W ) [H2O]/[HOT]) from H0 ≈ 0.6 at W > 20 to H0 ) -1.4 at W ) 2.4 For smaller nanodroplet sizes, the acid character is similar to the one that could be obtained in aqueous solutions of HCl, HNO3, or CH3SO3H at concentrations ranging from 3 to 7 M. Subsequently, the degree of counterion binding (β) has been determined starting from values of H0.5 The obtained values (β ) 0.92-0.93) are compatible with theoretical calculations and are slightly larger than those obtained in aqueous micellar media (β ) 0.80), which can be attributed to the small amount of water available for H+ solvation. HOT microemulsions offer us the advantage of studying reactions in isolated solvent pools where water or the H+ ion is one of the reagents involved in the reaction process. * To whom correspondence [email protected].

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SCHEME 2

Acyl group transfer reactions are important in biochemistry: esters are one of the major functional groups of lipid chemistry, amides constitute the peptide bond in proteins, and thioesters serve as acyl donors and as intermediates in enzymatic catalysis.6 This is the reason why the hydrolysis reactions of these compounds are one of the most studied in physical organic chemistry.6-12 The study of the possible reaction mechanisms of the acid hydrolysis of esters show that we can find A1- and A2 (or ASE2)-type reaction mechanism. In A2-type reactions, conversion to the transition state occurs by attack of a nucleophile on the protonated species. The intervention of water as a nucleophile is just one of the possible examples for this type of reactions. In the ASE2-type reaction mechanism, the slow step is represented by the proton transfer (Scheme 2). Direct conversion of the protonated substrate to the transition state (A1) is much less probable, though changes in acidity and water activity can lead to this mechanism.7

10.1021/jp900461d CCC: $40.75  2009 American Chemical Society Published on Web 06/11/2009

Uni-, Bimolecular Ester Hydrolysis Reactions

J. Phys. Chem. B, Vol. 113, No. 26, 2009 8829

SCHEME 3

Previous studies in our laboratory have shown a mechanistic change in the acid hydrolysis of 4-nitrophenylacetate and 4-nitrophenyllaurate13 in HOT-based microemulsions. For low water contents (W < 15), an A1 mechanism has been observed that changes to A2 for higher water contents. This change of mechanism is due to the fact that, on decreasing the water content of the system, the water is associated with the surfactant counterion and presents low availability for reactions. A similar mechanism change has been observed in the presence of a high percentage of H2SO4 (higher than 70% by weight or an H0 value smaller than H0 ) -5.9).14 In the presence of a HOT-based microemulsion, the mechanism change is detected for a much lower acidity, H0 ) 0.3, equal to a H2SO4 solution of 2% by weight. The reason for the mechanism change to be observed at a lower acidity is that there is a different availability of water to solvate in the microemulsion. To study in depth the influence of functionalized microemulsions on acid hydrolysis of esters, we investigate in the present work the decomposition of 4-nitrophenylpicolinate (NPP) and 2,4-dinitrophenylpicolinate (DNPP) in the presence of HOT microemulsions (Scheme 3). Unlike the nitrophenylacetate (pKa = -7),15 NPP and DNPP will be totally protonated (pKa = 1.5 for NPP and 0.7 for DNPP) in the HOT microemulsions, which implies that the reactivity differences observed when varying the microemulsion composition could be interpreted as a consequence of alterations in the physical properties of the reaction medium. The results obtained are consistent with the fact that the hydrolysis reactions take place via A2 mechanisms and the rate constant increases on increasing the water content of the microemulsion. This behavior is consistent with the enhancement of the water nucleophilic character of the HOT-based microemulsions on increasing the W parameter. There are various methods that allow us to determine the hydrolysis reaction mechanism, such as the Zucker-Hammett criterion18 and the methods developed by J. F. Bunnett in the 60s.19-22 Bunnett and Olsen extended the linear free energy relationship (LFER) to acid-catalyzed reactions23 by the introduction of the solvating parameter, Φ, that represents the response of the equilibrium to the increase in [H+] and subsequent decrease in water activity and measures the effects brought about by the changes of hydration. Scorrano et al.24 suggest that the slope parameter, Φ, is a measure of the solvation requirements of the species involved in the protonation equilibrium. Therefore, it is possible to obtain information on the solvation; on the amount of charge localization; and hence, on the structure of the transition state from the Φ parameter. The Bunnett-Olsen equation is still valid today since it is still used in the study of hydrolysis reactions in media of moderate or strong acidity.9,25,26 The results obtained in the present work allow us to propose the utilization of acid microemulsions as an efficient alternative to the linear free energy relationship for the determination of the reaction mechanism in the acid hydrolysis of esters. Such utilization is based on the fact that the rate constant of the reactions that proceed by an A1 mechanism decreases on increasing the water content of the microemulsion, inasmuch as the rate constant would increase if the reaction proceeded by an A2 mechanism.

