Toward Understanding the Mechanism of Phase Transfer Catalysis

Article pubs.acs.org/JPCC

Toward Understanding the Mechanism of Phase Transfer Catalysis with Surface Second Harmonic Generation Piotr Fita* Institute of Experimental Physics, Faculty of Physics, University of Warsaw, ul. Hoża 69, 00-681 Warsaw, Poland S Supporting Information *

ABSTRACT: Phase transfer catalysis (PTC) is nowadays an important technique of organic synthesis. There are numerous indications that its key processes take place at the boundary between immiscible phases; however, this has not been proved by direct observations of interfaces separating the phases. Here, we demonstrate how surface second harmonic generation (SSHG) can be used for direct observation and identification of species adsorbed at the interface during a PTC reaction. The model system consisted of an organic (chlorobenzene) and aqueous (saturated solution of NaOH) phases with pH indicator (thymolphthalein, ThPh) used as a reactant. SSHG measurements allowed monitoring the surface concentration of ThPh anions at the interface and recording the kinetics of adsorption and deprotonation of ThPh. The results confirmed that deprotonation indeed occurs in the interfacial region. The proposed technique is relatively simple and general and can be used in a variety of PTC systems; therefore, its application will unveil the microscopic mechanism of PTC in practically used reactions.

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INTRODUCTION Phase transfer catalysis1−3 (PTC), introduced in the sixties,4−6 has been already developed into a well-established technique of organic synthesis for laboratory and large-scale industrial use. Catalysts used in PTC facilitate reactions between reactants dissolved in immiscible phases, aqueous, and organic by transporting molecules normally insoluble in the latter through the interface. Due to this transferring activity of the catalysts, the reactants can meet in the organic phase, where the desired reaction takes place. There are numerous indications that key steps of PTC reactions take place at phase boundaries;7 however, the microscopic mechanism of the process is frequently depicted incorrectly. The reason for this lies in the fact that most of the studies of the PTC mechanism were indirect, based on sampling of bulk liquids constituting the system, not on direct observations of interfaces, due to the lack of appropriate experimental techniques. The latter have been developed only in the last decades and were scarcely applied to PTC systems. A nice example of interface-selective optical studies of a PTC system are works of Uchiyama et al.8,9 carried out by quasielastic laser scattering on capillary waves propagating at the interface. The authors studied the transfer of a phenoxide anion from water to nitrobenzene after forming an ion pair with a tetrabutylammonium cation. Unexpected conclusions of these works were the following: (1) ion pairs were formed in the bulk for high catalyst concentrations and at the interface, if the catalyst concentration was low; (2) excess of the catalyst adsorbed at the interface disturbed the transfer of ion pairs between the two phases. These results indicated the complicacy of PTC mechanisms and the need to carefully adjust parameters such as concentrations for optimizing the reaction © XXXX American Chemical Society

yield. The technique applied for these studies showed the importance of the direct observation of interfaces in PTC studies but had a significant limitation: it was unspecific to molecules adsorbed at the interface. The latter has been overcome by optical techniques which rely on second-order nonlinear processes to achieve surfaceselectivity. In recent years they became methods of choice for spectroscopic studies of liquid interfaces and are described in a number of reviews.10−15 One of these techniques is the surface second harmonic generation (SSHG), in which the interface under study is illuminated with a beam of short, intense laser pulses. Due to second-order hyperpolarizability of the medium light of twice the frequency of the incident beam is generated at the interface but not in the bulk liquid. In principle, SH light may be generated at interfaces of neat immiscible liquids,16 but the efficiency of the process is enhanced by orders of magnitude if the incident beam is 1-photon or 2-photon resonant with strong electronic transitions in molecules adsorbed at the interface.17,18 In such a case, the intensity of the generated light is proportional to the square of the surface concentration of the species in resonance. Such measurements were used for instance to study adsorption at interfaces,19−22 and a difference of the interfacial affinity between charged and neutral forms of the same molecule was used to investigate the acid−base equilibrium of p-nitrophenol at a water surface23 and azobenzene surfactant at a 1,2-dichloroethane/water interface.24,25 Reaction kinetics at interfaces can be followed by recording the SH light intensity in Received: July 17, 2014 Revised: September 10, 2014

