Ionizing Power and Nucleophilicity in Water in Oil AOT-Based

7672

Langmuir 2005, 21, 7672-7679

Ionizing Power and Nucleophilicity in Water in Oil AOT-Based Microemulsions Luis Garcıá-Rıó,* Pablo Hervella, and Jose´ Ramoń Leis Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, Universidad de Santiago, 15782 Santiago, Spain Received April 4, 2005. In Final Form: June 14, 2005 A study was carried out on the solvolysis of substituted phenyl chloroformates in AOT/isooctane/water microemulsions. (AOT is the sodium salt of bis(2-ethyhexyl)sulfosuccinate.) The results obtained have been interpreted by taking into account the distribution of the chloroformates between the continuous medium and the interface of the microemulsions, where the reactions take place. The values obtained for the rate constant in the interface, ki, decreases as the water content of the microemulsions increases, as a consequence of the decrease in its nucleophilic capacity. This behavior is consistent with a rate-determining step of water addition to the carbonyl group. The values of ki allow us to obtain the slopes of the Hammett correlations at the interface of the microemulsions, F ) 2.25, whose values are greater than those obtained in an aqueous medium, F ) 0.82. This increase in the Hammett slope is similar to that observed in ethanol/ water mixtures and is a consequence of a variation in the structure of the transition state of the reaction where there is a smaller extension of the expulsion of the leaving group. The values of the rate constants at the interface of the microemulsions have allowed us, by means of the Grunwald-Winstein equation, to obtain the solvent ionizing power and the nucleophilicity of the solvent. The values obtained for YCl increase together with the water content of the microemulsion, whereas the values of NT decrease. These variations are a consequence of the interaction between the AOT headgroups and the interfacial water, where the water molecules act like electronic acceptors. The intensity of this interaction is greater if the system has a small water content, which explains the variation of YCl and NT.

Introduction The use of microemulsions as a reaction medium has awakened great interest in recent years.1 From the point of view of organic synthesis, microemulsions allow us to put into contact hydrophilic and hydrophobic reactants and constitute an alternative to phase-transfer catalysts.2 From a mechanistic point of view, the microemulsions, because of their microheterogeneous nature, allow us to compartmentalize the reactants. This compartmentalization can give rise to a decrease in the reaction rate if it causes the reactants to separate, or it can bring about an increase in the rate if the local concentrations of the reactants increase. Both types of behavior have been obtained experimentally for reactions that take place in the aqueous microdroplet and at the interface.3 The study of the water properties of the microemulsions has shown that their behavior differs clearly from that which is observed in an aqueous medium.4 Many physical properties that can affect reactivity, such as electric percolation,5 microviscosity,6 the dielectric constant,7 the empirical solvent parameters,7-9 and so forth, have been determined experimentally; they differ appreciably from * Corresponding author. E-mail: [email protected]. (1) Handbook of Microemulsion Science and Technology; Kumar, P., Mittal, K. L., Eds.; Marcel Dekker: New York, 1999. (2) Holmberg, K. Adv. Colloid Interface Sci. 1994, 51, 137. (3) (a) Valiente, M.; Rodenas, E. J. Phys. Chem. 1991, 95, 3368. (b) Moya´, M. L.; Izquierdo, C.; Casado, J. J. Phys. Chem. 1991, 95, 6001. (c) Lo´pez-Cornejo, P.; Pe´rez, P.; Garcıá, F.; Vega, R.; Sańchez, F. J. Am. Chem. Soc. 2002, 124, 5154. (d) Garcıá Rıó, L.; Leis, J. R.; Penã, M. E.; Iglesias, E. J. Phys. Chem. 1993, 97, 3437. (e) Garcıá Rıó, L.; Leis, J. R.; Mejuto, J. C. J. Phys. Chem. 1996, 100, 10981. (4) De, T. K.; Maitra, A. Adv. Colloid Interface Sci. 1995, 59, 95. (5) (a) Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1989, 93, 10. (b) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990, 94, 387. (c) Lang, J.; Mascolo, G.; Zana, R.; Luisi, P. L. J. Phys. Chem. 1990, 94, 3069. (d) Moulik, S. P.; De, G. C.; Bhowmik, B. B.; Panda, A. K. J. Phys. Chem. B 1999, 103, 7122. (e) Hait, S. K.; Moulik, S. P.; Rodgers, M. P.; Burke, S. E.; Palepu, R. J. Phys. Chem. B 2001, 105, 7145.

those obtained in an aqueous medium. Likewise, it has been observed that these physical properties depend on the water content of the system, as is reflected through the quotient W, where W ) [H2O]/[AOT]. However the kinetic repercussions of these properties have not been very widely studied. The rate constants of spontaneous reactions in aqueous micelles depend on their properties as a reaction region and the sensitivity of the reaction to medium effects.10 In our laboratory, we have observed that both the rate of solvolysis of benzoyl chlorides and the mechanism by which the reaction occurs depend on W.11 To extrapolate the behavior observed in other systems, it would be necessary to determine the polarity parameters of the interface, which can be easily related to the reaction rate. In this sense, the most widely used parameters are the solvent ionizing power and the solvent nucleophilic power by means of the Winsten-Grunwald equation. This equation has been widely used to investigate different types of reactions in different solvents.12

()

log

k ) lNT + mYX + c k0

(1)

where k and k0 are the solvolysis rate constants in a given (6) (a) Keh, E.; Valeur, B. J. Colloid Interface Sci. 1981, 79, 465. (b) Hasegawa, M.; Sugimura, T.; Suzaki, Y.; Shindo, Y.; Kitahara, A. J. Phys. Chem. 1994, 98, 2120. (c) Zinsli, P. E. J. Phys. Chem. 1979, 83, 3223. (7) Lay, M. B.; Drummond, C. J.; Thistlethwaite, P. J.; Grieser, F. J. Colloid Interface Sci. 1989, 128, 602. (8) Wong, M.; Gra¨tzel, M.; Thomas, J. K. J. Am. Chem. Soc. 1976, 98, 2391. (9) Correa, N. M.; Biasutti, M. A.; Silber, J. J. J. Colloid Interface Sci. 1995, 172, 71. (10) Bunton, C. A. J. Phys. Org. Chem. 2005, 18, 115 and references therein. (11) Garcıá Rıó, L.; Leis, J. R.; Moreira, J. A. J. Am. Chem. Soc. 2000, 122, 10325. (12) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 3rd ed.; Wiley-VCH: Weinheim, Germany, 2003.

