Reactivity of Benzoyl Chlorides in Nonionic Microemulsions: Potential

Department of Physical Chemistry, Faculty of Chemistry, UniVersity of Vigo, 36310 Vigo, Spain, and. Department of Physical Chemistry, Faculty of Chemi...
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J. Phys. Chem. B 2005, 109, 22614-22622

Reactivity of Benzoyl Chlorides in Nonionic Microemulsions: Potential Application as Indicators of System Properties C. Cabaleiro-Lago,*,† L. Garcia-Rı´o,‡ P. Herve´ s,† and J. Pe´ rez-Juste† Department of Physical Chemistry, Faculty of Chemistry, UniVersity of Vigo, 36310 Vigo, Spain, and Department of Physical Chemistry, Faculty of Chemistry, UniVersity of Santiago de Compostela, 15782 Santiago de Compostela, Spain ReceiVed: July 29, 2005; In Final Form: September 28, 2005

The solvolysis reactivity of benzoyl chlorides entails a high sensitivity on medium properties. A systematic study of the reaction of a series of these substrates, varying the electron-withdrawing character of the substituent, has been performed in nonionic microemulsions. The kinetic effects due to variation of microemulsion compositions can be assigned to modifications in system properties, to be precise, to modifications in interface properties. Microemulsion properties that are obtained from kinetic analysis of solvolysis show a good agreement with the characterization of the microemulsion that was made via 1H NMR and solvatochromic fluorescence probes. Benzoyl chlorides with electron-donating groups react through a dissociative mechanism, whereas electron-withdrawing groups favor an associative mechanism. A comparative analysis of reactivity between the different substrates at the interface shows a variation in the contributions of both reaction pathways, associative and dissociative, to the whole reaction mechanism. The confined media shift the point where the mechanism changes from an associative to a dissociative pathway, far away from the turning point in water. Furthermore, the change in mechanism can be modulated by modification of the microemulsion composition.

Introduction

CHART 1

Microemulsions are thermodynamically stable, macroscopically homogeneous mixtures of at least three components: polar and nonpolar liquid phases (usually water and oil) and a surfactant that, at a microscopic scale, forms a film separating the two immiscible liquids into two subphases. The diversity of microemulsion structures depends on the chemical composition, temperature, and concentrations of the constituents. Different surfactants can stabilize different structures, from discrete spherical aggregates to interconnected bicontinuous structures.1 Water and oil subphases are separated by an interfacial region that is the locus of chemical reactivity in aggregated systems. Current interpretations of the effects of association colloids on chemical reactivity view aggregates as microreactors, i.e., reaction regions distinct from the bulk solvent but distributed throughout the solution. Microemulsions offer the intriguing capability of cosolubilizing high concentrations of water-insoluble and water-soluble reactants, and many studies of chemical reactivity have focused on potential large-scale applications of these systems as reaction media. Microemulsions have been evaluated as reaction media for a variety of chemical reactions,2 mainly to overcome solubility problems in organic and enzymatic reactions3 but also to catalyze or inhibit chemical reactions by compartmentalization or concentration of reactants,4 or for the preparation of inorganic nanoparticles.5 Although many authors have widely studied chemical reactivity in ionic microemulsions, nonionic microemulsions have been less explored as reaction media. We find nonionic microemulsions particularly interesting because the nonionic surfactant can be removed from the product after * Corresponding author. E-mail: [email protected]. † University of Vigo. ‡ University of Santiago de Compostela.

synthesis, either by heating or cooling,6 and additionally, no cosurfactant is required to solubilize water in the oil continuous phase. Further, the comparison with ionic microemulsions can be interesting to explore the influence of an ionic headgroup. In our case, the commercially available surfactant Brij 30 (Chart 1) was selected as the nonionic surfactant to prepare w/o microemulsions. Poly(oxyethileneoxide)alkyl ethers, CnEOm, are popular because they are economically and ecologically (biodegradability and lower aquatic toxicity) suitable. In fact, the most important type of nonionic surfactant comprises fatty alcohol ethoxylates. They are used in liquid and powder detergents as well as in a variety of industrial applications because they are good emulsifiers (no need of cosolutes to form emulsions). Therefore, the corresponding phase behavior has been reported in several works.7-10 Further, it is possible to select the length of the nonpolar and polar moieties to create a system with a particular shape of the micelles, and no other surfactant is required for solubilizing water in the oil continuous phase. On the basis of SANS studies of the structure of hydrated micelles formed from CnEOm surfactants in oil media, it was concluded that the extent of solubilization of water in the L2 (water in oil one-phase isotropic solution) phase does not exceed the maximum hydration requirement of the hydrophilic EO groups. The scattering spectral analysis is also consistent with the formation of small bilayer structures that become thicker and more extended as W ([water]/[surfactant]) increases.10,11 At low W the thickness of the bilayer micelles (hanks) was found to be less than twice the fully extended conformation of the