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dx.doi.org/10.1021/jp5071314 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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phenolic protons are removed and the lactone ring opens producing the deeply blue form ThPh2−. Absorption spectra of ThPh recorded in acetonitrile (Figure 1) exhibit strong

time elapsed after the reaction is initiated by bringing the phases into contact. This scheme has been demonstrated for monitoring adsorption kinetics of surfactants at a water surface26 or interaction of p-nitrophenol and tri-n-butyl phosphate at a water/dodecane interface.22 An attempt to apply SSHG measurements to study transport processes have been made in an electrochemical system where the transfer of crown ethers−Na+ complexes through a membrane at a liquid/liquid boundary was controlled by changes of the electric potential across the interface.27,28 However, to our knowledge, SSHG has never been applied for studies of PTC reactions in simple liquid/liquid systems. Therefore, in this article we report on preliminary experiments carried out in a model two-phase system consisting of a saturated NaOH aqueous solution and chlorobenzene (CB) used as an organic solvent. This is an important example of PTC systems, in which organic anions (carbanions) are formed in situ, in the initial step of deprotonation of anion precursors. The mechanism of carbanions generation and the exact location of this process (interface vs bulk) are key questions for this class of PTC reactions. There are two mechanisms of the catalytic process in which carbanions (denoted further C−) are generated from their precursors (CH) dissolved in the organic phase. In the first of them, the carbanion precursor CH reacts with NaOH at the interface. The resulting ion pair C−Na+ undergoes ion exchange with the catalyst, tetraalkylammonium (TAA) halide (Q+X−, where Q+ denotes the TAA cation), to form an ion pair C−Q+, located initially at the interface. Due to the lipophilicity of TAA cations such a pair can freely migrate into the organic phase, where it further reacts.1,7 In the alternative mechanism, it is the catalyst molecule, Q+X−, which undergoes ion exchange with NaOH to form Q+OH− at the interface. The latter extracts the hydroxide ion OH− into the organic phase and reacts with the carbanion precursor CH, so that the ion pair C−Q+ is formed in the bulk organic phase. Although experimental evidence indicates that the first mechanism (interfacial) is operating, the other one (extracting) cannot be completely ruled out, at least in certain systems. SSHG experiments carried out in organic liquid/NaOH systems may definitively answer the questions on the catalytic mechanism, thanks to the ability of SSHG to confirm the presence of particular molecules at the interface. For a demonstration of this scheme, we selected a pH indicator thymolphthalein (ThPh)29 as a carbanion precursor. ThPh is structurally similar to very commonly used phenolphthalein (PhPh), but contrary to PhPh, it does not bleach in strongly basic solutions after the initial color change. Most important forms of ThPh, deduced from electrochemical studies30,31 and comparison with PhPh,32,33 are shown in Chart 1. The colorless form ThPh0, with the closed lactone ring, is present in organic solutions and predominates in aqueous solutions at pH below approximately 9. At higher pH, both

Figure 1. Absorption spectra of thymolphthalein in acetonitrile (ACN) and water at pH ≈ 12.

absorption only below 250 nm. In basic aqueous solutions absorption bands in the visible range appear (Figure 1, see also the Supporting Information), in particular a strong band centered at around 600 nm, attributed to ThPh2− and responsible for its blue color. Thus, the presence of ThPh2− at the interface can be clearly detected by SSHG measurements if the probe light is tuned to be 2-photon resonant with this transition.