10.1021/la050882l CCC: $30.25 © 2005 American Chemical Society Published on Web 07/15/2005

Oil AOT-Based Microemulsions

solvent and in the standard solvent, respectively. m is the sensitivity of the specific rate of solvolysis to changes in the solvent ionizing power (YX). l indicates the substrate response to changes in solvent nucleophilicity (NT). Ethanol/water (80:20 (v/v)) is chosen as the standard solvent with NT ) 0 and YX ) 0. In this study, we have investigated the reaction of solvolysis of substituted phenyl chloroformates in AOT/ isooctane/water microemulsions. This reaction has been chosen because the determining step of the solvolysis of the 4-nitrophenyl chloroformate is the addition of water to the carbonyl in all solvents studied.13-18 Therefore, the study of this reaction will allow us to evaluate the nucleophilic capacity of the interfacial water. With the aim of more efficiently characterizing the properties of the interfacial water and investigating its repercussions on the transition state of the reaction, we have studied the solvolysis processes of the substituted phenyl chloroformates. The substrates used show different substituents in the aromatic ring: 4-CH3O, 4-CH3, 4-H, 4-Cl, 3-CF3, and 4-NO2. The results obtained, together with the solvolysis of the diphenylmethyl chloride, will allow us to obtain the values of the ionizing power and nucleophilicity of the solvent at the interface of the AOT-based microemulsions. The results obtained are in the vicinity of those that exist in the literature19 for ethanol/water mixtures with a high ethanol content.

Langmuir, Vol. 21, No. 17, 2005 7673

Figure 1. Influence of the concentration of AOT on kobs in the solvolysis of substituted phenyl chloroformates (b) 3-CF3 and (O) 4-H at W ) 18 and 25.0 °C. Scheme 1

experiments were carried out at 25.0 °C. The kinetic traces were fitted with one exponential equation using the software of the SF apparatus.

Experimental Section

Results

AOT (Aldrich) was dried in a vacuum desiccator in the presence of P2O5 for 2 days and then used without further purification. Substituted phenyl cloroformates (all from Aldrich) were of the highest available purity and were used as supplied; all of them were dissolved in isooctane (Aldrich). Microemulsions were prepared by mixing an isooctane, water, and 1.00 M AOT/ isooctane solution in appropriate proportions. The solvolysis reactions were followed by monitoring the UV absorbance of substrate solutions (concentration range (1-2) × 10-4 M with respect to the total volume of the microemulsion) using a Milton Roy 3000 spectronic diode array spectrophotometer fitted with thermostated cell holders at 25.0 °C. Kinetic studies have been performed at λ ) 290 nm (4-CH3O); 285 nm (4-CH3); 280 nm (4-H); 285 (4-Cl); 290 (3-CF3); and 340 nm (4-NO2). The kinetic absorbance versus time data always fit the first-order integrated rate equation satisfactorily (r > 0.999); in what follows, kobs denotes the pseudo-first-order rate constant. We were able to reproduce the rate constants with an error margin of (5%. In all cases, we verified that the final spectrum of the reaction products coincided with another obtained in pure water, guaranteeing that the presence of the microemulsions did not alter the products of the reaction. Reaction kinetics in water were carried out in an Applied Photophysics SF apparatus with unequal mixing. The chloroformate dissolved in dry acetonitrile was placed in the smaller syringe (0.1 mL). The larger syringe (2.5 mL) was filled with water. The total acetonitrile concentration was 3.85% (v/v). The solutions of the substrates for the kinetic experiments were freshly prepared in dry acetonitrile at the appropriate concentration such that the final concentration was 1.0 × 10-4 M. All

By way of example, Figure 1 shows the influence of the AOT concentration on kobs in experiments carried out while keeping the W relationship constant, W ) [H2O]/[AOT], for the solvolysis of the 3-CF3 and 4-H derivatives. The experiments carried out at W ) 18 show that kobs increases together with the concentration of the surfactant (similar behavior was obtained for the other substrates, not shown). This behavior is a consequence of the incorporation of the chloroformate into the interface of the microemulsion where the reaction takes place. Therefore, an increase in the AOT concentration gives rise to an increase in the concentration of chloroformate available for the reaction. Likewise, we can observe that kobs increases together with the electron-withdrawing substituents on the aromatic ring. This behavior is consistent with an additionelimination mechanism where the rate-determining step of the reaction is the water addition to the carbonyl group. To carry out a complete study of the solvolysis in microemulsions, it is necessary to obtain the true rate constants because the values of kobs are affected by the equilibrium constants of the distribution of the reactants through the different pseudophases of the system. The experimental behavior can be interpreted by considering the distribution of the reactants, which is shown in Scheme 2. We can consider the distribution of the chloroformates between the continuous medium and the interface through the following distribution constant

