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Heterogeneous Kinetics of the Dissolution of an Inorganic Salt, Potassium Carbonate, in an Organic Solvent, Dimethylformamide Claire L. Forryan,† Oleksiy V. Klymenko,† Colin M. Brennan,‡ and Richard G. Compton*,† Physical and Theoretical Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom, and Syngenta, Leeds Road, Huddersfield HD2 1FF, United Kingdom ReceiVed: December 13, 2004; In Final Form: February 18, 2005
Understanding the mechanisms of solid-liquid systems is fundamental to the development and operation of processes for the production of agrochemicals and pharmaceuticals. The use of a strong inorganic base in an organic solvent, typically, potassium carbonate in dimethylformamide, is often used to facilitate the formation of a required anionic organic nucleophile. In this paper, the dissolution kinetics of potassium carbonate in dimethylformamide at elevated temperatures is studied in the presence of ultrasound, as revealed via monitoring of the deprotonation of 2-cyanophenol by dissolved K2CO3. Two independent experimental methods were employed; the loss of 2-cyanophenol was detected electrochemically at a platinum microdisk working electrode, and the formation of the 2-cyanophenolate anion was monitored via UV/visible spectroscopic analysis. The results were modeled by fitting the experimental data to a theoretical model for the surface-controlled dissolution of solid particles. The dissolution rate constant, k, for the dissolution of K2CO3 in DMF was found to have a value of (1.3 ( 0.2) × 10-7 mol cm-2 s-1 at 100 °C, and the activation energy for the dissolution was 44.2 ( 0.4 kJ mol-1 over the temperature range of 70-100 °C studied.
1. Introduction The dissolution kinetics of solid particles suspended in a solvent is of great importance in both industrial and natural processes.1-11 Solid-liquid interfacial reactions may occur via several mechanisms, the focus of our studies being on that where the rate-determining step involves the dissolution of the solid substrate.12 In a previous paper, we developed the theory for the surface-controlled dissolution of solid particles over time as a function of the particle size distribution of the solid.13 This was effectively illustrated for the dissolution of potassium bicarbonate in dimethylformamide (DMF) at elevated temperature. In the kinetic treatment, we considered that after being placed into the solution, the particles start to dissolve with the rate constant k, with the rate of change of the number of moles of the species in a particle being proportional to k and the particle surface area, assuming that the particles are spherical. From the evaluation over all values of particle diameter, we obtained the time dependence of the total number of moles of the species in the particle mixture. Formerly, studies of the dissolutions of inorganic solids had been in aqueous solutions,7,14,15 such as the dissolution of limestone in aqueous electrolyte solutions,16-18 with particles of sizes in a small diameter fraction, the full distribution of particle sizes in an inorganic not being considered. Heterogeneous solid-liquid systems are widely used in the production of fine chemicals. Understanding the mechanisms of these heterogeneous reactions is fundamental to the development and operation of robust processes for the production of agrochemicals and pharmaceuticals. The use of a strong inorganic base in an organic solvent is often used to facilitate * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +44-01865-275413. Fax: +44-01865-275410. † University of Oxford. ‡ Syngenta.