Experimental Section HOT was prepared from NaOT by ion exchange through Amberlite IR 120 (plus) resin. Other authors have prepared the surfactant HOT in previous studies as an intermediate step for replacing the Na+ counterion of NaOT.27 The second step in replacing the counterion should be the neutralization of HOT with the appropriate base. The extent of Na+/H+ exchange was assessed by using two methods: namely, (a) atomic absorption spectroscopy, which confirmed the absence of Na+ from the HOT samples (the residual Na+ content was consistent with a degree of Na+/H+ exchange greater than 99%), and (b) acid-base titration, the results of which were also consistent with an extent of exchange exceeding 99%. The compositions of the w/o microemulsions were chosen in such a way as to span wide ranges; thus, the concentration of HOT ranged from 6 to 30 wt %, that of water ranged from 0.5 to 43 wt %, and that of isooctane ranged from 29 to 93 wt %. ATR-FTIR spectra were recorded on a Spectra-Tech Horizontal ATR instrument equipped with a 45° crystal mounted on a Volatile Liquid Cover. A resolution of 4 cm-1 and 1000 scans were used. ATR-FTIR spectra were recorded on a Mattson Cygnus 100 spectrometer. NMR spectra were recorded with the aid of a coaxial tube filled with DMSO-d6 (Aldrich, 99.9%) to lock onto the deuterium signal. The signals of tetramethylsilane were used as 1H NMR references. All spectra were recorded on a Bruker AM 500 MHz spectrometer. The NPP and DNPP were prepared by stirring picolinic acid (0.01 mol), 4-nitrophenol or 2,4-dinitrophenol (0.01 mol), and dicyclohexylcarbodimide (0.01 mol) in chloroform (100 mL) for 16 h. The solution was filtered to remove the precipitated dicyclohexylurea, and the solvent was removed by rotary evaporation. The obtained ester was treated with decolorizing carbon in methanol and recrystallized from methanol. NPP had a mp of 145-147 °C; DNPP, 151-153 °C.28,29 The purity of the final product was checked using 1H NMR spectroscopy. All NMR spectra were taken at a 500 MHz Bruker Unity instrument in CDCl3. These values were verified with those previously reported.28,29 The hydrolysis reactions were followed by monitoring the UV-vis absorbance of substrate solutions in the concentration range (3.0-4.0) × 10-4 M using a Cary 500 scan UV-vis-NIR spectrophotometer fitted with thermostatted cell holders. The wavelengths used for the kinetic studies fell in the range of 375-380 nm. The integrated first-order rate expression was fitted to the absorbance-time data by linear regression (r > 0.999) in all cases. The observed rate constants, kobs, could be reproduced with an error margin of 5%. All experiments were carried out at (25.0 ( 0.1)°C. Results 1. Influence of the Microemulsion Composition. To investigate the influence of the microemulsion composition on the rate constant of the hydrolysis of NPP and DNPP, a series of experiments were carried out by varying the surfactant concentration while keeping constant the droplet size. Thus, there was studied the way in which the distribution of the reagent

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Fernańdez et al. SCHEME 4

Figure 1. Influence of [HOT] on kobs for acid hydrolysis of NPP in water/HOT/isooctane microemulsions at 25 °C for W ) 3 (o), 6 (b), 10 (0), and 17 (9).

Figure 2. Influence of [HOT] on kobs for acid hydrolysis of DNPP in water/HOT/isooctane microemulsions at 25 °C for W ) 4 (o), 7 (b), 10 (0), and 15 (9).