(13) Kyong, J. B.; Yoo, J. S.; Kevill, D. N. J. Org. Chem. 2003, 68, 3425 and references therein. (14) (a) Kyong, J. B.; Park, B.-C.; Kim, C.-B.; Kevill, D. N. J. Org. Chem. 2000, 65, 8051. (b) Kevill, D. N.; D’Souza, M. J. J. Chem. Soc., Perkin Trans. 2 1997, 1721. (15) Kevill, D. N.; Bond, M. W.; D’Souza, M. J. J. Org. Chem. 1997, 62, 7869. (16) Kevill, D. N.; D’Souza, M. J. Can. J. Chem. 1999, 77, 1118. (17) Kevill, D. N.; Kim, J. C.; Kyong, J. B. J. Chem. Res., Synop. 1999, 150. (18) Kevill, D. N.; D’Souza, M. J. J. Org. Chem. 1998, 63, 2120. (19) (a) Bentley, T. W.; Llewellyn, G. Prog. Phys. Org. Chem. 1990, 17, 121. (b) Kevill, D. N. Adv. Quant. Struct.-Prop. Relat. 1995, 1, 79.

Koi )

[chloroformate]i

Z [chloroformate]o

(2)

where subindices o and i refer to the continuous medium and the interface, respectively, and the concentrations are referenced to the total volume of the system. Parameter Z of the composition of the microemulsion is defined as Z ) [isooctane]/[AOT], by analogy with parameter W. Considering that the total concentration of the chloro-

7674

Langmuir, Vol. 21, No. 17, 2005

Garcıá-Rıó et al.

4-NO2 where the ki values are higher than in bulk water for W ) 2-4. Discussion

Figure 2. Representation of 1/kobs against Z in accordance with eq 4 for the solvolysis of (b) 3-CF3 and (O) 4-H. Scheme 2

formate will be the sum of that which is found in the continuous medium and in the interface of the microemulsion, we can obtain the following expression for kobs:

kiKoi kobs ) Koi + Z

(3)

This equation predicts the existence of a linear dependency between 1/kobs and parameter Z according to eq 4.

1 1 1 ) + Z kobs ki kiKoi

(4)

As an example, the results of Figure 2 show the good fit of eq 4 for the solvolysis the 3-CF3 and 4-H derivatives (similar plots, not shown, were obtained for the other substrates) at W ) 18. From the relationship between the ordinate and the slope of the representations of Figure 2, we can obtain the values of Koi for the different chloroformates studied.20 The obtained values are shown in Table 1. As we can observe, the substituents of the aromatic ring barely alter the values of Koi. From the values of Koi and the observed rate constant at different W values, we can obtain the true rate constants at the interface of the microemulsion. The obtained results are shown in Table 121,22 together with those obtained in an aqueous medium.23 As we can observe, the values of ki obtained at the interface of the microemulsion are lower than those obtained in an aqueous medium, except for (20) The possibility of the distribution of the substituted phenyl chloroformates between the three pseudophases of the microemulsion and the fact that the solvolysis reaction takes place simultaneously at the interface and in the water microdroplet have previously been discussed and discarded. See ref 11.

1. Influence of the Water Content of the Microemulsion on the Rate of Solvolysis. Table 1 shows the obtained values for the solvolysis of substituted phenyl chloroformates in AOT/isooctane/water microemulsions. As can be observed, the rate constant at the interface decreases in all cases, as the value of W increases. To correctly interpret the influence of the composition of the microemulsion on the solvolysis rate, it is necessary to understand the mechanism whereby the reaction takes place. The solvolysis of chloroformates has been widely studied in an aqueous medium and in different solvent mixtures, and two mechanisms for the reaction have been proposed: an addition-elimination reaction, where the water addition step determines the reaction rate, and an ionization mechanism.13-18 The solvolysis mechanism of phenyl chloroformate is well established over a wide range of hydroxylic solvents by an addition-elimination mechanism, with the addition step being rate-determining. Figure 3 shows the obtained values for the logarithm of the solvolysis rate constant of the 4-NO2 derivative at the interface, log ki, according to the water content of the microemulsion, as well as the obtained value in an aqueous medium. It can be observed that the log ki values for small water contents are in the region of the value obtained in pure water and subsequently decrease as the water content of the system increases. Figure 3 also shows the results obtained by Bentley24 et al. on studying the solvolysis of 4-nitrophenyl chloroformate in mixtures of methanol and water. It can be noted that the rate constant increases together with methanol, reaching a maximum value for (21) The experimental results show that the rate constants of the solvolysis of the substituted phenyl chloroformates in the interface decrease as W increases as a consequence of the variation in the physical properties of the water. However, this change in the physical properties is not reflected in the distribution constants, Kai, which remain independent of W. The reason for this different sensitivity resides in the enthalpies of solvent transfer of the chloroformate and of the transition state. These results are not available in the literature, and the extent to which they can be determined experimentally is likely to be affected by the solvolysis of the chloroformates. For the purposes of comparison, we can compare the results of the solvolysis of chloroformates with those obtained for the basic hydrolysis of the ethyl acetate in 60% aqueous DMSO and 60% aqueous ethanol (ref 14). The activation enthalpies for the alkaline hydrolysis are ∆Hq ) 10.9 and 14.9 kcal/mol for aqueous DMSO and aqueous ethanol, respectively. It is found that a factor of δ∆Hq ) 4.0 kcal/mol favors the aqueous DMSO. This factor should be compared with the desolvation of the OH- by the DMSO, OHshowing that the enthalpy of solvent transfer is δ∆HDMSOfethanol ) -14.24 kcal/mol. Likewise, the experimental results show that the enthalpy of solvent transfer from 60% aqueous DMSO to 60% aqueous ethylacetate ethanol for ethyl acetate is δ∆H DMSOfethanol ) 0.23 kcal/mol. As might be expected, the enthalpies of transfer of the esters do not contribute significantly to the differences in the enthalpies of the reactants in these solvents. A lower degree of sensitivity of the enthalpy of transfer of the chloroformate in comparison with the transition state must be responsible for the fact that the distribution constant, Koi, is not modified as the water content of the microemulsion varies, whereas the rate constants are modified. (22) Haberfield, P.; Friedman, J.; Pinkston, M. F. J. Am. Chem. Soc. 1972, 94, 71. (23) The values obtained for the solvolysis constant in an aqueous medium are close to those obtained previously by Lee et al. (Koo, I. S.; Yang, K.; Kang, K.; Lee, I. Bull. Korean Chem. Soc. 1998, 19, 968). The values obtained by these authors (7.66 × 10-2; 2.25 × 10-2; 1.33 × 10-2; 1.07 × 10-2; 8.76 × 10-3)s-1 for (4-NO2; 4-Cl; 4-H; 4-CH3; 4-CH3O), respectively, are slightly different from those obtained in the present study. These differences are attributable to the fact that in the present study the experiments in water were carried out in the presence of the 3.8% (v/v) acetonitrile. (24) Koo, I. S.; Yang, K.; Kang, K.; Lee, I.; Bentley, T. W. J. Chem. Soc., Perkin Trans. 2 1998, 1179.