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EXPERIMENTAL METHODS The model system consists of a layer of a saturated aqueous NaOH solution, on which a solution of ThPh in chlorobenzene (CB) is poured. CB is used as a solvent because it is stable in contact with NaOH and forms an optically smooth interface with its saturated solution. A detailed description of sample preparation procedures is presented in the Supporting Information. The experimental setup for surface second harmonic generation (SSHG) utilizes a commercial titanium-sapphire femtosecond amplifier (Coherent Legend Elite Duo) working at 5 kHz repetition rate and delivering pulses centered at 800 nm as a primary laser source. The amplifier pumps a travelingwave optical parametric amplifier (Coherent Opera Solo) configured to deliver the second harmonic of the idler beam used as a probe in SSHG measurements. The central wavelength of the Opera Solo output pulses is tuned to 1040 or 1070 nm in the presented studies. The probe beam is attenuated using a set of fixed and variable reflective neutral density filters and passes a thin-layer polarizer and a broadband half-wave plate. The half-wave plate can be rotated around its axis in order to control the direction of linear polarization of the probe beam. Part of the probe beam is reflected onto a photodiode which monitors the intensity of the probe pulses. A lens with 300 mm focal length is used to focus the probe beam on the interface. Right before the sample there is a long-pass absorption filter used to remove second harmonic light generated on surfaces of optical elements. The incidence angle of the probe beam onto the studied interface θ is set to approximately 70 degree (Figure 2). The sample is contained in a 3 × 3 × 3 cm glass cubic cell. Second harmonic light generated at the interface is collected using a 100 mm focal length lens and passes a second thin-layer polarizer. Presented measurements were carried out with ppolarization of the probe beam and p-polarization of the detected light. The generated light is filtered with short pass glass absorption filters in order to remove the probe light and

Chart 1. Dominant Forms of Thymolphthalein in Solutions

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below the detection threshold). Kinetics recorded at low ThPh concentrations exhibit significant fluctuations of the SH intensity with amplitude exceeding 20% and a pattern which changes from experiment-to-experiment. Their origin can be attributed to spatiotemporal fluctuations of the surface concentration occurring in a nonequilibrated system where a chemical reaction takes place at the interface.34,35 Such a phenomenon has been typically observed in photochemical reaction systems driven off-equilibrium by light,36 whereas in the presented case, the sample is far from the equilibrium at the moment of preparation, which satisfies conditions required for random spatiotemporal pattern formation. This effect, undesired here, can be partially reduced by covering the sample cell and limiting evaporation-induced convective flow of the solvent; nevertheless, it is intrinsic to the heterogeneous system in which a chemical reaction takes place. The time needed to reach the saturation very strongly depends on the ThPh concentration (for concentrations of ThPh in CB equal to 5 × 10−6 M and higher, the initial rise of the SH intensity is faster than mechanical stabilization of the interface, thus it cannot be reliably recorded). Because only ThPh2− is involved in the resonant enhancement of the SH generation, through the 2-photon resonance of the probe light with the transition responsible for the absorption band around 600 nm, the square root of the measured SH light intensity is proportional to the surface concentration of ThPh2−. Therefore, the kinetics of the signal can be interpreted as an increase of the surface concentration of ThPh2−, which is formed through deprotonation of ThPh0 at the interface. Anions ThPh2− cannot leave the interfacial area because they can neither migrate into the organic phase (this has been confirmed by UV/vis spectroscopy) nor into the NaOH phase due to the strong salting-out effect of the latter. The concentration of ThPh0 in the area of the organic phase adjacent to the interface, depleted by adsorption, is constantly restored by diffusion from the bulk solution. Therefore, ThPh2− accumulates at the interface, and its surface concentration increases until the saturation is reached (analogical measurements carried out with PhPh show an initial increase of the SH signal followed by its decay due to the removal of the third proton leading to formation of a colorless triple anion33 (see Figure S3 in the Supporting Information). The above interpretation has been verified by fitting a simple model to the collected data. Although many advanced analytical and numerical models of adsorption from solutions have been published, they are not directly applicable to the presented experiments because they assume the equilibrium between the surface and the subsurface layer and are usually developed for open systems.37−42 Therefore, a model better corresponding to the experimental conditions has been proposed. In this model, the dynamics of ThPh0 concentration in the organic phase [ThPh0] is described by one-dimensional diffusion with the diffusion coefficient D:

Figure 2. Geometry of the SSHG experiment.

directed to a monochromator (Oriel Cornerstone 130) with a photomultiplier module attached (Hamamatsu H9305-04). The photomultiplier is connected to a lock-in voltmeter (Stanford Research Systems SR830) synchronized with the femtosecond amplifier. The analog output of the lock-in is digitized with a general purpose data acquisition card (National Instruments USB-6216) and recorded using a personal computer. The probe beam power is adjusted to be, typically, in the range of 0.5−2 mW, depending on the expected intensity of the generated second harmonic light for a given sample. Care is taken to be within the linearity range of the detection system, which can be checked by testing the square dependence of the detected light intensity on the probe beam power (Figure S1 of the Supporting Information). Absorption spectra of solutions were recorded with a PerkinElmer Lambda 35 spectrophotometer using 1 cm fused silica cells and slits set to 1 nm.