the formation of a required anionic organic nucleophile for coupling with a dissolved electrophile. Typically, the site of reaction is in the homogeneous media, but the rate of the reaction can be controlled by the interfacial kinetics. Frequently, the inorganic base/organic solvent system of initial choice is potassium carbonate in DMF. The use of potassium carbonate in DMF promotes many organic processes such as epoxide synthesis from sulfur ylides,19,20 synthesis of primary amines,21,22 reduction of allylic nitro compounds to oximes,21 and several alkylations.20,23,24 Should the reaction not work, a combination of stronger base and/or alternative polar aprotic solvents often turned on the assumption that the nucleophile is not acidic enough to have been deprotonated by the dissolved carbonate in the solvent. This quite often neglects the heterogeneous nature of the reaction. In this paper, we expand our previous studies of the dissolution of solid particles in organic solvents by investigating the dissolution of K2CO3 in DMF at elevated temperatures and consider possible further reactions in the DMF solution. We monitor the dissolution of K2CO3 via the homogeneous deprotonation of 2-cyanophenol. As shown in Scheme 1, dissolved K2CO3 deprotonates the 2-cyanophenol to form the 2-cyanophenolate anion and KHCO3, which can further deprotonate the 2-cyanophenol. Consequently, the loss of 2-cyanophenol over time provides information on both the dissolution kinetics of K2CO3 (left-hand side of Scheme 1) and the homogeneous deprotonation of 2-cyanophenol. We employed a real-time electrochemical method based upon the voltammetric detection of 2-cyanophenol at a platinum microdisk electrode, combined with UV/visible spectroscopic analysis of samples removed from the reaction vessel. The solutions were heated via a heated microdisk method developed by Coles et al.,25 which utilizes an electronically controlled heat gun that allows for greater precision in controlling the solution temperature. Power ultrasound was incorporated into the system to induce mixing, which has been shown to facilitate
10.1021/jp0407573 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/06/2005
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SCHEME 1: Scheme for the Reaction of K2CO3 with 2-Cyanophenol in DMF
reproducible microelectrode responses in heterogeneous systems.26 The results show that initially there is rapid deprotonation of 2-cyanophenol via predissolved inorganic solid in solution. With a slight excess of K2CO3, over longer time the observed kinetics of loss of 2-cyanophenol were controlled by the rate of dissolution of K2CO3. The dissolution kinetics of K2CO3 in DMF at elevated temperatures were successfully analyzed using our surface-controlled dissolution model. With an excess of 2-cyanophenol as compared to K2CO3, the loss of 2-cyanophenol is initially from the surface-controlled dissolution of K2CO3 in DMF. Once the solid is exhausted, there is a further, slower formation of 2-cyanophenolate, possible controlled by a homogeneous deprotonation of 2-cyanophenol in the DMF solution. 2. Experimental Section 2.1. Chemical Reagents. All voltammetric experiments were carried out in N,N-dimethylformamide (DMF, Aldrich, 99.9 +%, HPLC grade) with tetra-n-butylammonium perchlorate (TBAP, Fluka, puriss. electrochemical grade) as a supporting electrolyte. The DMF was carefully treated by drying over Linde 5-Å molecular sieves (Aldrich) for a minimum of 48 h, and then, prior to use, it was shaken with ICN alumina N-super 1 (ICN Biomedicals GmbH, Germany) and the solvent was decanted off. The water content of the solvent was determined by Karl Fischer titration (Metrohm, 758 KFD Titrino),27 and it was found that the DMF drying procedure outlined above yielded DMF with a water content of ca. 0.04 wt % (2.3 × 10-2 mol L-1), compared to DMF as supplied, which has a water content of ca. 0.15 wt % (8.6 × 10-2 mol L-1). 2-Cyanophenol (Aldrich), potassium carbonate (AnalaR), and potassium bicarbonate (Aldrich) were of the highest commercially available grade, and the potassium 2-cyanophenolate salt was supplied by Syngenta; all were used without further purification. 2.2. Instrumentation. 2.2.1. Electrochemical Measurements. Cyclic voltammetry was recorded using a computer-controlled µAutolab potentiostat (Eco Chemie, Netherlands) and carried out in a cell with a solution volume of 15 cm3. A three-electrode design was employed consisting of a platinum counter electrode, a platinum pseudoreference electrode, and a platinum microdisk working electrode (radius 13.2 µm) manufactured in house according to a published procedure.28 Prior to each electrochemical measurement, the microdisk electrode was polished on soft lapping pads (Kemet Ltd., U.K.) using alumina (Kemet Ltd., U.K.) of sizes 1 and 0.3 µm8,29 and then rinsed in ultrapure water before being carefully dried and rinsed in the dried solvent. The electrode diameter of the microdisk was calibrated electrochemically using a 1 mM ferrocene solution in acetonitrile containing 0.1 M TBAP, adopting a value for the diffusion coefficient of 2.3 × 10-5 cm2 s-1 at 298 K.30 2.2.2. Heated Microdisk Cell and Ultrasound Setup. The ultrasonic generator used was a model VCX 5000 (Sonics and materials) horn equipped with 3-mm-diameter titanium microtip emitting 25-kHz ultrasound. The power output of the transducer was calorimetrically measured in DMF,31,32 for which an