Langmuir, Vol. 21, No. 17, 2005 7675

Table 1. Values of the Kinetic Parameters Obtained for the Solvolysis of Substituted Phenyl Chloroformates in AOT/ Isooctane/Water Microemulsions at 25.0 °Ca

a

4-CH3O

4-CH3

4-H

4-Cl

3-CF3

4-NO2

Koi

4.6 ( 0.7

5.9 ( 0.9

2.1 ( 0.4

8.5 ( 0.7

2.2 ( 0.3

7.6 ( 0.5

W 2 3 4 5 6 7 10 13 18 23 28 30 35 40 45 50 water

6.00 × 10-4 6.05 × 10-4 6.00 × 10-4 5.53 × 10-4 5.14 × 10-4 4.94 × 10-4 4.59 × 10-4 4.59 × 10-4 4.17 × 10-4 4.03 × 10-4 3.48 × 10-4 3.50 × 10-4 3.03 × 10-4 3.23 × 10-4 2.98 × 10-4 2.87 × 10-4 1.16 × 10-2

4.41 × 10-4 4.09 × 10-4 3.70 × 10-4 3.54 × 10-4 2.92 × 10-4 2.85 × 10-4 2.35 × 10-4 2.31 × 10-4 2.06 × 10-4 1.60 × 10-4 1.45 × 10-4 1.54 × 10-4 1.40 × 10-4 1.25 × 10-4 1.19 × 10-4 1.11 × 10-4 9.70 × 10-3

1.38 × 10-3 1.28 × 10-3 1.23 × 10-3 1.10 × 10-3 9.57 × 10-4 8.94 × 10-4 8.96 × 10-4 8.58 × 10-4 7.85 × 10-4 7.61 × 10-4 6.65 × 10-4 6.28 × 10-4 6.09 × 10-4 5.77 × 10-4 5.63 × 10-4 5.48 × 10-4 1.30 × 10-2

3.79 × 10-3 3.35 × 10-3 3.32 × 10-3 3.10 × 10-3 2.59 × 10-3 2.14 × 10-3 1.80 × 10-3 1.89 × 10-3 1.61 × 10-3 1.20 × 10-3 9.80 × 10-4 1.09 × 10-3 9.91 × 10-4 6.35 × 10-4 8.90 × 10-4 6.86 × 10-4 2.26 × 10-2

1.35 × 10-2 1.34 × 10-2 1.02 × 10-2 9.35 × 10-3 7.91 × 10-3 6.92 × 10-3 6.92 × 10-3 5.51 × 10-3 4.26 × 10-3 3.87 × 10-3 3.40 × 10-3 3.35 × 10-3 2.91 × 10-3 2.66 × 10-3 2.42 × 10-3 2.33 × 10-3 3.21 × 10-2

7.35 × 10-2 6.50 × 10-2 5.76 × 10-2 4.89 × 10-2 4.01 × 10-2 4.00 × 10-2 3.62 × 10-2 3.39 × 10-2 3.10 × 10-2 2.95 × 10-2 2.83 × 10-2 2.55 × 10-2 2.74 × 10-2 2.31 × 10-2 2.20 × 10-2 2.11 × 10-2 5.64 × 10-2

ki/s-1

Values in water were obtained in the presence of 3.85% (v/v) acetonitrile.

Scheme 3

Figure 3. (b) Influence of W on log ki for the solvolysis of 4-nitrophenyl chloroformate in AOT/isooctane/water microemulsions at 25.0 °C. (O) Rate constants for the solvolysis of 4-nitrophenyl chloroformate in aqueous methanol at 25.0 °C. (Data are from ref 24.)

70% methanol/water (v/v), and subsequently decreases. The value obtained for the rate constant in methanol is approximately double that of pure water. The observed behavior in methanol/water mixtures must be related to the properties of these reaction mediums according to the solvent ionizing power (YCl) and its nucleophilic power (NT).19 The water is a solvent with high ionizing power (YCl ) 4.57) and low nucleophilic power (NT ) -1.38), whereas methanol presents the opposite behavior: it has low ionizing power (YCl ) -1.17) and high nucleophilic power (NT ) 0.17).19,25 The fact that the rate of solvolysis of the 4-NO2 derivative in methanol is greater than in water indicates that the reaction is not very sensitive to the ionizing power of the solvent and that the extent of bond breaking in the transition state is of little importance. Figure 3 shows how the solvolysis rate constant at the interface of AOT-based microemulsions also decreases as (25) (a) Kevill, D. N.; Anderson, S. W. J. Org. Chem. 1991, 56, 1845. (b) Bentley, T. W.; Carter, G. E. J. Am. Chem. Soc. 1982, 104, 5741. (c) Kevill, D. N.; D’Souza, M. J. J. Chem. Res., Synop. 1993, 174. (d) Koo, I. S.; Bentley, T. W.; Kang, D. H.; Lee, I. J. Chem. Soc., Perkin Trans. 2 1991, 296.