10.1021/jp0542219 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/05/2005

Benzoyl Chlorides in Nonionic Microemulsions CHART 2

surfactant, suggesting that the EO chains of the surfactant were interdigitated in the interior of the micelle. As W increases, the bilayer thickness increases, until at Wmax the EO chains are fully hydrated and extended. It was concluded that the limit of stability was reached at the point when the surfactant became fully hydrated and it is not possible to form a discrete water droplet within the micelles.10-12 According to this picture we can consider the water present in the ternary system as interfacial water. Moreover, DSC studies in similar systems13 conclude that the water is weakly bound to EO groups as expected for an uncharged nonionic surfactant. The solvolysis of benzoyl chlorides, which are known to be sensitive to the physical properties of the medium, has been used to study the effects of additives on the structure of water,14-16 and its behavior in ionic microemulsions has been reported.17,18 In this study, the chemical reactivity of benzoyl chlorides (see Chart 2) has been used as a chemical probe to investigate the effect of microemulsion composition on interfacial water properties. The study of a nonionic microemulsion allowed an evaluation of the effects of charged or noncharged headgroups from a comparison with previous results using AOT microemulsions.17,18 Experimental Section Poly(oxyethylene) dodecyl ether (Brij 30), ethoxyethanol, and isooctane were supplied by Aldrich and used without further purification. Benzoyl chlorides (all from Aldrich) were of the highest purity and were used as supplied. Nile Red was supplied by Aldrich. Kinetic Measurements. Microemulsions were prepared by mixing isooctane, water (distilled and deionized), and 2 M Brij 30/isooctane solution in the appropriate ratios. Solvolysis reactions were monitored in a Hewlett-Packard 8453 UV-vis spectrophotometer and a Varian Cary 5000 UV-vis-NIR spectrophotometer, both fitted with thermostated cell holders (all experiments were performed at (25.0 ( 0.1 °C), following the absorbance of the aromatic reactants. The wavelengths used for the kinetic studies fell between 285 and 300 nm for 4-MeO, 3,4-(MeO)2, 4-Me, 4-H, 4-Cl, 3-Cl, 3-CF3, and 4-CF3 and λ ) 245 nm for 3-NO2 and 4-NO2. The concentration was 3 × 10-4 for 4-H, 4-Cl, 3-Cl, 3-CF3, and 4-CF3 and 4 × 10-5 for 4-MeO, 3,4-(MeO)2, 4-Me, 3-NO2, and 4-NO2. Kinetic data always fitted the first-order integrated rate equations satisfactorily; in what follows, kobs denotes the pseudo-first-order rate constant. Experiments were reproducible to within 3%. Fluorescence Measurements. Nile Red was dissolved in the microemulsion by addition of a stock solution of Nile Red in isooctane. Fluorescence measurements were performed in a FluoroMax-3 spectrofluoremeter. Samples were excited at 490 and 575 nm. 1H NMR Measurements. NMR spectra were obtained in DMSO-d6 supplied by Aldrich in 99.9% purity. Spectra were recorded with the aid of a coaxial tube filled with DMSO-d6 to lock on the deuterium signal. The signals of tetramethylsilane

J. Phys. Chem. B, Vol. 109, No. 47, 2005 22615 were used as 1H NMR references. All spectra were recorded on a Bruker AM 500 MHz spectrophotometer. Characterization of Microemulsions of Brij 30. SolVatochromic Fluorescent Probes. Since substrate behavior is closely related to medium properties and, more precisely, to medium polarity, the variation of polarity with microemulsion composition has been studied. To obtain an empirical measurement of solvent polarity, solvatochromic compounds have been widely used in binary solvent mixtures as well as microhetereogeneous media.19-22 In fact their use in noncontinuous media provides further advantages since the partitioning in the different environments of the systems allows a simultaneous study of the different regions. Reichartd20 developed an empirical polarity scale based on the pyridinium N-phenolate betaine (ET(30)), the absorption maximum of which shifts depending on the polarity of the solvent in which it is dissolved. ET(30) shows a large shift in absorption maxima with polarity providing a high sensitivity to small changes in solvent strength. However, the presence of acid traces deactivated the solvatochromic activity of the Reichardt probe, ET(30), since the protonated form of the betaine dye has no solvatochromic properties. ET(30) in isooctane/Brij 30/water microemulsions changes color to pale yellow which corresponds to the protonated form. For this reason, the use of the Nile Red solvatochromic probe was considered. Besides being soluble in a wide range of solvents (it has a hydrophobic nature, low solubility in water), Nile Red is stable in acidic media, and it is not susceptible to a loss of absorption in the presence of acids.21 Nile Red presents solvatochromism in both absorption and fluorescence. In this fact relies the main advantage of using this probe in microhetereogeneous media since, depending on medium polarity, the absorption or emission maxima of Nile Red can change. Selecting an appropriated excitation wavelength for fluorescence studies allowed us to monitor the fluorescence spectra that correspond to the probe within a specific microemulsion environment. The λabs max of the lowest energy band of Nile Red shifts from 520 nm in isooctane to 550 nm in water. Thus, exciting the sample at λ’s higher than 520 nm, where the absorption of Nile Red in the hydrocarbon phase is negligible, one can selectively excite and study the Nile Red located on the interface and the water pool. Due to the low solubility in pure water, we consider that Nile Red dissolved in the water pool has a negligible contribution to the emission band.23 In pure isooctane, Nile Red exhibits a well-defined and highly structured emission band (Figure 1) with peaks at 528 and 565 nm and a shoulder at 610 nm when it is excited at λexc ) 490 nm, while at λexc > 550 nm emission is negligible. Figure 1 shows the normalized emission spectra for microemulsions with constant W and varying the concentration of Brij 30. The spectra support the idea that the probe is distributed along the different environments. The emission spectra of the ternary system show a weak contribution of the Nile Red in isooctane, the main contribution being for Nile Red in Brij 30, at the interface. As the surfactant concentration increases the spectrum is red shifted and becomes less structured indicating that more and more Nile Red is associated to the interface and the contribution of the Nile Red in isooctane peak decreases. The dependence of the emission maximum wavelength with surfactant concentration supports the previous idea since the emission maximum of Nile Red progressively tends to the value of 628 nm as the concentration of surfactant increases (see Figure 1, inset) which corresponds to the emission value when Nile Red is completely associated to interface (see below for details). To avoid the contribution of the Nile Red in the oil phase, and therefore study

