Investigation of the Kinetics and Mechanism of Acid Chloride

Acid chloride (ROCl) hydrolysis in a 1,2-dichloroethane (DCE)/water system has been investigated over a wide range of conditions using microelectroche...
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Investigation of the Kinetics and Mechanism of Acid Chloride Hydrolysis in an Oil/Water System Using Microelectrochemical Measurements at Expanding Droplets (MEMED) Jie Zhang,†,§ John H. Atherton,‡ and Patrick R. Unwin*,† Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom, and Avecia Huddersfield Works, P.O. Box A38, Leeds Road, Huddersfield HD2 1FF, United Kingdom Received August 27, 2003. In Final Form: December 4, 2003 Acid chloride (ROCl) hydrolysis in a 1,2-dichloroethane (DCE)/water system has been investigated over a wide range of conditions using microelectrochemical measurements at expanding droplets (MEMED). Hydrolysis occurred spontaneously when DCE droplets containing either butyryl chloride, valeryl chloride, hexanoyl chloride, or decanoyl chloride were expanded into an aqueous buffer solution (pH 4-11). The reaction was monitored using a potentiometric Ag/AgCl ultramicroelectrode to measure local changes in Cl- concentration in the aqueous phase near the surface of the droplets during the reaction. For butyryl chloride and decanoyl chloride, the hydrolysis rate increased gradually with increasing pH. For valeryl chloride and hexanoyl chloride, the hydrolysis rate increased with pH in the range of 4-9 but decreased at higher pH values, due to the blocking of the liquid/liquid interface by products. A mechanism for the ROCl hydrolysis reaction, involving rapid (Langmuirian) adsorption of ROCl at the interface followed by rate-limiting interfacial hydrolysis, was found to account for the experimental observations.

Introduction Acid chlorides (ROCl) are used extensively in organic synthesis.1,2 For example, ROCl can be used to produce acids via hydrolysis, when the use of free acid has to be avoided.2 Consequently, the chemical and physical factors affecting the rates of hydrolysis of ROCl and related compounds have been the subject of many studies,2-18 with a focus on homogeneous systems.3-12 ROCl hydrolysis with * To whom correspondence should be addressed. E-mail: [email protected]. † University of Warwick. ‡ Avecia Huddersfield Works. § Present address: School of Chemistry, Monash University, Victoria 3800, Australia. (1) March, J. Advanced Organic Chemistry: Reactions, Mechanisms and Structure; McGraw-Hill: Auckland, 1977; p 361. (2) Chaudry, I.; Albery, W. J. Unpublished results (ICI confidential report). (3) Hudson, R. F.; Moss, G. E. J. Chem. Soc. 1962, 5157. (4) Cairns, E. J.; Prausnitz, J. M. J. Chem. Phys. 1960, 32, 169. (5) Kevill, D. N.; Kim, C. B. J. Chem. Soc., Perkin Trans. 2 1988, 1353. (6) Koo, I. S.; Bentley, T. W.; Kang, D. H.; Lee, I. J. Chem. Soc., Perkin Trans. 2 1991, 175. (7) Bentley, T. W.; Jones, R. O.; Koo, I. S. J. Chem. Soc., Perkin Trans. 2 1994, 753. (8) D’Souza, M. J.; Kevill, D. N.; Bentley, T. W.; Devaney, A. C. J. Org. Chem. 1995, 60, 1632. (9) Johnson, J. E.; Riesgo, E. C.; Jano, I. J. Org. Chem. 1996, 61, 45. (10) Bentley, T. W.; Llewellyn, G.; McAlister, J. A. J. Org. Chem. 1996, 61, 7927. (11) Johnson, J. E.; Riesgo, E. C.; Jano, I. J. Org. Chem. 1996, 61, 45. (12) King, J. F.; Gill, M. S. J. Org. Chem. 1998, 63, 808. (13) Albery, W. J.; Couper, A. M.; Hadgraft, J.; Ryan, C. J. Chem. Soc., Faraday Trans. 1 1974, 70, 1124. (14) Silhanek, J.; Kondradova, L.; Simeckova, O.; Horak, J. Collect. Czech. Chem. Commun. 1982, 47, 2904. (15) Atherton, J. H. Res. Chem. Kinet. 1994, 2, 193. (16) Tam, K. Y.; Compton, R. G.; Atherton, J. H.; Brennan, C. M.; Docherty, R. J. Am. Chem. Soc. 1996, 118, 4419. (17) Slevin, C. J.; Unwin, P. R.; Zhang, J. In Liquid Interfaces in Chemical, Biological, and Pharmaceutical Applications; Volkov, A. G., Ed.; Marcel Dekker: New York, 2001; p 325. (18) Slevin, C. J.; Unwin, P. R. Langmuir 1997, 18, 4799.