the water content of the system increases. This behavior, together with that obtained on studying the reaction in methanol/water mixtures, indicates that as the water content of the microemulsion decreases there should be an increase in its nucleophilic power. It is well known that the properties of the water of AOT-based microemulsions26 are different from those of bulk water and as the water content of the microemulsion varies a gradual variation in its properties occurs. The experimental behavior is compatible with a model that considers the existence of four types of water in the microemulsions:27 free water, water bonded to the counterion, water bonded to the headgroup of the surfactant, and normal water. These four types of water coexist and interchange rapidly. The free water is found to be dispersed between the hydrocarbon chains of the surfactant, existing as monomers or dimers, and does not hydrogen bond to its surroundings. Normal water is found in the center of the aqueous microdroplet and undergoes strong interactions by means of the hydrogen bond. In addition to these two types of water, there exist other molecules of bonded water in the vicinity of the ionic tensioactives. The local interactions of the water molecules with the counterions and with the headgroups of the surfactants have opposite effects on the water structure. The hydration of the anionic headgroups of the surfactants increases the electronic density on the hydrogen atoms in the water molecules (Scheme 3), with the consequent breakage of the hydrogen bonds of the normal water. The intensity of the O-H bonds increases, and this factor means that the 1 H NMR chemical displacement of H atoms in water (26) (a) Boissiere, B.; Brubach, J. B.; Mermet, A.; Marzi, G.; Bourgaux, C.; Prouzet, E.; Roy, P. J. Phys. Chem. B 2002, 106, 1032. (b) Zhou, G. W.; Li, G. Z.; Chen, W. J. Langmuir 2002, 18, 4566. (c) Venables, D. S.; Huang, K.; Schmuttenmaer, C. A. J. Phys. Chem. B 2001, 105, 9132. (d) Li, Q.; Weng, S.; Wu, J.; Zhou, N. J. Phys. Chem. B 1998, 102, 3168. (e) Temsamani, M. B.; Maeck, M.; Hassani, I.; Hurwithz, H. D. J. Phys. Chem. B 1998, 102, 3335. (27) Zhou, N.; Li, Q.; Wu, J.; Chen, J.; Weng, S.; Xu, G. Langmuir 2001, 17, 4505.

7676


molecules bonded to the headgroups is found in higher fields than for normal water and the vibrational frequency of the O-H bond is centered upon higher frequencies. This type of interaction causes a reduction in the electrophilic character of the water and, consequently, an increase in its nucleophilic character. On the contrary, the counterions accumulated within the aqueous microdroplets can polarize the water molecules, giving rise to a lower electronic density in the vicinity of the protons and consequently a reduction in the strength of the O-H bond of the water. This effect causes the resonance signals of 1H NMR of the H atoms in water to be displaced to lower fields and the vibration frequencies of the O-H bond to be displaced to lower frequencies. This type of interaction causes an increase in the electrophilic character of the water and a reduction of the nucleophilic character. Consequently, the hydration of the AOT headgroup gives rise to an increase in the nucleophilic character of the water and a reduction in its electrophilic character. This behavior is due to the fact that in the case of the AOT the hydration of the headgroup of the surfactant is more important than that of the counterion, contrary to what happens with other tensioactive agents such as NaDEHP.28 It could be mentioned that the increases in solvent nucleophilicity by interaction with the anionic center are supported by the enormous acceleration caused by chloride ion addition to methanolyses of acyl halides in acetonitrile.29 Indirect evidence of this type of behavior exists, such as the values of the polarity parameter ET(30) according to the water content of the microemulsions of AOT.7,9 The reduction of the ET parameter when W decreases is a measurement of the reduction of the electrophilic capacity of the water of the microemulsion. However, the increase in the degree of structuring of the interfacial water, as the water content of the microemulsion decreases, increases the capacity of the oxygen atom of the water to participate in the formation of hydrogen bonds. More precisely, the interfacial water molecules will be involved in the solvation of the SO3- and carbonyl groups of the AOT. Experimental results of the variation of the properties of the microemulsions of AOT show that the charge density on the AOT headgroup increases as the water content of the microemulsion decreases.30 This increase in the charge density on the surfactant as W decreases will mean that the interactions between the surfactant headgroup and the water molecules increase as the water content decreases. Therefore, a reduction occurs in the electrophilic character, and an increase occurs in the nucleophilic character of the water bonded to the headgroup as W decreases. This variation in the water properties will be responsible for the kinetic behavior observed in the solvolysis of 4-nitrophenyl chloroformate.31 2. Hammett Correlation for Solvolysis of Substituted Phenyl Chloroformates in Microemulsions. Table 1 shows the values of the solvolysis rate constants (28) Li, Q.; Li, Tao; Wu, J. J. Phys. Chem. B 2000, 104, 9011. (29) Kevill, D. N.; Foss, F. D. J. Am. Chem. Soc. 1969, 91, 5054. (30) Moran, P. D.; Bowmaker, G. A.; Cooney, R. P.; Bartlett, J. R.; Woolfrey, J. L. J. Mater. Chem. 1995, 5, 295. (31) As mentioned by one of the reviewers, the local pH at the interface between the bulk water and the microemulsions certainly decreases. (See Hasegawa, M. Langmuir 2001, 17, 1426.) However this decrease in the local pH will have no kinetic influence because the solvolysis of phenyl chloroformate is not acid-catalyzed in moderate H2SO4, HClO4, or HCl solutions (Moodie, R. B.; Towill, R. J. Chem. Soc., Perkin Trans. 2 1972, 184). It should be mentioned that the acid concentrations vary as follows: [H2SO4] ) 2.94-8.23 M; [HClO4] ) 1.72-7.63 M; and [HCl] ) 3.27-10.41 M. These values are much higher than those reached at the interface of the microemulsions.