along the different microemulsion microenvironments affects reactivity without varying the nanopool acidity. In Figures 1 and 2, one can observe how an enhancement of the surfactant concentration supposes a slight increase in the reaction rate constant (kobs) in experiments in which the W parameter is kept constant. On increasing the volume of the interphase, due to the increment of the [HOT] concentration, the distribution equilibrium of the ester between the continuous medium and the interphase causes an increase in its concentration in the last zone and, therefore, an increase in the hydrolysis rate. Comparison of the results obtained for similar surfactant concentrations and different W values shows that the rate constant increases with W. Because the acidity of HOT-based microemulsions4 ranges from H0 ) 0.6 to -1.4, we can conclude that both NPP and DNPP are fully protonated. In this way, the rate-limiting step for the reaction is the water attack on the protonated substrate. The increase in the rate constant with W should reflect the increase in the nucleophilic character of water with W. This change in the nucleophilic character of water was previously reported in our laboratory for HOT-based microemulsions on studying solvolytic reactions.30 2. Kinetic Analysis Method. Kinetic studies of reactions in water in oil (w/o) microemulsions can be interpreted in terms of reactivity only if local reagent concentrations and intrinsic rate constants in the various microphases of these organized media can be obtained from the overall apparent rate data. To apply the pseudophase formalism, we must consider the microemulsion formed by three strongly differentiated pseudophases: an aqueous pseudophase (w), a continuous medium formed fundamentally by organic solvent (o), and an interface formed fundamentally by the surfactant (i). As usual, upon

studying the reactivity in colloidal systems (micelles, vesicles, and microemulsions), the activity coefficients of the components in these systems are independent of their concentrations. There is a controversy in the scientific community about the microstructure of water in oil microemulsions, mainly at very low water contents. In fact, some authors have suggested that for low water contents, the aggregate microstructure will deviate from the droplet model. Therefore, the pseudophase model considering the existence of three pseudophases should be wrong. However, recent results from our laboratory studying the effect of AOT-based microemulsions on the reaction between crystal violet and the sulfite ion have shown irrefutable evidence of the existence of three well-differentiated microenvironments in the AOT-based microemulsions, even at very low water contents.31 The existence of three well-differentiated microenvironments is also supported by the degree of counterion binding to the surfactant,32-34 FT-IR studies,30,35 photon correlation spectroscopy,36 SAXS, and SANS, as well as by the use of microemulsions as templates for the synthesis of nanoparticles.37 Because of the very low water solubility of NPP and DNPP, they will partition between the continuous medium and surfactant film, where the reaction is taking place. In Scheme 4, we show schematically the distribution of the substrates between the continuous medium and the interface of the microemulsion. Moreover, as we have already mentioned in the previous section, in this type of reaction, it is necessary to take into account the existence of the protonation equilibrium of the ester at the interface of the microemulsion. The equilibrium constant Koi is the partition constant of the substrate between the continuous medium and the interface, and Kc is the protonation equilibrium constant of NPP,

[NPP]i Z, Koi ) [NPP]o

Kc )

[NPP - H+]i [NPP]i[H+]i

(1)

where Z is the composition parameter for the microemulsion, defined as Z ) [isooctane]/[HOT] by analogy with W. On the basis of the kinetic diagram of Scheme 4, we obtained the following expression for the pseudo-first-order rate constant, kobs, as a function of the microemulsion composition (see the Supporting Information for a complete derivation of eq 2):

kobs ) kiKc

Koi β j HOT Koi + Z V

(2)

where ki is the rate constant of solvolysis at the microemulsion j HOT is the molar volume of HOT, and β is the degree interface, V of counterion binding. Equation 2 can be rewritten as


j HOT j HOT V V 1 ) + Z kobs kiKcβ kiKcKoiβ


(3)