22616 J. Phys. Chem. B, Vol. 109, No. 47, 2005

Figure 1. Emission spectra of Nile Red in isooctane/Brij 30/water microemulsion for constant W. (s) Emission spectra of Nile Red in pure isooctane. The dotted line corresponds to the emission maximum of Nile Red in water. The excitation wavelength λ ) 490 nm. The inset shows the λemission dependence on the concentration of Brij 30.

Cabaleiro-Lago et al.

Figure 3. Maximum wavelength emission spectra of Nile Red and the parameter ET(30) vs W.

Figure 4. Dependence of the H1 NMR chemical shift on the water content of the isooctane/Brij 30/water microemulsion at 25 °C.

Figure 2. Overlapped curves correspond to the emission spectra of Nile Red in the isooctane/Brij 30/water microemulsion for increasing concentration of Brij 30 and constant W. (- - -) Emission spectra of Nile Red in pure Brij 30. The excitation wavelength λexc ) 575 nm.

more in detail the properties of the interface, fluorescence experiments were carried out using a higher excitation wavelength. Figure 2 shows the emission spectra of the same samples as before when the oil contribution is eliminated (λexc ) 575 nm). No shift with composition is observed indicating that variation of surfactant concentration does not affect the polarity of the interface. The spectra are red shifted with respect to the emission peak in Brij30/isooctane solution showing that the interface provides a more polar environment than that of the pure Brij30. Considering the peak emission values and on the basis of the data reported by Hungerford et al.,22 the fluorescence data can be transformed into ET(30) values. For λ ) 628 nm we can assign a value in the ET(30) polarity scale of 48.2 that corresponds to a polarity similar to that of pure 2-propanol. On increasing water concentration (Figure 3) the spectra of the Nile Red in the interface gradually red shifts from 620 to 630 nm, with a variation related to the increase of polarity of the interface region. Values of ET(30), calculated in the same way as before, fall between 47 and 49, which indicates a change in polarity from an environment similar to that of 2-propanol to a more polar media similar to that of n-butanol. NMR Spectra. Nuclei chemical shifts arise from differences in their electronic environments. Therefore, they are susceptible to local interactions and can provide information about changes in their environment. The chemical shift of water protons is very sensitive to the local properties of water molecules in terms

of electron density in the surroundings of the proton. The electron density is affected by the environment in which water molecules are immersed. As seen in Figure 4 the chemical shifts for water protons vary to downfield as the water content increases and approaches that of bulk water. The hydration of the headgroup of the surfactant tends to increase the electron density of protons in water molecules and consequently breaks the hydrogen-bonding net between the water molecules. For low contents in water this effect is statistically more pronounced, shifting the peak to higher field, increasing more drastically the electron density of protons, and therefore the nucleophilic character of water. Results The influence of surfactant concentration was examined through a systematic variation of the concentration of Brij 30 within the concentration range where the microemulsion is stable keeping W constant. As examples, Figures 5 and 6 show the variation of kobs with surfactant concentration for the solvolysis of two substrates with different behaviors (see the Supporting Information for data of all compounds). For both benzoyl chlorides, kobs increases with Brij 30 concentration due to the incorporation of benzoyl chlorides at the interface. Whereas for 4-MeO, kobs increases with water content, the opposite behavior is observed for 3-CF3. To carry out a quantitative analysis of the influence of the microemulsion on reactivity we need to determine the true rate constants and concentration of reactants in each pseudophase. The pseudophase formalism, originally developed for aqueous micellar systems, has been extended to other colloidal aggregates, such as vesicles and microemulsions, with satisfactory

Benzoyl Chlorides in Nonionic Microemulsions

J. Phys. Chem. B, Vol. 109, No. 47, 2005 22617

Figure 5. Observed rate constant for the solvolysis of 4-methoxybenzoyl chloride at 25 °C in isooctane/Brij 30/water microemulsions at several [water]/[Brij 30] molar ratios (W): (b) W ) 2.5; (O) W ) 3.5; (9) W ) 4.5. The lines are guides for the eye.

Figure 7. Plot of the data from Figure 5 according to 1/kobs vs Z (eq 4) for the solvolysis of 4-MeO for W: (b) W ) 2.5; (O) W ) 3.5; (9) W ) 4.5.