amplitude of 5% was found to correspond to 8 W cm-2 and was employed for all experiments. The microelectrode high-temperature experiments were carried out in a cell with a solution volume of 15 cm3 by hot air circulation from an electronically controlled heat gun within a small box of insulating material with a front glass wall.25 A Pt resistance thermometer controlled the air temperature, and a thermocouple in contact with the solution was used to read the temperature during the voltammetric scans. The platinum microdisk electrode, reference and counter electrodes, and ultrasound horn were inserted from above into the cell through precision-bored apertures in the Teflon cell lid, which was manufactured in house to minimize heat losses. Temperature control was most important, and care was taken to ensure that all experiments were carried out at the required temperatures to (1 °C. To maintain solution temperatures of 60, 80, 90, or 100 °C under the application of the power ultrasound, external heating from the heated microdisk setup required hot air circulation of ca. 55, 65, 80, and 86 °C, respectively. Two regimes of inorganic solid and 2-cyanophenol addition were adopted for this study: (a) the 2-cyanophenol solutions in DMF/0.2 M TBAP were heated under ultrasound to the required elevated temperature, and upon stabilization of the temperature the K2CO3 was added and (b) solutions of K2CO3 in DMF/0.2 M TBAP were heated under ultrasound at elevated temperatures for a known time period (1-5 h) before the addition of 2-cyanophenol to the solution. The solutions were thoroughly degassed with nitrogen (BOC gases) throughout their initial heating, and a continuous flow of nitrogen over the solution was maintained throughout the electrochemical experiments to ensure that no oxygen was in contact. After the addition of either the inorganic solid (regime a) or the 2-cyanophenol (regime b), the platinum microdisk electrode was introduced into the cell, and voltammetric scans were recorded at a scan rate of 50 mV s-1 1 min after the addition and subsequently every 5 min for up to 1 h. After each measurement, the electrode was removed and polished meticulously by the procedure outlined above. The steady-state limiting currents of the elevated temperature responses were deduced by subtracting the observed limiting current from that of a blank background scan of 0.2 M TBAP/DMF at the elevated temperature concerned. 2.2.3. UV/Visible Spectroscopy. UV/visible spectra were recorded on a Unicam UV2 series UV/visible spectrophotometer (Unicam, Cambridge, U.K.) using a quartz cuvette of path length 1 cm, scanning over a wavelength of 275-400 nm. Smallvolume samples (ca. 100 µL) were removed at regular time intervals from the ultrasound-heated solutions, over a 1 h period, and allowed to cool to room temperature. These were diluted with DMF by a factor of 100, and their UV/visible spectra were recorded. Each spectrum was background subtracted from that of blank DMF. 2.2.4. Particle Size Measurements. The particle size distribution was determined using a Malvern Mastersizer 2000 (GLP no. 1345). The samples were analyzed in duplicate with a dry powder Scirocco 2000 accessory using a measurement time of
Dissolution Kinetics of K2CO3 in DMF
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5 s, a background time of 5 s, a vibration feed rate of 60%, and a dispersive air pressure of 3 bar. 2.2.5. ThermograVimetric Analysis. The water content analysis of solid K2CO3 was carried out using a Mettler TGA (thermogravimetric analyzer) controlled by their STAR software. Samples were placed in a 70-mL alumina crucible and heated, with a nitrogen purge, from 25 to 150 °C at a rate of 10 °C/min and held at 150 °C for a further 60 min. The weight loss of the solid over time was monitored. There was no decomposition of the solid at this temperature. 2.2.6. X-ray Powder Diffraction. X-ray powder diffraction patterns were recorded on a Bruker D8 in the angular range of 4-50° 2θ. 3. Theoretical Model of the Surface-Controlled Dissolution of Solid Particles In the kinetic treatment of the dissolution of solid particles previously reported,13 the number of moles of solid remaining undissolved in solution over time is modeled. Let us consider a sample of particles of mass m and density F with the size distribution given by the distribution function f(d), where d is the particle diameter. We assume that the particles are spherical. After being placed into the solution, the particles start to dissolve with the rate constant k. The rate of change of the number of moles of the species in a particle is proportional to the rate constant k and the particle surface area S1(d0) ) πd02. Hence, the following differential equation can be written for a single particle of an initial diameter d0:
dn1(d0, t) ) -kS1(d0, t) ) -Rn12/3(d0, t) dt
(1)
where R ) kπ1/3(6M/F)2/3. The solution of this differential equation is given by
R 3 n1(d0, t) ) n11/3(d0, 0) - t 3
[
]
(2)
From an evaluation of the number of moles in all particles that initially had a diameter of d0 as a function of time, we find that
dn(d0, t) ) n1(d0, t) dN(d0) )
[
] (
)
d0F 6V R 3 -t n11/3(d0, 0) - t H f(d0) dd0 (3) 3 2kM πd03
Integrating equation (3) over all values of the particle diameter, we obtain the time dependence of the total number of moles of the species in the particle mixture:
n(t) )
∞ πF ∫(2kM/F)t ) [(6M
6m πF
1/3
R 3f(d0) d0 - t dd0 3 d3
]
(4)
0
For a more detailed derivation, ref 13 should be consulted. 4. Results and Discussion 4.1. Particle Size Distribution of K2CO3. The size distribution of a sample of particles in a solid is given by the particle distribution function f(d), where d is the particle diameter. The particle size distribution of a sample of the K2CO3 used in all experiments, measured as described in section 2.2.4, is displayed in Figure 1. The volume weighted mean particle diameter was calculated to be 515 ( 10 µm. 4.2. Electrochemical Analysis. The electrochemical behavior of 2-cyanophenol in DMF and the response to the addition of K2CO3 was investigated at elevated temperatures to develop a
Figure 1. Particle size distribution for K2CO3 used experimentally.