Equation 3 predicts the existence of a linear dependence between the inverse of the rate constant and the Z parameter of the microemulsion composition. Figures 3 and 4 show some examples of the good linear adjustments that are obtained when representing graphically the inverse of kobs opposite to Z, maintaining W constant. From the slopes and intercepts of Figures 3-4 and according to eq 3, we obtain Koi values for NPP (Koi ) 17 ( 2) and DNPP (Koi ) 24 ( 3). The fact that the partition constants between the continuous medium and the interface of HOT-based microemulsions are W-independent is a proof29 (see Supporting Information) that neither NPP nor DNPP is distributed into the water nanodroplet. 3. Reactivity at the Interface of HOT-Based Microemulsions. Once the distribution of ester among the different system pseudophases is quantitatively determined, we can analyze the experimental kinetic results. Likewise, in eq 3, the degree of counterion binding, β ≈ 0.92,5 and the molar volume of HOT, j HOT ) 0.418 M-1, are known. Therefore, the product kiKc could V be obtained either from the ordinates or the slopes of eq 3. The error associated with the intercepts is much smaller; thus, from the data presented in Tables S-3 and S-4 of the Supporting Information, we can determine the variation of kiKc with W for NPP and DNPP. The results are displayed in Figure 5, where it can be observed that kiKc increases with the nanodroplet size of the microemulsion. The relatively facile hydronium ion-catalyzed reaction at low pH very likely reflects protonation of the pyridine nitrogen; that is, the reaction involves attack of H2O on the conjugate acid species. There is little or no C-O bond-breaking in the transition state, and electron withdrawal in the leaving group has little influence upon the ease of nucleophilic attack. In contrast, changes in the structure of the acyl group bring about large changes in the rate constants. Thus, the cinnamic picolinic acid anhydrides are at least 100-fold faster than those of DNPP, even though the leaving groups in both cases have pKa values near 1.28 As can be observed in Figure 5, the product kiKc increases with W. To explain the variation of kiKc with W, it is necessary to know the influence of the water content on the protonation equilibrium constant, Kc, and on the rate constant, ki. Moreover, Figure 5 shows that the product kiKc is higher for DNPP than for NPP, which is consistent with the higher electrophilic character of the carbonyl group of DNPP. Discussion 1. Reactivity in the Interphase of the Microemulsion. To explain the influence of microemulsion composition on kiKc (see Figure 5), we need to understand independently its influence on the protonation equilibrium constant and the rate constant. From previous studies38 on metal-ligand complexation in NaOT microemulsions, we can evaluate the influence of W on the protonation equilibria constants for NPP and DNPP. In our laboratory, the complexation of Ni2+ and Co2+ with the bidentate ligand pyridine-2-azo-p-(N,N-dimethylaniline) (PADA) in AOT/isooctane/water microemulsions has been studied (see Scheme 5). The results indicate that the values of the formation constants of the complexes Ni2+-PADA and Co2+-PADA increase as the water content of the microemulsions decreases. The complexation constant, Ni2+-PADA, in microemulsions of AOT/

Figure 3. Influence of Z on 1/kobs for acid hydrolysis of NPP in water/ HOT/isooctane microemulsions at 25 °C for W ) 3 (o), 6 (b), 10 (0), and 17 (9). The solid line corresponds to the linear adjustment to eq 3.

Figure 4. Influence of Z on 1/kobs for acid hydrolysis of DNPP in water/HOT/isooctane microemulsions at 25 °C for W ) 4 (o), 7 (b), 10 (0), and 15 (9). The solid line corresponds to the linear adjustment to eq 3.

Figure 5. Variation of kiKc with W for acid hydrolysis reaction of NPP (o) and DNPP (b) in water/HOT/isooctane at 25 °C. The solid line is a guide for the eye.

isooctane/water varies with W between 13 000 M-1 for W)1 and 4000 M-1 for W ) 45. For the complex Co2+-PADA, the complexation constant is equal to 1000 M-1 for W ) 1 and 400 M-1 for W ) 45. We can observe that in both cases, the complexation constant increases three times on decreasing the nanodroplet size and approaches the values obtained in bulk water. In HOT microemulsions, we can consider that the equilibrium constant of the protonation of ester will increase on decreasing the water content of the system, as in the case of the constant of formation of the complex PADA-Ni2+. Therefore, the enhancement of the reaction rate in the interphase with W would be more important than the one shown in Figure 5. If we compare, for example, the reactivity in the interphase for W ) )/(kiKW)2 ) 2 and W ) 25, we observe that the quotient (kiKW)25 c c

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Figure 6. Variation with W of the resonance signal for the hydrogen atoms in the water molecules in (o) water/NaOT/isooctane and (b) water/HOT/isooctane microemulsions.

is equal to 3.0 for NPP and 16.3 for DNPP. This quotient would indirectly measure the catalysis that takes place in the interphase core on increasing the nanodroplet size. Considering that Kc decreases approximately 3 times on increasing W (as with the M2+-PADA), we find that (kiKcW)25)/(kiKcW)2) ) (kiW)25)/ ), and therefore, on increasing the size of the aqueous (0.33kW)2 i core of the microemulsion, ki would have to increase some 10 and 15 times for NPP and DNPP, respectively. 2. Influence of the Nature of Water. The obtained results show that the acid hydrolysis reaction of NPP and DNPP is favored by the enhancement of the nanodroplet size of the system. Water molecules confined in organized assemblies control the structure, function, and dynamics of these systems in a unique way. In bulk water, the dielectric relaxation time is 8 ps, whereas the solvation dynamics occurs on the subpicosecond time scale. Microemulsions39 display a minor component on the 100-1000 ps time scale. Concerning the different types of water, we must conclude that it is difficult to extrapolate results from solvation dynamics (usually on the time scale of nanoseconds) to the ester hydrolysis (usually with half-life times of 700 s). On the time scale of the ester hydrolysis reactions, solvation dynamics can be seen as a stationary equilibrium reflecting different states of water. Recently, we have studied the properties of water in HOT microemulsions by the techniques of FT-IR and 1H NMR. 1H NMR spectroscopy revealed that water is structured differently from in water/NaOT/isooctane microemulsions. The addition of water to a microemulsion has no significant effect on the chemical shifts of hydrogen atoms other than those in H2O. The proton magnetic resonance of water exhibits a single peak (δwater) that reflects fast exchange between the different types of water (water trapped between the surfactant alkyl chains, water bound to headgroups, free water, and water bound to the counterion). Figure 6 shows the variation of the resonance signal for the hydrogen atoms in the water molecules (δwater) with droplet size in water/NaOT/isooctane and water/ SCHEME 5