Figure 4. Hammett plot of the solvolysis of substituted phenyl chloroformates in (b) water and AOT-based microemulsions of (9) W ) 2, (2) 18, and ([) 50 at 25.0 °C. Open symbols are for 4-CH3O phenyl chloroformate and are excluded from the correlations.

of the substituted phenyl chloroformates according to the water content of the microemulsions. In all cases, it can be observed that the rate constant at the interface, ki, decreases as W increases. It is important to confirm whether the solvolysis mechanism of the substituted phenyl chloroformates with electron-donating or -attracting groups is the same as that which operates in the case of the 4-NO2 derivative or whether the ionization mechanism predominates. Figure 4 shows a Hammett correlation for the solvolysis of the 4-CH3O, 4-CH3, 4-H, 4-Cl, 3-CF3, and 4-NO2 derivatives in an aqueous medium and in AOT/isooctane/water microemulsions for different W values: W ) 2, 18, and 50. In all cases, we can observe very satisfactory correlations (r > 0.995) with the sole exception of the 4-CH3O phenyl chloroformate. (Note from data in Table 1 that the 4-CH3O derivative always reacts faster than the 4-CH3 substituted phenyl chloroformate.) It is important to note that the slope of the correlation, F, increases when passing from an aqueous medium to the interface of the microemulsions. The fact that the substituted phenyl chloroformates 4-CH3, 4-H, 4-Cl, 3-CF3, and 4-NO2 satisfactorily confirm a Hammett correlation indicates that they all react by means of the same mechanism. Therefore, we can consider that in all cases an associative mechanism operates for all of the W values, where the rate-determining stage is the water addition to the carbonyl group. The observed behavior with the 4-CH3O derivative diverges from those correlations found for the other chloroformates. In all cases, it can be observed that the 4-CH3O derivative is more reactive than would be expected on the basis of its σ value. The divergence from the Hammett correlation (Figure 4) is too great to be attributed to an error in obtaining the value of ki through the distribution constant of 4-CH3O, Koi. The values obtained for ki are 2-4 times greater than expected by virtue of the Hammett correlation. These divergences from the Hammett correlation had already been found for the solvolysis of substituted phenyl chloroformates in ethanol/water and methanol/water mixtures32a and in methanol/acetonitrile mixtures.32b The reactivity of chloroformates is strongly influenced by the stabilization of the initial state by resonance33 (Scheme 4) This resonating effect increases because of the presence of electron-donating substituents in the aromatic ring. (32) (a) Koo, I. S.; Yang, K.; Kang, K.; Lee, I. Bull. Korean Chem. Soc. 1998, 19, 968. (b)Yew, K. H.; Koh, H. J.; Lee, H.; Lee, I. J. Chem. Soc., Perkin Trans. 2 1995, 2263.


Langmuir, Vol. 21, No. 17, 2005 7677

Scheme 4

Figure 6. Grunwald-Winstein correlation for the solvolysis of diphenylmethyl chloride in ethanol/water and methanol/ water mixtures. log(k/k0) ) (0.74 ( 0.03)YCl + (0.27 ( 0.07). Data are from ref 37.

Figure 5. F values from Hammett correlations for the solvolysis of substituted phenyl chloroformates in AOT-based microemulsions and in water/ethanol mixtures. (Data are from ref 32a.)

The rate constant for the methanolysis of phenyl chloroformate, 6.94 × 10-3 s-1, is much lower than that of the cinnamoyl chloride,34 63.7 × 10-3 s-1. The greater rate of solvolysis of the cinnamoyl chloride is attributed to the low resonance electron donation of the cinnamoyl group leading to a low stabilization of the initial state. The values of the Hammett slope are positive, so the electron-donating groups should destabilize the transition state and inductive electron withdrawal stabilizes the transition state. In this way, the chloroformates with electron-donating groups in the aromatic ring should be less reactive because of the stabilization of the initial state and the destabilization of the transition state. However, it can be observed that the 4-CH3O derivative is more reactive than predicted by the Hammett correlation. This behavior suggests a change in the structure of the transition state to a more dissociative type for the electron-donating substituents. Figure 5 shows the obtained values for the slopes of the Hammett correlation in AOT/isooctane/water microemulsions according to the water content of the system. Generally, a value of F ) 2.25 is obtained independently of the value of W. At the same time, in Figure 5 we can observe that the value of the Hammett slope increases along with the percentage of ethanol present in the reaction medium from F ) 0.82 to 1.77 for pure water and pure ethanol, respectively. The F values at the interface of AOTbased microemulsions are greater than in mixtures of ethanol and water, indicating that bond formation tends to be advanced in the transition states. If we assume virtually separate F values for bond formation (F > 0) and bond breaking (F < 0), then positive F values obtained suggest the predominance of bond formation in the transition state. The increase of F on going from bulk water to pure ethanol or to the interface of AOT-based microemulsions implies a slight increase in bond formation in the transition state. (33) (a) Crunden, E. W.; Hudson, R. F. J. Chem. Soc. 1961, 3748. (b) Queen, A.; Nour, T. A.; Paddon-Row, M. N.; Preston, K. Can. J. Chem. 1970, 48, 522. (c) McKinnon, D. M.; Queen, A. Can. J. Chem. 1972, 50, 1401. (34) Kim, T. H.; Huh, C. Lee, B. S.; Lee, I. J. Chem. Soc., Perkin Trans. 2 1995, 2257.