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RESULTS AND DISCUSSION The intensity of the SH light generated at the interface recorded in time elapsed after the preparation of the sample is shown in Figure 3. For concentrations of ThPh in the range from 2 × 10−7 M to 2 × 10−6 M, the initial SH intensity is practically undetectable but subsequently rises in time and finally reaches a saturation level (without ThPh in the organic phase the signal remains

∂[ThPh0] ∂ 2[ThPh0] =D ∂t ∂x 2

Adsorption of ThPh0 at the NaOH/organic interface is described by the Langmuir kinetics, 38,42 in which the adsorption rate is proportional to the number of free adsorption sites. It is assumed that only adsorbed ThPh0 molecules undergo deprotonation to form ThPh2−, which do not react anymore and do not leave the interface. Further, it is assumed that ThPh0→ThPh2− conversion is instantaneous,

Figure 3. Temporal dependence of the intensity of the second harmonic light generated at the chlorobenzene/NaOH interface with thymolpthalein present in the organic phase. C



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χ2 is a measure of the difference between experimental and fitted curves. The number of the first time step taken into account, nχ, is greater than 1 in order to omit data points at early times, when the interface is not yet stabilized. Due to the scaling (division of experimental values Γn by S calculated in step 3), the value of χ2 is mainly affected by the difference of the two curves in the range of times when the surface concentration rapidly increases. (5) Summing of χ2 calculated for each ThPh concentration in order to obtain X2. This quantity describes the global difference of the experimental data and curves simulated for the tested set of parameters. X2 is minimized in the fitting procedure as a function of D, k0, and Γmax using built-in procedures in the MATLAB environment. It has been tested that the results only very weakly depend on the choice of ns and nχ. The above algorithm allows reliable and unequivocal fitting of the experimental data, in spite of their relative, not absolute character. It should be pointed out that this is possible because the rate of the initial rise of the surface concentration of the adsorbate (before the saturation is reached) strongly depends on its bulk concentration, and the data for various concentrations are fitted globally with one set of parameters. The best-fit curves are shown together with the experimental data in Figure 4. It can be seen that the kinetics of the surface

compared to diffusion and adsorption timescales. This is welljustified because deprotonation of ThPh in basic aqueous solutions is much faster than the time scale of the rise of the surface concentration of ThPh2− recorded in SSHG experiments. Under the above assumptions, the surface concentration [ThPh2−]int of interfacial ThPh2− and the bulk concentration of ThPh0 near the interface are related by equations:38,42 d[ThPh2 −]int ∂[ThPh0] = −D dt ∂x

x=0

⎛ d[ThPh2 −]int [ThPh2 −]int ⎞ ⎟[ThPh0] = k 0 ⎜1 − Γmax dt ⎝ ⎠

x=0

where k0 describes the initial adsorption rate and Γmax is the saturation surface concentration of ThPh2−; the interface is located at x = 0. The equations are solved numerically using the random-walk approach,43,44 in order to find the temporal dependence of the surface concentration of ThPh2− for a given set of system’s parameters: D, k0, and Γmax (details of the numerical solution are presented in the Supporting Information). The set of parameters which best describe the studied system is found by fitting the calculated temporal dependence of [ThPh2−]int to the experimental data. This procedure, however, is not straightforward, for SSHG experiments do not allow absolute measurements of the surface concentration of ThPh2−. Nevertheless, the fact that the surface concentration of the adsorbate and the measured SH light intensity reach the saturation can be utilized for evaluating the agreement between the model and the experiment. If the experimental data are scaled so that the values recorded at long times (in the saturation regime) are identified with the simulated asymptotic surface concentration, the agreement can be evaluated by comparison of experimental and simulated curves at earlier times (before the saturation is reached). Therefore, during the fitting procedure for each set of tested parameters, D, k0, and Γmax, the following operations are done: (1) Calculation of the simulated temporal dependence of [ThPh2−]int for the set of ThPh concentrations used in the experiments (2 × 10−7 M, 4 × 10−7 M, 6 × 10−7 M, 1.2 × 10−6 M, and 2 × 10−6 M). (2) Interpolation of the simulated [ThPh2−]int kinetics to time points (tn) of the experimental data. (3) Calculation of the scaling factor S for each experimental curve, using the formula. S=