SCHEME 1

Figure 6. Observed rate constant for the solvolysis of 3-trifluoromethylbenzoyl chloride at 25 °C at several [water]/[Brij 30] molar ratios (W): (b) W ) 2.5; (O) W ) 3.5; (9) W ) 4.5. The lines are guides for the eye.

results.4,24-28 The microemulsion is assumed to be divided into three regions corresponding to the oil-rich domain, the central aqueous core, and the surfactant interfacial region. Reagents are distributed among the three pseudophases according to their solubility, and the global rate will be the sum of the rates in the different microenvironments. In our case, due to low solubility of benzoyl chlorides in water, substrates are preferentially distributed between the continuous medium and the interface with a distribution constant Koi. However, a small amount might still access the water pool, where the reaction should be much faster, thereby compensating for the differences in relative abundance. In a first approximation, we can assume that the substrate is distributed along the three microemulsion pseudophases (see Scheme 1), so that the solvolysis process can take place both at the interface and in the aqueous pseudophase. On the basis of this reactant distribution, we can obtain the following equation:

r ) ki[substrate]i + kw[substrate]w

(1)

where ki and kw are the rate constants for the solvolysis at the interface and in bulk water, respectively. The equilibrium distribution of substrate between the phases can be defined in terms of the molar ratio. The distribution

constants between the oil and the interface, Koi, and between the water and the interface, Kwi, are expressed as eq 2:

Koi )

[S]i [S]o

Z

Kwi )

[S]i [S]w

W

(2)

where concentrations are referred to the total volume of the system and Z and W are defined as the molar ratios Z ) [isooctane]/[Brij 30], W ) [water]/[Brij30]. Taking into account that the total concentration of the substrate is the sum of the concentrations in the different pseudophases and on the basis of the Scheme 1, we obtain

kobs )

kiKoiKwi + kwKoiW KoiKwi + KoiW + KwiZ

(3)

which can be transformed as

KoiKwi + KoiW Kwi 1 ) + Z kobs kiKoiKwi + kwKoiW kiKoiKwi + kwKoiW

(4)

Figures 7 and 8 show the linearized plots of 1/kobs versus Z for the solvolysis of 4-MeO and 3-CF3 for several W values. From eq 4, the intercept/slope ratio becomes

Koi intercept W ) Koi + slope Kwi

(5)

Table 1 displays the results of the intercept/slope ratios of the plots of 1/kobs versus Z. The invariance of the intercept/

22618 J. Phys. Chem. B, Vol. 109, No. 47, 2005

Cabaleiro-Lago et al. TABLE 2: Mean Distribution Constant Values for the Incorporation of Substrates to the Interface substrate

Koi

4-MeO 3,4-(MeO)2 4-Me 4-H 4-Cl 3-Cl 3-CF3 4-CF3 3-NO2 4-NO2

21.0 ( 1.3 38.1 ( 4.4 4.2 ( 1.2 2.2 ( 0.3 1.5 ( 0.1 1.5 ( 0.2 1.6 ( 0.3 a 9.4 ( 0.8 5.9 ( 0.2

a The Koi value for 4-CF3 is not present since large errors in the intercept lead to a too small value of the distribution constant.

Figure 8. Plot of the data from Figure 6 according to 1/kobs vs Z (eq 4) for the solvolysis of 3-CF3 for W: (b) W ) 3.5; (O) W ) 4.5; (9) W ) 5.5.

TABLE 1: Influence of W on the Intercept/Slope Ratio According to Eq 5 for the Solvolysis of Benzoyl Chlorides 4-MeO, 4-H, and 3-Cl in Isooctane/Brij 30/Water Microemulsions at 25 °C intercept/slope W

4-MeO

4-H

3 3.5 4 4.5 5 6 7

25 ( 2 18 ( 2

2.0 ( 0.2 2.2 ( 0.2 2.6 ( 0.3

23 ( 2 22 ( 2 20 ( 4 19 ( 9

2.9 ( 0.3

3-Cl 1.3 ( 0.5 1.3 ( 0.3 1.7 ( 0.3 1.8 ( 0.5

slope ratio with W suggests that the substrate is distributed between the continuous medium and the interface only. A similar result was found in AOT microemulsions.18 Previous results indicate that the reaction takes place only at the interface since this is the area where the benzoyl chloride and the water molecules come into contact. Furthermore, water can be considered mainly as interfacial water as has been noted in the Introduction. Therefore, the model shown in Scheme 1 can be simplified, and we can obtain the following expression for the pseudo-first-order rate constant, kobs, according to the composition of the microemulsion.

kobs )

kiKoi Koi + Z

(6)

which predicts a linear dependence between 1/kobs and Z

1 Z 1 ) + kobs ki kiKoi

(7)