Fernańdez et al. HOT/isooctane microemulsions. Most of the water present in NaOT microemulsions at low W values interacts with the SO3groups of NaOT. In HOT microemulsions, water molecules exhibit a stronger affinity for H+ ions than they do for the surfactant headgroups. δwater changes from 3.9 ppm (NaOTbased microemulsiones) or 8.3 ppm (HOT-based microemulsions) at low W values to levels close to that for bulk water (4.8 ppm) at high W values. Apart from confirming the results obtained for 1H NMR, the FT-IR technique allows us to differenciate between the four types of water present in a microemulsion. In Figure 7, we can see the variation of the percentage of free water and bounded water in the HOT-based microemulsion with W. On increasing the nanodroplet size, the percentage of free water increases from 20% to 40%, while the counterion-bounded water, which was initially 65% of the total, decreases to 50%. In any case, the H+-bounded water remains the major water type in the W range. This aspect supposes an important divergence with respect to NaOT-based microemulsions in which the free water is more abundant, followed by the surfactant headgroup-bounded water. The difference also justifies the behavior observed in the signal of 1H NMR. These changes in the properties of water in HOT microemulsions have important kinetic repercussions. In HOT microemulsions, the dominant interaction is that of water with the counterion (H+). Nucleophilicity in these systems decreases with decreasing water content of the microemulsion. Increasing the water content of the microemulsion increases the number of free water molecules. These structural changes have been kinetically demonstrated when studying the solvolysis reaction of phenyl chloroformate and nitrophenyl chloroformate, for which the reaction rate is governed by the nucleophilicity of water. In the reaction mechanism of the solvolysis of aryl chloroformates, the limiting step is the addition of water.40,41 The results obtained when studying these solvolysis reactions in these microemulsions have demonstrated that the rate constants increase on increasing the size of the aqueous nanodroplet, showing the increase in the nucleophilic character with W. For the acid hydrolysis of NPP and DNPP, the reaction mechanism is obviously different from that of the hydrolysis of chloroformates, but when comparing Figures 8 and 5, we can observe how the effect of the change in the nucleophilicity of water in the aqueous core of the microemulsion similarly affects the reactivity of both processes. Therefore, the reaction rate of picolinates in HOT microemulsions will be determined by changes in the medium nucleophilicity. 3. Mechanistic Criterium. To conclude, we must emphasize the difference in behavior found when comparing the influence of HOT-based microemulsions on the hydrolysis reactions of picolinates (which take place via an A2 mechanism) and nitrophenylacetates (which take place mainly via an A1 mechanism). In the case of the hydrolysis of nitrophenylacetates, reactivity increases



Figure 7. Variation of the percentage of free water (o), H+ bounded water (9), SO3- bounded water (4) and trapped water (2) with W in water/ HOT/isooctane microemulsions. The solid lines are a guide for the eye.

SCHEME 6

Figure 8. Variation of log ki with W for the solvolysis of O2NPhOCOCl in water/HOT/isooctane. The solid line is a guide for the eye.