SCHEME 2: Reaction Scheme for the Electrochemical Reduction of 2-Cyanophenol
strategy for following the dissolution of K2CO3 as the 2-cyanophenol “titrates” with the dissolved solid. The steady-state voltammetric responses for solutions of 5 mM 2-cyanophenol (0.2 M TBAP/DMF) at temperatures of 80 and 100 °C at a platinum microdisk electrode are detailed in Figure 2a and b, respectively. These reveal one reduction wave for both temperatures of 80 and 100 °C at half potentials of -0.9 V versus Pt and -1.2 V versus Pt with limiting currents of 3.7 × 10-8 and 5.5 × 10-8 A, respectively. This wave can be attributed to the irreversible reduction of 2-cyanophenol to form the radical anion, which is consistent with previous studies of the reduction of 2-cyanophenol in DMF at room temperature33 and elevated temperatures up to 100 °C34 at gold microdisk electrodes. As shown in Scheme 2, the radical anion formed on reduction is rapidly protonated by the parent molecule, the rate constant for which was found to be g1 × 107 M-1 s-1, and hence the net number of electrons per cyanophenol reduced is approximately one-half.33,34 Previously reported ESR studies on 2-cyanophenol have given evidence that the protonated radical formed in Scheme 2 undergoes further reactions generating other radical species.35 Solid K2CO3 (0.025 g) was added to the 5 mM 2-cyanophenol solution at 80 °C, and 0.012 g of K2CO3 was added to that at 100 °C. The voltammetric response for the 2-cyanophenol/ radical anion reduction wave was measured 1 min after the addition and subsequently over a 40-min time period at 80 °C and for 90 min at 100 °C, and the resulting responses are overlaid in Figure 2a and b. Upon the addition of the solid, the half potential of the reduction wave shifts to less negative potentials of ca. -0.7 V versus Pt, indicative of a chemical reaction of the 2-cyanophenol. It can be seen at both temperatures that after the introduction of K2CO3 to the solutions the limiting current for the reduction wave decreases over time. This corresponds to the deprotonation of 2-cyanophenol by K2CO3 to form the 2-cyanophenolate anion as depicted in Scheme 1. The decrease in the limiting current corresponds to the loss of 2-cyanophenol from solution as it reacts over time with the K2CO3. It should be noted that the 2-cyanophenolate anion was found to be voltammetrically inactive in DMF; the electrochemical reduction of solutions of potassium 2-cyanophenolate
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Figure 2. Steady-state voltammograms for the reductions at a platinum microdisk working electrode of a solution of 5 mM 2-cyanophenol in 0.2 M TBAP/DMF at temperatures of (a) 80 and (b) 100 °C, with responses after the additions of 0.025 and 0.012 g K2CO3, respectively.