On the basis of proposed model, the ratio between the intercept and slope yields the values for the distribution constant, Koi, for each microemulsion composition (eq 7). Consistent values were obtained in all cases irrespective of microemulsion composition. Discrepancies between the values are due to the uncertainty in the determination of the intercept. Mean values for the distribution constants are shown in Table 2. The distribution constants are similar and independent of substituents for all substrates. Only substrates with highly electron-donating substituents (4-MeO and 3,4-(MeO)2) show slightly higher values. Once the value of the association constant is known, true constants for the solvolysis at the interface can be obtained from the experimental rate constants kobs (see Table S11 in the Supporting Information) on the basis of eq 6. Table 3 shows

the effect of water content in the microemulsion on the values of the rate constant, ki. Two extreme behaviors can be identified. The rate constant at the interface, ki, increases with water content for substrates with electron-donating groups (4-MeO (Figure 9) and 3,4(MeO)2). Both substrates show the highest variation of the rate constant along with water content, about a 10-fold increase of the rate constant from W ) 2 to W ) 8 (Table 3), and reactions for high water contents are still several orders of magnitude lower than the value in pure water. On the other hand, substrates with electron-withdrawing character (4-Cl to 4-NO2) show the opposite behavior. The rate constants decrease about 5 times when the water content varies from W ) 2 to W ) 8. A trend can thus be observed for theses substrates: as the electronwithdrawing character of the substituent increases, the rate constant values get closer to the values in pure water, and for some benzoyl chlorides and microemulsion compositions, the reaction is even faster than in pure water. For example 4-NO2 solvolysis is catalyzed by the microemulsion. The rate constant in bulk water is 0.062 s-1, whereas the rate constants in the microemulsions range from 0.339 to 0.065 s-1. Within the whole range of water content used, the solvolysis reaction is faster than in bulk water (see Figure 10). An intermediate pattern is observed as well (see Table 3). 4-Me shows both behaviors; ki decreases along with water content (as 4-NO2) for low values of W, whereas for higher values of W, solvolysis shows a behavior similar to that of 4-MeO, i.e., the rate constant increases as the water concentration does. The maximum value of 6.79 × 10-4 s-1 is again lower than the value in pure water, 7.1 s-1. 4-H shows a similar behavior, ki decreasing as W increases at low water contents and a slight increase with W for higher W values. The change of trend occurs at higher W values than in 4-Me case, indicating again that the change in the experimental behavior is related to the electron-withdrawing character of the susbtituent. To explain these results, the balance between the two mechanisms for the solvolysis of benzoyl chlorides must be taken into account. Discussion The solvolysis of benzoyl chlorides is a well-known process, both in water and in solvent mixtures. The reaction can occur by means of three mechanisms: (a) dissociative, through an acylium ion intermediate; (b) concerted displacement, which can have dissociative or associative character; and (c) associative, or addition-elimination, through a tetrahedral intermediate.29 These mechanisms are well defined with a clear borderline between them. In a reaction series a small change in the structure of the reactants or in reaction conditions can lead to changes

Benzoyl Chlorides in Nonionic Microemulsions

J. Phys. Chem. B, Vol. 109, No. 47, 2005 22619

TABLE 3: Influence of W on ki (s-1) Values for Solvolysis of Benzoyl Chlorides in Isooctane/Brij 30/Water Microemulsions at 25 °C ki/s-1 W

4-MeO

2.00 2.50 3.00 3.50 4.00 5.00 6.00 7.00 8.00

4.54 × 10 5.75 × 10-4 7.21 × 10-4 9.75 × 10-4 1.20 × 10-3 1.92 × 10-3 2.81 × 10-3 3.52 × 10-3 4.66 × 10-3

water 58.2 a

-4

4-Me

6.52 × 10 8.65 × 10-4 1.09 × 10-3 1.36 × 10-3 1.77 × 10-3 2.71 × 10-3 3.74 × 10-3 5.00 × 10-3 6.03 × 10-3

5.56 × 10 4.60 × 10-4 4.05 × 10-4 3.90 × 10-4 3.82 × 10-4 4.09 × 10-4 4.86 × 10-4 5.63 × 10-4 6.79 × 10-4

2.43 × 10 1.90 × 10-3 1.44 × 10-3 1.35 × 10-3 1.17 × 10-3 1.03 × 10-3 1.00 × 10-3 1.02 × 10-3 1.06 × 10-3

1.95 × 10 1.50 × 10-2 1.16 × 10-2 9.25 × 10-3 7.91 × 10-3 6.25 × 10-3 5.30 × 10-3 4.75 × 10-3 4.38 × 10-3

7.1

1.1

1.89 × 10-1 4.90 × 10-2 3.10 × 10-2 3.56 × 10-2 3.63 × 10-2 6.20 × 10-2

-4

50.1

4-H -4

4-Cl -3

3-Cl

4-CF3a

3,4-(MeO)2

-2

3-CF3

3.12 × 10 2.33 × 10-2 1.87 × 10-2 1.46 × 10-2 1.20 × 10-2 9.19 × 10-3 7.58 × 10-3 6.78 × 10-3 6.29 × 10-3 -2

6.11 × 10 4.71 × 10-2 3.86 × 10-2 5.22 × 10-2 3.49 × 10-2 2.74 × 10-2 1.44 × 10-2 1.21 × 10-2 1.08 × 10-2 -2

3-NO2

9.48 × 10-2 7.20 × 10-2 1.40 × 10-1 5.16 × 10-2 1.20 × 10-1 8.81 × 10-2 3.31 × 10-2 6.42 × 10-2 2.74 × 10-2 5.19 × 10-2 2.26 × 10-2 4.05 × 10-2 2.00 × 10-2 3.46 × 10-2 1.84 × 10-2 4.00 × 10-2

4-NO2 3.39 × 10-1 2.69 × 10-1 2.29 × 10-1 1.81 × 10-1 1.44 × 10-1 1.19 × 10-1 9.56 × 10-2 6.53 × 10-2

A mean distribution constant value of Koi ) 1.5 has been used to calculate rate constants.