1 N − nS + 1

N

∑ n = nS

Figure 4. Data points: temporal dependence of the surface concentration of ThPh2− at the NaOH/chlorobenzene interface for various concentrations of ThPh in the organic phase. Solid lines: signal kinetics calculated from the model and fitted to the data. Vertical scale corresponds to the results of calculations, and the experimental data are plotted after being scaled as described in the text.

concentration are well-reproduced by the model, but the initial rise of experimental signals for two highest ThPh concentrations is slightly faster than predictions of the model. Still, the agreement between the experiment and the simulation is satisfactory and the differences can be attributed to factors not taken into account such as the exact mechanism of deprotonation, interactions between molecules at the interface, convective transport in the organic phase, and orientational reorganization of molecules at the interface, all of which can affect signal kinetics. The best fit was obtained with the following parameters: D = (6.4 ± 0.5) × 10−10 m2/s, k0 = (6.3 ± 0.5) × 10−6 m/s, and Γmax = (1.1 ± 0.1) × 10−6 M/m2. D very well agrees with its estimation based on measurements of diffusion coefficients for dyes of similar molecular volume in water. Recent values measured at 25 °C for Rhodamine 6G45 and Fluorescein46 lie in the range of (3.9−4.5) × 10−10 m2/s which, after recalculation to the conditions of presented SSHG experiments, lead to the expected value of D for ThPh in CB in the range of (5.1−5.9) × 10−10 m2/s. On the other hand, the saturation surface concentrations reported for a variety of surfactants at an air/

Γn [ThPh2 −]nint

where Γn is a square root of the second harmonic intensity at nth time step and N is the total number of time steps. The first time step taken for this calculation, nS, is selected in such a way that mainly data points from the saturation range are taken into account for scaling. A number of points (N − nS + 1) is used for calculating S in order to reduce the influence of noise and fluctuations. When an experimental kinetics is divided by S, it approximately overlaps with a corresponding simulated curve in the saturation range. (4) Calculation of χ2 using the formula N 2

χ =

∑ n = nχ

⎛ Γn ⎞ ⎜ − [ThPh2 −]nint ⎟ ⎝S ⎠

2

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water interface38,42 are in the range of (1−10) × 10−6 M/m2 and measured for a series of hemicyanine dyes47 in the range of (1−2) × 10−6 M/m2. Given the size of ThPh molecules, Γmax obtained from the simulation of the experimental data is very reasonable. Therefore, the resulting values of fit parameters further confirm the general validity of the applied model and the interpretation of the experimental data. Nevertheless, due to the simplicity of the model one has to treat the parameters obtained from the analysis as estimates rather than exact values. Behavior of the PTC mechanism in the model system was checked by adding a phase transfer catalyst, benzyltriethylammonium chloride (BTACl) either to the organic or the diluted NaOH phase. In both cases, after shaking the system and waiting for phase separation, the organic phase turns blue, which results from appearance of ThPh2− characteristic bands in its absorption spectrum (Figure 5, curves 1 and 2).