3. Solvent Ionizing Power and Nucleophilicity in Microemulsions. The Grunwald-Winstein equation35 using empirical polarity parameters of the solvent has been used to correlate values of rate constants obtained under very diverse conditions. The extended GrunwaldWinstein equation has been explicitly evaluated using two standard substrates, one with a high sensitivity to solvent nucleophilicity (bromomethane36a,b or methyl tosylate;36c,d l ) 1.00) and the other with a low sensitivity to solvent nucleophilicity (2-adamantyl tosylate;36c,d l ) 0.00; m ) 1.00). Kevill and co-workers realized that considerable improvement of the nucleophilic scale could be realized by a fundamental change in the type of substrate undergoing solvolysis. Replacement of the anionic leaving group by a neutral molecule leaving group can be achieved by using the rates of solvolysis of the S-methyldibenzothiophenium ion. A new solvent nucleophilicity scale (NT) can derived in this way.19b Using values of the solvent ionizing power,19,25b YCl, and nucleophilicity,25a NT, we have applied the GrunwaldWinstein equation for the solvolysis of diphenylmethyl chloride.37 Figure 6 shows the Grunwald-Winstein correlation for rate constants obtained in ethanol/water and methanol/water.38 The values of the rate constants correlate very well with the simplified Grunwald-Winstein equation without the need to include the nucleophilicity of the solvent, resulting in the correlation shown in eq 5. As has been established,19b the analysis of the solvolysis rate constants of diphenylmethyl chloride in a wide range of solvents did show a negligible sensitivity to changes in NT.

()

log

k ) (0.74 ( 0.03)YCl + (0.27 ( 0.07) k0

(5)

By using ki values for the solvolysis of diphenylmethyl chloride at the interface of AOT-based microemulisons,37 (35) (a) Winstein, S.; Grunwald, E.; Jones, H. W. J. Am. Chem. Soc. 1951, 73, 2700. (b) Winstein, S.; Fainberg, A. H.; Grunwald, E. J. Am. Chem. Soc. 1957, 79, 4146. (36) (a) Peterson, P. E.; Waller, F. J. J. Am. Chem. Soc. 1972, 94, 991. (b) Peterson, P. E.; Vidrine, D. W.; Waller, F. J.; Henrichs, P. M.; Magaha, S.; Stevens, B. J. Am. Chem. Soc. 1977, 99, 7968. (c) Bentley, T. W.; Schadt, F. L.; Schleyer, P. v. R. J. Am. Chem. Soc. 1972, 94, 992. (d) Bentley, T. W.; Schadt, F. L.; Schleyer, P. v. R. J. Am. Chem. Soc. 1976, 98, 7667. (37) (a) Garcıá Rıó, L.; Leis, J. R.; Igleisas, E. J. Phys. Chem. 1995, 97, 3437. (b) In this study, we have obtained the values of ki ) 3.31 × 10-5s-1 and ki ) 8.86 × 10-5s-1 for W ) 2 and 3, respectively. (38) Bentley, T. W.; Ryu, Z. H. J. Chem. Soc., Perkin Trans. 2 1994, 761.

7678



philicity according to W (Figure 7). The values of NT can be interpolated or extrapolated using eq 8:

NT )

Figure 7. (O) NT and (b) YCl values of nucleophilicity and solvent ionizing power for the interface of AOT-based microemulsions.

a value of k0 ) 1.72 × 10-3 s-1 for the solvolysis rate constant in mixtures of 80% ethanol/water (v/v), and eq 5, we can determine YCl for any value of W at the interface of AOT/isooctane/water microemulsions.39 Figure 7 shows the obtained values, from which we can interpolate or extrapolate the values of YCl according to the following equation:

YCl )

-(4.4 ( 0.4) + (0.40 ( 0.04)W 1 + (0.26 ( 0.04)W

(6)

Equation 6 (line in Figure 7) is the equivalent of a graphical interpolation, where the value of -4.4 extrapolated to W ) 0 is quite speculative. The obtained values for YCl vary between -2.32 for W ) 2 and 1.02 for W ) 50. These values are close to those obtained for 100% ethanol (YCl ) -2.52) and 70% ethanol/ water (YCl ) 0.78). The existing studies in the literature14b show a Grunwald-Winstein correlation for the solvolysis of phenyl chloroformate (eq 7) in mixtures of ethanol/water, methanol/water, acetone/water, trifluoroethanol/water, trifluoroethanol/ethanol, hexafluoro2-propanol/water, acetone/ water, and dioxane/water, resulting in the following values: l ) 1.68 ( 0.10, m ) 0.57 ( 0.06, and c ) 0.12 ( 0.41. The very large sensitivity (l value) to changes in solvent nucleophilicity suggests a very pronounced involvement of the solvent as a nucleophile in the ratedetermining step, consistent with the first step of an addition-elimination mechanism being rate-determining.

log

()

k ) (1.68 ( 0.10)NT + (0.57 ( 0.06)YX + k0 (0.12 ( 0.41) (7)

From the correlation of eq 7 and using the values of YCl previously obtained for the interface of the AOT-based microemulsions and the value of k0 ) 5.03 × 10-3 s-1 for the solvolysis rate constant in a mixture of 80% ethanol/ water, we can obtain the values of the solvent nucleo(39) As one of the reviewers suggests, the intercept of eq 5 should be simply due to the “dispersion” of the data. As shown by Bentley and co-workers (Bentley, T. W.; Dau-Schmidt, J. P.; Llewellyn G.; Mayr, H. J. Org. Chem. 1992, 57, 2387), the response of solvolysis rate constants to changes in solvent ionizing power show deviations from the behavior predicted by the Grunwald-Winstein equation that are attributed to differences in solvation effects adjacent to the reaction site. Experimental results show that the dispersion is consistent with proposals that this effect is due to the specific solvation of the π system. Because the correct intercept for AOT/isooctane/water microemulsions is not known, it might be better to use only the slope in calculating YCl and NT values.