SCHEME 2

Figure 9. Log(ki) vs W for the solvolysis of 4-MeO in a nonionic microemulsion at 25 °C. The solid line is a guide for the eye.

Figure 10. Log(ki) vs W for the solvolysis of 4-NO2 in a nonionic microemulsion at 25 °C. The dotted line means the value for solvolysis in pure water. The solid line is a guide for the eye.

from a concerted to a stepwise mechanism or vice versa. The change from a dissociative to an associative mechanism can be analyzed in terms of a Jencks-More-O’Ferrall diagram (see Scheme 2). Benzoyl chlorides with electron-donating groups react through a dissociative mechanism giving rise to an acylium intermediate, whereas electron-withdrawing groups favor an associative mechanism with a tetrahedral intermediate. Microemulsion composition and substituent effects can be explained in terms of a duality of reaction paths (associative and dissociative). Extreme behaviors, such as those of 4-MeO that reacts mainly by means of a dissociative reaction and 4-NO2 that reacts through an associative reaction, are analyzed. We discuss the behavior of 4-H and 4-Me in more detail since both

pathways seem to be present. The relationship between reactivity and microemulsion properties is analyzed as well. Influence of Microemulsion Composition. The influence of the composition of the microemulsion on the reaction depends on the operating reaction mechanism. For reactions occurring through a dissociative mechanism a determining factor is the solvation of the leaving group. The reduction of water electrophilic character causes a reduction in reaction rate since the system shows lower ability to solvate the leaving group. However, at the same time, water nucleophility increases favoring the associative pathway. In the case of the Brij 30 microemulsion, polar groups are hydrated with water molecules. For low water content, the molecules must be mainly distributed around polar groups with the subsequent breakage of the hydrogen bonds of the bulk water. Further additions of water yield more hydrated surfactants, and the mean interaction should be lower for each water molecule. Therefore, the water structure is progressively more similar to the structure of bulk water. SolVolysis of 4-MeO. Solvolysis of benzoyl chlorides has been widely studied in water and solvent mixtures. Comparison of the reactivity in solvent mixtures with the reactivity in microemulsions can be used to elucidate water properties for different microemulsion compositions. For example, 4-MeO reacts predominantly through a dissociative mechanism that is strongly affected by the properties of medium, in particular polarity and the capability to solvate the leaving group.29-31 Studies regarding the effects of solvent mixtures on solvolysis reactions show that the dissociative pathway is favored in highly aqueous alcohol/

22620 J. Phys. Chem. B, Vol. 109, No. 47, 2005

Cabaleiro-Lago et al. CHART 3

Figure 11. Linear relationship between the maximum excitation band wavelength for Nile Red and the parameter ET(30) with the natural logarithm of the rate constant (ki) for 4-MeO.

water mixtures when the ionizing power, i.e., the tendency of a particular solvent to promote ionization, is higher.31 Even further, the analysis of the kinetic effect on solvolysis in AOTbased microemulsions on the basis of the Winsten-Grunwald equation suggests that the solvent ionizing power of the medium (YCl) increases as the water content increases, whereas the nucleophilic character of the medium (NT) decreases as W increases.32 This behavior is a consequence of the interaction between the interfacial water and the headgroups of the surfactant, AOT, which promotes an increase in the electron density of water molecules and, therefore, decreases its ability to solvate anions, with the resulting decrease in 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. 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 water, which increases its nucleophilicity. These results can be extrapolated to the Brij 30 microemulsion. In this medium we observe a decrease of the rate constant for the hydrolysis of 4-MeO with decreasing water content, and therefore it can be concluded that, for low W, the microemulsion interface is less polar, water located in that region has a lower electrophilic character than that of bulk water, and consequently has a lower ability to solvate the leaving group probably due to the interactions between the polar headgroups and the water molecules. For a low content of water molecules, these must be mainly distributed around polar groups with corresponding breakage of the hydrogen bonds of the bulk water. This type of interaction causes a reduction in the electrophilic character and its capability to solvate leaving group. As previously reported, Nile Red experiments corroborate the decrease in the polar character of water as W decreases, i.e., the microemulsion interface provides a less polar environment than water in a continuous aqueous phase. In addition, a linear free energy relationship can be observed between the rate constant of the solvolysis of 4-MeO and the solvent polarity scale ET(30) (Figure 11) supporting the assumption that the increase in reactivity for benzoyl chloride that reacts predominantly by means of a dissociative pathway is related to an increase in interface polarity. SolVolysis of 4-NO2. The behavior of 4-NO2 is opposite to that observed for electron-donating substituents. Figure 10 shows the variation of log ki versus W. The rate constant decreases as water concentration increases. A similar pattern was observed for 4-Cl, 3-Cl, 3-CF3, 3-CF4, and 3-NO2. This behavior is difficult to explain since benzoyl chlorides always show a