Figure 3. Plots of the limiting current/nA against reaction time/min for a 5 mM solution of 2-cyanophenol in 0.2 M TBAP/DMF at (a) 80 and (b) 100 °C after the addition of (×) 0.012, (4) 0.025, (9) and 0.053, and (2) 0.111 g of K2CO3.
in 0.2 M TBAP/DMF gave no responses at the platinum microdisk electrode. The limiting currents for the responses of 5 mM solutions of 2-cyanophenol (0.2 M TBAP/DMF) at temperatures of 80 and 100 °C after the addition of (×) 0.012 g, (4) 0.025 g, (9) and 0.053 g, and (2) 0.111 g of K2CO3 were examined over a 40min period. Assuming hypothetically that the complete dissolution of the solid was possible, these masses would correspond to K2CO3 concentrations of 5.8, 12.0, 25.6, and 53.5 mM. Graphs of the steady-state limiting current against time after the addition of the inorganic solid are given in Figure 3a and b for the 80 and 100 °C heated solutions, respectively. The limiting current for each mass addition decreases over time, indicating the loss of 2-cyanophenol and subsequent formation of the 2-cyanophenolate anion. At both temperatures, the larger mass additions of 0.025, 0.053, and 0.111 g give complete consumption of the 2-cyanophenol after ca. 20 min, with the initial rate of loss of 2-cyanophenol increasing with increasing mass of the solid. For the smaller mass addition of 0.012 g K2CO3 at 80 and 100 °C, there is a slower, gradual loss of 2-cyanophenol, which after 90 min at 100 °C reaches completion. 4.2.1. Effects of Ultrasound Time. The experiments at 80 °C were repeated with the solutions of K2CO3/DMF being heated under ultrasound for 3 h in DMF/0.2 M TBAP and then subsequent addition of 5 mM 2-cyanophenol. Figure 4 gives the plots of the steady-state limiting current of the 2-cyanophenol/radical anion reduction wave against the time. A comparison with Figure 3a, for the addition of K2CO3 to the already heated 2-cyanophenol at an analogous temperature, shows that over the initial first minute there is a much reduced limiting current
Figure 4. Plots of the limiting current/nA against reaction time/min for (×) 0.012, (4) 0.025, (9) 0.053, and (2) 0.111 g of K2CO3 in 0.2 M TBAP/DMF heated under ultrasound for 3 h followed by the addition of 5 mM 2-cyanophenol.
for 2-cyanophenol and hence increased loss of 2-cyanophenol with pre-ultrasound of the K2CO3/DMF solution. These results suggest that the initial mechanism for 2-cyanophenol loss is via a homogeneous reaction with dissolved K2CO3 in the DMF solution. For the higher masses of K2CO3 of 0.111 and 0.053 g, it appears that after applying ultrasound over a 3-h time period there is sufficient dissolution of the inorganic solid in the DMF solution to give complete reaction of the 2-cyanophenol to form the phenolate. For 0.012 and 0.025 g of K2CO3, after the initial rapid current decrease, at longer time the limiting current of the 2-cyanophenol reduction wave decreases gradually, as observed in Figure 3a, showing a slow loss of 2-cyanophenol
Dissolution Kinetics of K2CO3 in DMF
Figure 5. Plots of limiting current/nA against reaction time/min for the addition of 5 mM 2-cyanophenol to 0.025 g of K2CO3 in 0.2 M TBAP/DMF heated under ultrasound at 80 °C for times of (×) 0, (4) 1, (9) 3, (2) 5, and (O) 10 h.