Figure 9. Variation of ki or kiKc with W for acid hydrolysis reaction of (o) NPL and (b) DNPP in water/HOT/isooctane at 25 °C. The solid line is a guide for the eye.

on decreasing the water content of the system, whereas for the hydrolysis of picolinates, an enhancement of reactivity takes place on increasing W (Figure 9). The variation of the bimolecular rate constant of hydrolysis with W must be interpreted as a consequence of the effects of the medium derived from the high ionic content present in the interior of the aqueous nanodroplet and its effect on both the rate and equilibrium constants. The reason for this difference in the experimental behavior lies in the different mechanism by which both reactions proceed and in the manner in which the changes in the composition of HOT-based microemulsions can affect the different reaction steps. In Scheme 6, the differences between A1 and A2 mechanisms for the hydrolysis of esters can be observed in more detail. In mechanism A2, the determining rate step will be the nucleophilic attack of water on the protonated ester. This process is controlled by the nucleophilic character of water that, in the case of HOT-based microemulsions, increases on increasing the

W parameter. Consequently, the reactivity of the hydrolysis processes that proceed by A2 mechanisms increases with the water content of the microemulsion. For NPL and NPA, a decrease in the droplet size implies a reactivity increase. It is important to emphasize that, in aqueous medium, a marked difference between the behavior of NPA and NPP regarding their acid hydrolysis can be observed. Thus, for the acid hydrolysis of NPP, the rate constant increases on decreasing the pH up to pH ) 1, remaining constant for higher acidity values. However, in the case of acid hydrolysis of nitrophenyl acetate, the observed rate constant increases with the medium acidity up to values of H0 ) -4. This difference in behavior is closely related to the different protonation facility of the two esters: pKa ) -7 for NPA and pKa ) 1.5 and 0.7 for NPP and DNPP, respectively. As previously mentioned, the equilibrium constant for the formation of the PADA-Ni2+ complex increases on decreasing the water content of the microemulsion due to the insufficient solvation of Ni2+. These results may be extrapolated to the acid hydrolysis of esters. The ease of protonating the esters derived from the picolinic acid (pKa ) 1.5) determines their complete protonation in HOT-based microemulsions. Therefore, an enhancement of the protonation equilibrium constant on decreasing W has no repercussion on the percentage of the protonated ester. However, in the case of NPA, the increase in the protonation equilibrium constant on decreasing the water content of the microemulsion must cause an increase in the

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SCHEME 7

Fernańdez et al. compositions and derivation of eq 2 are reported. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

concentration of the protonated ester. The percentage enhancement of the protonated ester can be interpreted as an increase in reactivity on decreasing W. Another determining factor in the hydrolysis of NPA is the effect of the medium on the step in which the protonated ester unimolecularly dissociates to form an acylium ion. Since the positive charge is more located in the acylium ion, it seems logical that a stabilization of that charge would lead to a reactivity increase (Scheme 7). In this respect, the stabilizing/ destabilizing electrostatic effect exerted by the surfactant headgroups on organic moieties in reactions that take place in aqueous micelles42 and in oil microemulsions43 is well-known. In this case, in HOT microemulsions, the acid hydrolysis reaction of NPA and NPL is favored by the electrostatic interaction between the anionic headgroup and the acylium ion, RC+O. This electrostatic interaction is higher when the droplet size decreases, since the decrease causes an increase in the negative charge42 on the sulfonate group of the surfactant as a consequence of its insufficient solvation. Conclusions A kinetic study of the hydrolysis of esters of picolinic acid in water/HOT/isooctane microemsulsions has been carried out. The special physicochemical properties of these strongly acidic microemulsions play a fundamental role in the observed experimental behavior. The influence of the aqueous nanodroplet size on the acid hydrolysis of NPP and DNPP shows an increase in the reaction rate as W increases. This result is relevant, since it opposes the one previously found for acid hydrolysis of nitrophenylacetate (NPA) and nitrophenyllaurate (NPL). Unlike the nitrophenylacetate (pKa = -7),15 NPP and DNPP will be fully protonated (pKa = 1.5 for NPP and 0.7 for DNPP) in the HOT microemulsions. The reason for the different behavior should be sought in the reaction mechanism; namely, A2 type for NPP and DNPP, and A1 for NPA and NPL. It implies that the reactivity differences observed when varying the microemulsion composition should be interpreted as a consequence of alterations in the physical properties of the reaction medium. As we have shown previously,30 an increase in the nanodroplet size gives rise to an enhancement of the water nucleophilicity, and since water addition is the determining step in the A2 mechanism, an increase in the water nucleophilicity causes an increase in the reaction rate. Acknowledgment. This work was supported by Ministerio de Ciencia y Tecnologıá (Project CTQ2008-04420/BQU) and Xunta de Galicia (PGIDIT07-PXIB209041PR and 2007/085). MMP thanks Ministerio de Educación y Ciencia for a FPU fellowship. Supporting Information Available: A compilation of observed rate constants and partition constants between the continuous mediun and the interface for different microemulsion

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