decrease in rate constant when polarity and ionizing power decrease. These effects could be minimized, i.e., decrease less sharply, when the second possible mechanism, the associative pathway (independent of polarity), is present and contributes to the rate constants.30 Since the associative pathway is strongly dependent on solvent nucleophilicity, an unusual enhancement of the nucleophilic character can be postulated to explain the observed behavior. The fact that benzoyl chlorides with a higher tendency to react via an associative mechanism (more electron-withdrawing substituents) show a sharper effect on reaction rate supports the previous idea. Breakdown of the bulk water structure for low W, and the interaction with the polar headgroup of the surfactant could be the reason for the enhancement of the ability of water molecules to act as a nucleophile (see Chart 3). A previous NMR study33 suggests that the water molecules interact preferentially with the hydroxyl group and only at high water content is interaction with the EO groups established. This preferential binding is expected due to the relative electronegativity of the oxygen of both groups although, statistically, the four EO groups should have a more favorable interaction. Additionally, we can consider that the EO groups are folded in the oil phase, having restricted access to water. As W increases hydrogen bonds can be formed between water molecules stabilizing water and therefore reducing its nucleophilicity.33 This hypothesis is supported by the NMR data as has been noted above. The variation of proton chemical shift to downfield with W stems from a decrease in nucleophilicity as the water content of the microemulsion increases, due to hydrogen bonding getting reestablished. If we compare these results with previously obtained data for the same reaction in isooctane/AOT/water ionic microemulsions where the substrates react mainly through an associative mechanism, we observe that the values of ki are higher in the presence of the nonionic microemulsion, showing that water molecules could be more nucleophilic in the presence of Brij 30 than AOT. The hydration of the EO headgroups of the Brij 30 increases the electronic density on the hydrogen atoms in the water molecules, causing a reduction in the electrophilic character of water and consequently an increase in its nucleophilic character (as we mentioned earlier for AOT-based microemulsions).18 The hydration of the anionic headgroup of AOT causes the same effect, but we have to take into account the presence of the counterion (Na+) and its interaction with water molecules, decreasing the electronic density on the oxygen atoms and also decreasing the nucleophilic character of the water: The balance of these two opposite effects of AOT results in an increase in the nucleophilic character of water, but this increase is lower than that produced in the presence of Brij 30, leading to lower values of ki. Moreover, with other anionic surfactants such as NaDEHP, counterion hydration is more important than the effect of the headgroup.34-36 Comparing our results with those obtained in water,29 we observe that the rate constants increase with the electronwithdrawing character of the substrate. The values of ki are closer to the values in pure water, and for some benzoyl chlorides and microemulsion compositions, the reaction rate is

Benzoyl Chlorides in Nonionic Microemulsions

J. Phys. Chem. B, Vol. 109, No. 47, 2005 22621

Figure 12. Log(ki) vs W in a nonionic microemulsion at 25 °C for (b) 4-Me and (O) 4-H. The lines are guides for the eye.

even higher than in pure water (see Table 3). Similar behavior has been noted before for the solvolysis of 4-NO2 in alcohol (ethanol and methanol)/water mixtures, i.e., pseudo-first-order constants increase as the water content decreases.37 To check the effect of surfactant hydroxyl groups on the rate constant, kinetic measurements were performed in ethoxyethanol/water mixtures. If we focus only on the alcoholic moiety of Brij 30, ethoxyethanol mimics the effect of the surfactant on the water molecules. A similar kinetic pattern is observed in mixed ethoxyethanol/water systems. The observed rate constant increases from kobs ) 0.056 s-1 for a highly aqueous mixture (99% water) up to kobs ) 0.073 s-1 for 57% water mixtures, i.e., the rate constant increases as the amount of water decreases (see the Supporting Information, Figure S1). Therefore, we can conclude that the catalysis observed in the microemulsions originates from a similar effect as that for the alcohol/water mixtures. The ability of the Brij 30 EO groups to act as a basic catalyst could be an alternative answer of the high values of ki at low W. This possibility can however be ruled out considering that, in the case of AOT microemulsions with an anionic headgroup, this process does not follow general base catalysis. SolVolysis of 4-Me and 4-H. 4-Me shows both types of behavior as can be seen in Figure 12. For low water content the rate constant decreases along with W as 4-MeO does until W ) 5. For W > 5, k1 increases as the water concentration increases, showing a similar behavior to that for 4-NO2. As observed for electron-donating substituted benzoyl chlorides, the maximum value of the rate constant is lower than the value for pure water as a result of the lower interface polarity. To explain this pattern again the two reaction channels should be taken into account. Two reaction pathways are present: a dissociative pathway, hardly dependent on water polarity, and an associative pathway, less sensitive to polarity but with a sharp dependence on nucleophilicity. The balance of both mechanisms is responsible for the observed experimental behavior. A mechanistic change can be observed as the solvent composition is varied. The decrease of the rate constant along with W points toward an associative mechanism that becomes less and less predominant as the water concentration increases. If W is increased up to W ) 5, the dissociative pathway, very sensitive to variations on interface polarity, becomes relevant, and an increase of the rate constant with water content is observed. A similar behavior is observed in the case of 4-H, though the percentage of the associative channel is higher, and thus the change to a dissociative pathway occurs at higher W values. Hammet Correlations. Plots of log ki versus σ+ (Figure 13) go through a minimum over the range of W, microemulsion

Figure 13. Hammet plots for the solvolysis of substituted benzoyl chlorides in nonionic microemulsion at (b) W ) 2.5 and (O) W ) 8. The lines represent the mechanism change point for (- - -) water and (- - -) the isooctane/AOT/water microemulsion W ) 8 (ref 18).