to form the phenolate. This implies that at longer time the loss of 2-cyanophenol is controlled by the rate of solid dissolution. To examine the initial loss of 2-cyanophenol via the homogeneous reaction of predissolved solid further, the K2CO3/ DMF solutions were heated and agitated under power ultrasound for increasing time periods to increase the initial dissolution of the solid. The solubility of K2CO3 in DMF is extremely low and thought to be around 2-3 ppm at room temperature.36 No information is available on the solubility at elevated temperatures. The effects of applying ultrasound to the inorganic solid in DMF/0.2 M TBAP solutions at a temperature of 80 °C over a range of time periods from 1 to 10 h before the addition of 5 mM 2-cyanophenol are shown in Figure 5. Included are the results for the addition of the inorganic solid to the preheated solution of 5 mM 2-cyanophenol (×). It can clearly be seen that the initial rate of loss of 2-cyanophenol increases with preultrasound time of the inorganic solid/DMF solutions. For 1, 3, and 5 h of pre-ultrasound, the initial homogeneous reaction of predissolved solid is followed over longer time by the slow consumption of 2-cyanophenol as controlled by the rate of solid dissolution. A time period of 10 h of ultrasound gives sufficient dissolution of K2CO3 for all of the 2-cyanophenol to react rapidly with the solid already in solution; consequently, the slower dissolution-controlled phase is not observed. 4.3. UV/Visible Spectroscopic Analysis. UV/visible spectroscopy was employed as a further method for following the dissolution of K2CO3 in DMF. The dissolved solid deprotonates the 2-cyanophenol to produce the 2-cyanophenolate anion, which is detected spectroscopically. The UV/visible spectrum of a 0.1 mM solution of potassium 2-cyanophenolate in DMF is known to display the phenolate absorption peak at 358 nm with an absorbance of 0.764 mol2 dm-6.35 The absorbance of this peak was analyzed over the reaction time for the addition of 2-cyanophenol to K2CO3 solutions after predissolution ultrasound times of 2 to 3 h at elevated temperatures, and the concentration of 2-cyanophenolate formed at each reaction time was determined. 4.3.1. Determination of the Dissolution Rate Constant, k, for K2CO3 in DMF. The dissolution of K2CO3 in DMF was studied by following the deprotonation of 2-cyanophenol by K2CO3 for a 1:1 mole ratio of 2-cyanophenol added to the initial amount of K2CO3 or a slight excess of solid, assuming that complete dissolution of the solid was possible. Solutions of DMF with 0.015 g, 0.020, 0.043, and 0.050 g of K2CO3 were heated under ultrasound for 2 h at 100 °C, followed by the addition of (×) 5 mM, (b) 10 mM, (2) 15 mM, and (9) 25 mM 2-cyanophenol, and samples of the reaction solution were analyzed by UV/
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Figure 6. Plots of 2-cyanophenolate concentration/mM against reaction time/s for solutions of DMF with K2CO3 heated under ultrasound for 2 h at 100 °C followed by the addition of 2-cyanophenol at concentrations of (9) 5 mM (0.015 g K2CO3 in DMF), (0) 10 mM (0.020 g K2CO3 in DMF), (b) 15 mM (0.043 g K2CO3 in DMF), and (O) 25 mM (0.050 g K2CO3 in DMF).
Figure 7. Plots of n(t)/mmol against time/s for theoretical UV/visible spectroscopic experimental results at 100 °C dissolution-rate-controlled process of the addition of (4) 2-cyanophenol to 0.020 g of K2CO3 in DMF and (O) 2-cyanophenol to 0.050 g of K2CO3 in DMF.
(-) and for the 10 mM 25 mM
visible spectroscopy. The corresponding plots of the concentration of 2-cyanophenolate formed against time are given in Figure 6. The results are consistent with those found voltametrically; the initial production of 2-cyanophenolate was from the deprotonation of predissolved K2CO3, followed over a longer time by the slower surface-controlled dissolution of the solid. These results were analyzed using our surface-controlled rate of dissolution model previously reported13 to model the dissolution of KHCO3 in DMF at elevated temperatures successfully. Equation 4 was solved numerically, incorporating the particle size distribution function for our sample of K2CO3 (Figure 1) to develop plots of the number of moles of undissolved inorganic solid, n(t)/moles, against the reaction time/ s, where the following parameters were used: mass of the inorganic solid/g, density of the inorganic solid/g cm-3, molecular weight of the inorganic solid /g mol-1, time/s, and k/mol cm-2 s-1. The mass of solid used was calculated via subtraction of the mass predissolved in solution from the initial mass of solid added and the first minute of reaction not included in the fits. Taking a 1:1 mole ratio of 2-cyanophenolate produced to K2CO3 reacted, plots of n(t) against time give very good fits of theoretical to experimental data. This is shown in Figure 7 for the addition of 10 and 25 mM 2-cyanophenol to K2CO3 in DMF at 100 °C. The mean value determined for the dissolution rate