(1.10 ( 0.05) - (1.26 ( 0.01)W 1 + (0.22 ( 0.01)W

(8)

In addition to eq 6, eq 8 (line in Figure 7) is the equivalent of a graphical interpolation, and values extrapolated to W ) 0 are quite speculative. The obtained values decrease as the water content of the microemulsion increases, varying from NT ) 0.40 to -1.02 for W ) 2 and 50, respectively. These values of NT vary between values greater than for 100% ethanol (NT ) 0.37) and for mixtures of 40% ethanol/water (NT ) -0.74). Therefore, the values obtained for the solvent ionizing power and the nucleophilicity of the interface of AOT-based microemulsions are in the region of those existing for mixtures of ethanol/water with low water contents. It is important to note that the solvent ionizing power increases together with the water content of the microemulsion. This behavior is a consequence of the interaction between the interfacial water and the headgroups of the surfactant, AOT, where the water molecules act like electronic acceptors. This interaction causes an increase in the electron density of the water molecules and, therefore, decreases its capacity to solvate anions, with the resulting decrease in the solvent ionizing power. This interaction is stronger when the water content of the system is lower, which is compatible with the variation of YCl with W. Previously, we had obtained evidence of the capacity of AOT to form charge-transfer complexes with Br2 and evidence that this capacity increases as the value of W decreases.40 The behavior observed for NT is the opposite, increasing as W decreases. This variation is due to the electron donation of the AOT headgroup to the water, which increases its nucleophilicity. 4. Comparison with Solvent Mixtures. The obtained results allow us to characterize the interface of AOT-based microemulsions as a new reaction medium where the properties of the solvent vary together with W. This new reaction medium can constitute an alternative to the solvent mixtures for the study of reaction mechanisms. The special properties of nucleophilicity and solvent ionizing power in the interface of the microemulsion must be considered together with the advantage that in all cases the only species present is water with strongly differentiated properties from those of normal water and with values of YCl and NT that can be modulated through parameter W. The absence of solvent mixtures greatly simplifies the study of reactions of water addition to carbonyl compounds or carbocations. It is believed that these reactions occur by means of a nucleophilic attack by a solvent molecule assisted by a second solvent molecule acting as a general base. In this way, in alcohol/water mixtures there will be four competing product-determining steps (Scheme 5) defined by the following third-order rate constants:41 (i) kaa for the mechanism where an alcohol molecule acts as a nucleophile and a second molecule acts as a general base; (ii) kaw where the alcohol acts as a nucleophile and the water acts as a general base; (iii) kwa where water acts as a nucleophile (40) Garcıá Rıó, L.; Mejuto, J. C.; Ciri, R.; Blagoeva, I.; Leis, J. R.; Ruasse, M. F. J. Phys. Chem. B 1999, 103, 4997. (41) a) Bentley, T. W.; Jones, R. O. J. Chem. Soc., Perkin Trans. 2 1993, 2351. (b) Bentley, T. W.; Jones, R. O.; Koo, I. S. J. Chem. Soc., Perkin Trans. 2 1994, 753. (c) Bentley, T. W.; Jones, R. O. J. Chem. Soc., Perkin Trans. 2 1992, 743.

Oil AOT-Based Microemulsions Scheme 5

and the alcohol acts as a general base; and (iv) kww where water acts as both a nucleophile and a general base. This mechanism has been applied to acyl and sulfonyl transfer reactions as well as solvolysis reactions of chloroformates. One of its drawbacks is that it is necessary to determine the four rate constants in alcohol/water mixtures. Two of them, kww and kaa, are easy to determine in pure solvents. However, the other two present greater difficulties. In this sense, the advantage of the interface of the microemulsions is obvious. In this case, the only reactive species is water, the properties of which can be modulated at will. Conclusions The obtained results in the present study lead us to the following conclusions:

Langmuir, Vol. 21, No. 17, 2005 7679

(1) The solvolysis of substituted phenyl chloroformates in AOT-based microemulsions occurs at the interface of the microemulsions. The rate constant at the interface, ki, in all cases increases as the water content of the system decreases. This behavior is similar to that obtained in methanol/water mixtures and is a consequence of the increase in the nucleophilicity of the solvent. (2) The experimental results obtained for substituted phenyl chloroformates can be correlated using the Hammett equation. The values of the Hammett slope, F, do not vary with W and are greater than the value obtained in bulk water. This result is in accordance with the increase in the F value that is observed in ethanol/water mixtures as the percentage of ethanol increases. The high values of F have been interpreted as a consequence of the displacement of the transition state to a situation where bond making is more advanced. (3) The use of the Grunwald-Winstein equation and the values of the solvolysis rate constants of diphenylmethyl chloride and phenyl chloroformate have allowed us to obtain the values of the solvent ionizing power and nucleophilicity of the interface of AOT-based microemulsions. These values are close to those obtained for ethanol/ water mixtures with high percentages of ethanol. The values of YCl and NT vary together with the composition of the microemulsion because of the interactions between the AOT headgroups and interfacial water. This variation in the properties of the medium leads us to suggest the use of microemulsions as an alternative to solvent mixtures for the study of reaction mechanisms. Acknowledgment. Financial support from the Xunta de Galicia (PGIDT03-PXIC20905PN and PGIDIT04TMT209003PR) and Ministerio de Ciencia y Tecnologıá (project BQU2002-01184) is gratefully acknowledged. LA050882L

Ionizing Power and Nucleophilicity in Water in Oil AOT-Based

Recommend Documents