transfer catalysis has been demonstrated in the model system, in which thymolphthalein is used as a reactant and chlorobenzene (organic phase) with saturated NaOH solution (aqueous phase) constitute the reaction environment. SSHG measurements prove that the neutral form of thymolpthalein ThPh0, dissolved in the organic phase, is first adsorbed to the NaOH/chlorobenzene interface and is subsequently deprotonated to form the double anion ThPh2−. The surface concentration of this anion is monitored by measuring the intensity of second harmonic light generated at the interface illuminated by a beam of intense laser pulses. In the absence of a phase transfer catalyst, ThPh2− anions do not migrate into the organic phase, therefore they are trapped at the interface and their surface concentration increases in time until saturation is reached. Only upon addition of a phase transfer catalyst to the system, the presence of ThPh2− anions in the bulk organic phase is observed. These facts are consistent with the interfacial model of PTC, according to which the anions are created in the interfacial region, where they form ion pairs with cations of the catalyst and only such pairs migrate into the organic phase. The interpretation of SSHG measurements is also positively verified by comparison with a simple model based on Langmuir adsorption kinetics. Global fitting of the experimental data collected for various concentrations of the reagent allows estimation of the surface concentration of adsorbed anions, even though the data have only relative character. The demonstrated technique is simple and general and can become a tool of choice for studying the microscopic mechanism of phase transfer catalysis. Further work will involve practically used PTC systems, and SSHG measurements will be carried out in the presence of phase transfer catalysts. In particular, the influence of various catalysts and their concentration on the surface concentration of anions will be investigated. The studies should unveil the microscopic mechanism of important PTC reactions and show how it is affected by parameters of the system.

Figure 5. Solid lines: absorption spectra of thymolphthalein recorded in chlorobenzene over a layer of NaOH aqueous solution with the catalyst dissolved in the aqueous phase (1), organic phase (2), and without the catalyst (3). Dashed line: absorption spectrum of the organic phase from system 1, recorded 15 min after separation from NaOH.

This observation can be interpreted as formation of ion pairs between ThPh2− and BTA+ cations. Because of the lipophilicity of the latter such a pair migrates into the organic phase, as expected for a PTC process. Assuming the same extinction coefficient of ThPh2− in the ion pair as in the aqueous solution, a fraction of ThPh2− which migrate into the organic phase can be estimated. At the initial concentration of ThPh in CB equal to 1.1 × 10−4 M, this fraction equals approximately 25% for BTACl added to the aqueous phase (at approximately 5 × 10−3 M) and 10% for the catalyst added to the organic phase. Without BTACl in the system, the absorption spectrum of the organic phase did not show the bands of ThPh2− within limits of the spectrophotometer’s sensitivity (Figure 5, curve 3). When the organic phase is removed from over the NaOH layer, the equilibrium between ThPh0 and ThPh2− shifts back to prefer the neutral form, and the solution quickly becomes colorless (Figure 5, dashed line, see also the Supporting Information). This process is reversible because after mixing the colorless organic phase with the NaOH solution and BTACl and shaking the sample, absorption bands of ThPh2− appear again. This effect confirms that discoloration results from the ThPh2− → ThPh0 reaction (most probably due to protonation by trace amounts of water dissolved in CB). The above observations also correspond to the expected behavior of a PTC system.

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ASSOCIATED CONTENT

S Supporting Information *

Linearity test of the SSHG setup; sample preparation procedures; numerical methods; supplementary experimental data: absorption spectra of thymolphtalein at various pH, kinetics of the surface concentration of the double anion of phenolphthalein, and discoloration of thymolpthalein in the isolated organic phase of the PTC system. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: fi[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author would like to express his gratitude to Prof. Mieczysław Mak̨ osza and Prof. Michał Fedoryński for the inspiration for the research topic and valuable discussions on phase transfer catalysis and to Piotr Szymczak for advice on the numerical model. This research was supported by the Polish Ministry of Science and Higher Education through Project IP2011 012771. P.F. acknowledges the support of the

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CONCLUSIONS A scheme of application of the surface second harmonic generation for studying the microscopic mechanism of phase E



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Foundation for Polish Science TEAM project cofinanced by the EU European Regional Development Fund.

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ABBREVIATIONS PTC, phase transfer catalysis; SSHG, surface second harmonic generation; SH, second harmonic; ThPh, thymolphthalein; PhPh, phenolphthalein; CB, chlorobenzene; BTACl, benzyltriethylammonium chloride

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REFERENCES

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