8268 J. Phys. Chem. B, Vol. 109, No. 16, 2005 constant, k, for the dissolution of K2CO3 in DMF at 100 °C is (1.3 ( 0.2) × 10-7 mol cm-2 s-1. From the successful fits of experimental to theoretical data utilizing the 1:1 mole assumption in the analysis, it appears that for the deprotonation of 2-cyanophenol with a slight excess of initial K2CO3 the formation of 2-cyanophenolate is predominantly via K2CO3 deprotonation of 2-cyanophenol, the left-hand side of Scheme 1. Considering that the surface-controlled dissolution rate constant for KHCO3 in DMF was found previously to be (1.1 ( 0.3) × 10-8 mol cm-2 s-1,13 which is an order of magnitude smaller than that mentioned above for K2CO3, the deprotonation of 2-cyanophenol in this scenario is likely to be via the fast dissolution of K2CO3 available. This was validated from water content analysis by Karl Fischer titration of the solutions after complete deprotonation of a 1.0 M solution of 2-cyanophenol by K2CO3 to form the anion. Any deprotonation of the 2-cyanophenol by KHCO3, right-hand side of Scheme 1, would produce 2-cyanpohenolate with the molar equivalent of water. The water content of the K2CO3 was determined from thermogravimetric analysis, and a weight loss of ca. 1.73% was measured. The initial water from the solid added and the known initial water content of the DMF were subtracted from the final water content of the solution to give an accurate indication of the water produced during the deprotonation. The solvent after complete deprotonation was found to contain ca. 0.14% water by weight, which would correspond to only ca. 80 mM of the total 1.0 M 2-cyanophenolate being formed from KHCO3 deprotonation. However, some water may be produced via the disproportionation of bicarbonate, which forms the carbonate anion, carbon dioxide, and water. 4.3.2. Effect of Temperature on the Dissolution Rate Constant, k, for K2CO3 in DMF. The addition of 2-cyanophenol to solutions of K2CO3/DMF heated under ultrasound was examined for three further elevated temperatures of 70, 80, and 90 °C. K2CO3/DMF was heated for 2.5 h under ultrasound at 90 °C and heated for 3 h at the lower temperatures of 70 and 80 °C before the 2-cyanophenol additions. Figures 1S, 2S, and 3S, displayed in the Supporting Information, show the plots of 2-cyanophenolate concentration against reaction time for the additions of 2-cyanophenol to K2CO3/DMF at 70, 80, and 90 °C, respectively. Again, taking a 1:1 mole ratio of 2-cyanophenolate produced to K2CO3 deprotonation of the 2-cyanophenol, plots of n(t) against time for the theoretical model and the experimental results were compared. Figure 8 details the plots, and overlaid is that for the similar addition of 2-cyanophenol to K2CO3 in DMF at 100 °C. Good fits of theoretical to experimental data are seen for all temperatures, with the rate of dissolution of the solid increasing with increasing temperature. The values of k found are (3.9 ( 0.1) × 10-8 mol cm-2 s-1 at 70 °C, (5.5 ( 0.4) × 10-8 mol cm-2 s-1 at 80 °C, and (9.5 ( 0.5) × 10-8 mol cm-2 s-1 at 90 °C. A graph of ln k against 1/T over the temperature range of 70-100 °C studied, inset of Figure 8, yields a straight line with an R2 value of 0.989. From analysis in terms of an Arrhenius-type relation, the dissolution of K2CO3 in DMF was calculated to be 44.2 ( 0.4 kJ mol-1. 4.3.3. Further ObserVations. Next we investigated the surfacecontrolled dissolution of K2CO3 when less than a molar equivalent of K2CO3 is used, followed by further homogeneous deprotonation of 2-cyanophenol once the solid is exhausted. This was studied by monitoring the deprotonation of 2-cyanophenol by K2CO3 for an excess of 2-cyanophenol at elevated temperatures under power ultrasound, the mole ratio of 2-cyanphenol
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Figure 8. Plots of n(t)/mmol against time/s for theoretical (-) and UV/visible spectroscopic experimental results for the dissolution-ratecontrolled process of the addition of 25 mM 2-cyanophenol to 0.062 g of K2CO3 in DMF at (b) 70, (0) 80, (O) 90, and (9) 100 °C. The inset shows a graph of ln(k/mol cm-2 s-1) against 1/(T/K) over the temperature range of 70-100 °C.