composition, studied. The change of the Hammet slope indicates a change in the solvolysis mechanism. Hammet plots for reactions in water show a change in slope from negative values (F+ ) -3) to positive values which is indicative of a change of mechanism at about σ+ ) 0.55, between 3-Cl and 4-Cl.29 In nonionic microemulsions the same pattern can be observed (Figure 13). F+ for low σ+ values cannot be calculated precisely, but it seems that F+ is less negative than for solvolysis in water. For high σ+ values, the slope has a positive value around F+ ) 2.3. The change from one curvature to another depends on water concentration. For W ) 2.5, the change of mechanism occurs around σ+ ) -0.45 between 3,4-(MeO)2 and 4-Me, whereas for higher water content (W ) 8), the mechanism switches at about σ+ ) -0.05 between 4-Me and nonsubstituted benzoyl chloride. The effect of the switch in the reaction mechanism is highlighted by comparing the rate constants for 4-MeO with the expected values for an associative mechanism at this σ+ value: the actual rate constant exceeds the expected value by a factor of about 15 for microemulsions with W ) 2.5 and by a factor of 100 with W ) 8. As reported above, a decrease in microemulsion water content leads to a decrease of the σ+ value at which the mechanism change of solvolysis occurs. The change from a dissociative to an associative reaction pathway with increasingly electronwithdrawing substituents occurs much later in water than in isooctane/Brij 30/water microemulsions (see the small-dashed line of Figure 13). This can be related to the change in the properties of micelle-borne water.38,39 The rate constant of the associative mechanism depends on the nucleophilic character of the water, while the dissociative reaction does on the leaving group solvation. In this type of nonionic microemulsion, water is solvating the surfactant headgroups and thus has a limited ability to assist the leaving group. At the same time, the interactions between the headgroups and water increase the nucleophilic character of the water. Consequently, this microemulsion favors the associative mechanism, and this effect is sharper as W decreases. If we compare these results with those obtained in isooctane/AOT/water microemulsions, we see that the change from a dissociative to an associative mechanism occurs a little later (see the small-dashed line in Figure 13). The interactions between the counterions (Na+) and water molecules reduce the nucleophilic character of water, leading to a small increase in the dissociative character of the reaction.

22622 J. Phys. Chem. B, Vol. 109, No. 47, 2005 Conclusions A study has been carried out on the solvolysis of substituted benzoyl chlorides in isooctane/Brij 30/water microemulsions. All the substrates under study are poorly soluble in water. The solvolysis reaction occurs only at the interface, where the reactants can meet. Our interpretation of the reaction kinetics is simplified, since the distribution of only one of the reactants needs to be considered, and changes in reactions kinetics can be directly related to changes in the properties of the interfacial water and interface. We have characterized our microemulsions using fluorescence and 1H NMR spectroscopy techniques. Thus, using the Nile Red solvatochromic probe the variation of polarity with microemulsion composition has been studied. On decreasing microemulsion water concentration we observed a decrease of polarity at the interface region. 1H NMR spectroscopy allowed the properties of the interfacial water in the microemulsions to be elucidated. The chemical shift for water protons varies to upfield as the water content of the microemulsion decreases, indicating an increase in the nucleophilic character of the interfacial water. The application of the pseudophase formalism allowed us to obtain the reactant partition coefficients and the rate constant at the interface. The former were basically constant for a given reactant, regardless of the microemulsion composition. The reaction takes place simultaneously by means of dissociative and associative mechanisms in which the microemulsion exerts an opposite effect. We can observe two limiting behaviors; for example, 4-MeO reacts predominantly by a dissociative mechanism which is strongly affected by properties of the medium, in particular, polarity, and consequently the capability to solvate leaving group. In the presence of the Brij 30 microemulsions, we observed a decrease of the rate constant for hydrolysis of 4-MeO with decreasing water content of the microemulsion. The microemulsion interface provides a less polar environment for reactions, the interfacial water molecules having lower electrophilic character than that of bulk water, and consequently the medium has less capability to solvate the leaving group, probably due to the interactions between polar headgroups and water molecules. The behavior of 4-NO2 (which reacts mainly by means of an associative mechanism) is opposite to that observed for electron-donating substituents. Decreasing W accelerates the reaction rate due to the water molecules having a higher nucleophilic character as the water content decreases, probably due to the disruption of the water structure promoted by the insufficient number of water molecules to form the bulk water structure and the interactions between the polar headgroups and water molecules. The change from a dissociative to an associative reaction path with increasing electron-withdrawing substituents occurs much later in water than in isooctane/Brij 30/water microemulsions. The absence of positive counterions in our microemulsion favors the associative pathway more than in AOT microemulsions. Microemulsion properties that are obtained from kinetic analysis of solvolysis show a good agreement with the characterization of the microemulsion that was made via 1H NMR and solvatochromic fluorescence probes. Acknowledgment. Financial support from the Ministerio de Educacio´n y Ciencia (MAT2004-02991) and from Xunta de Galicia (PGIDIT04 TMT209003PR) is gratefully acknowledged. C.C.L. is thankful for a FPU fellowship. Supporting Information Available: Tables containing observed rate constants (kobs) and experimental conditions for

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