Figure 9. Plots of 2-cyanophenolate concentration/mM against reaction time/s for solutions of DMF with K2CO3 heated under ultrasound followed by the addition of 25 mM 2-cyanophenol (0.026 g of K2CO3 in DMF) at (2) 70 and (b) 100 °C and the addition of 50 mM 2-cyanophenol (0.051 g of K2CO3 in DMF) at (9) 100 °C.
added to the initial amount of K2CO3 being 2:1, assuming that complete dissolution of the solid was possible. Figure 9 shows the plots of the concentration of 2-cyanophenolate formed against reaction time for the addition of 25 mM 2-cyanophenol to 0.026 g of K2CO3 in DMF at 70 and 100 °C and the 50 mM addition of 2-cyanophenol to 0.052 g of K2CO3 in DMF at 100 °C, which correspond to K2CO3 concentrations of 12.5 and 25 mM, respectively, assuming complete dissolution. It can be seen that at 100 °C, after 7200 s the total concentrations of 2-cyanophenolate produced are 20 and 38 mM for the 25 and 50 mM 2-cyanophenol additions, respectively. Hence, the deprotonation of 2-cyanophenol is not via a 1:1 mole ratio of 2-cyanophenolate formed to K2CO3 because 2-cyanophenolate production of only one-half the starting concentration of 2-cyanophenol would be expected from the amount of K2CO3 present. The loss of 2-cyanophenol is likely to occur via the mechanism shown in Scheme 1, where initially the deprotonation is controlled by the rate of K2CO3 dissolution, and once the solid is exhausted there is further deprotonation of the 2-cyanophenol. Similar results were seen for the further elevated temperatures of 80 °C and 90 °C. Again, the initial formation of 2-cyanophenol is being controlled by the faster surface-controlled dissolution of K2CO3. It is likely that the further formation of 2-cyanophenolate, once the solid is
Dissolution Kinetics of K2CO3 in DMF exhausted, is via slower homogeneous deprotonation of 2-cyanophenol in the DMF solutions. To ascertain if the KHCO3 produced remains dissolved in the DMF solution, with no solid precipitation, X-ray powder diffraction was utilized. The X-ray diffraction patterns of standard samples of K2CO3 and KHCO3 and of a mixture by equal weights of the two solids were obtained. An experimental sample of solid, filtered and dried under vacuum, obtained after the deprotonation of 2-cyanophenol by the initial K2CO3 added was analyzed by the X-ray powder diffraction method. The pattern was compared to that of the standards and was observed to be in excellent agreement with that of K2CO3, none of the peaks unique to the patterns of KHCO3 or the mixture being present. These results suggest that from the deprotonation of 2-cyanophenol by K2CO3 the KHCO3 produced remains dissolved in solution, being available for further homogeneous reaction with the 2-cyanophenol. 5. Conclusions The dissolution of K2CO3 in DMF at elevated temperatures was studied by following the deprotonation of 2-cyanophenol by the dissolved solid over time (left-hand side of Scheme 1). The dissolution is surface-controlled, and the rapid follow up deprotonation with 2-cyanophenol to form the 2-cyanophenolate anion allows us to monitor the dissolution rate. Two independent experimental methods were employed to study the dissolution process comprehensively; the loss of 2-cyanophenol was detected voltametrically, and the formation of 2-cyanophenolate was followed via UV/visible spectroscopic analysis. It was shown that initially the 2-cyanophenol deprotonation is by predissolved K2CO3 in the DMF solution, and at longer times the observed kinetics are controlled by the rate of K2CO3 dissolution. The results were compared with the mathematical model for the surface-controlled dissolution of solid particles, and good fits were seen between experimental and theoretical data. It was found that the dissolution rate constant, k, for the dissolution of K2CO3 in DMF has a value of (1.3 ( 0.2) × 10-7 mol cm-2 s-1 at 100 °C, and the activation energy for the dissolution was 44.2 ( 0.4 kJ mol-1 over the temperature range of 70 to 100 °C. With an excess of 2-cyanophenol as compared to K2CO3, the loss of 2-cyanophenol is initially from the surfacecontrolled dissolution of K2CO3 in DMF. This is followed by a slower formation of 2-cyanophenolate, which is likely to be from homogeneous deprotonation of the 2-cyanophenol in the DMF solutions. Acknowledgment. C.L.F. expresses her gratitude to the EPSRC for a DTA studentship and Syngenta for a CASE award. We also thank Syngenta for the preparation of the potassium 2-cyanophenolate salt and for the use of their facilities for carrying out the Karl Fischer titrations, particle size measurements, and thermogravimetric analysis. O.V.K. thanks the Clarendon fund for partial support. Supporting Information Available: UV/visible spectroscopic results detailed as plots of 2-cyanophenolate concentration against reaction time for the additions of 2-cyanophenol to K2CO3/DMF heated under ultrasound at 70, 80, and 90 °C,
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