pure water occurs rapidly,3 and so kinetic measurements have typically been made in organic solvents or binary aqueous mixtures with organic solvents. As detailed later, investigations of ROCl hydrolysis in two-phase (oil/water) systems are scarce, but there is information on related processes. Albery and co-workers13 studied the hydrolysis of p-methylbenzyl chloride and methyl nicotinate in water and at the water/isopropyl myristate interface, established on the sinter in a rotating diffusion cell (RDC). For p-methylbenzyl chloride, the ratelimiting step was shown to be the transfer of substrate from the organic phase to water, while methyl nicotinate hydrolysis was limited by diffusion across the sinter. Silhanek et al.14 studied the hydrolysis of triphenylmethyl chloride (TPMCl) using a stirred toluene/water contactor. It was proposed that the reaction took place in the laminar layer on the aqueous side of the interface, but it has also been pointed out that the reaction may have occurred at the liquid/liquid interface.15 Compton and co-workers16 studied the hydrolysis of solid TPMCl in aqueous solution using the channel flow cell method. The reaction was shown to occur at the solid/ liquid interface, with release of triphenyl cations and chloride anions directly from the solid surface as the ratelimiting step. The rate of the interfacial process depended on the density of exposed chlorine atoms in the crystal plane, and the hydrolysis product had a negligible effect on the overall reaction rate. The only previous study of acid chloride hydrolysis in a two-phase system is due to Chaudry and Albery, who used the RDC to investigate the hydrolysis of decanoyl chloride and an acid chloride intermediate used in pyrethroid synthesis in an n-heptane/water system.2 Based on experimental observation, they proposed a model that considered interfacial hydrolysis and Langmuirian adsorption of the various reactants and products at the interface. However, as they pointed out, some of the features of the experimental results were not in agreement with the predictions of this model. A potential difficulty

10.1021/la0355951 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/27/2004

Acid Chloride Hydrolysis in an Oil/Water System

in this application of the RDC is that the acid product of hydrolysis may be surface-active, leading to a timedependent inhibition of the reaction. The problem of the stagnant interface, inherent in the RDC, is lifted with microelectrochemical measurements at expanding droplets (MEMED), where the interface is continuously refreshed.17 We have recently shown MEMED to be a powerful technique for studying the kinetics of reactions that occur at liquid/liquid interfaces.17-19 MEMED involves growing droplets of one liquid (feeder) phase in a second immiscible (receptor) phase in a controlled fashion. The developing concentration profile adjacent to the drop surface in the receptor phase, due to interfacial reaction, is monitored with a stationary ultramicroelectrode (UME), positioned directly opposite the expanding droplet. By solving the convective-diffusion equation, with appropriate boundary conditions, theoretical concentration profiles can be generated for comparison with experiment, allowing the interfacial reaction to be investigated quantitatively. Mass-transport models for MEMED have been developed,18,19a,g and the methodology has been applied to a wide variety of systems.17-19 Hydrolysis of TPMCl at the water/1,2-dichloroethane (DCE) interface18 is among the model systems considered previously. This paper presents a series of detailed investigations of acid chloride hydrolysis. The hydrolysis process of interest can be represented as follows:

CH3(CH2)nCOCl + H2O (or OH-) f CH3(CH2)nCOOH + HCl (or Cl-) (1) where n ) 2, 3, 4, and 8 in the studies herein. The kinetics of the process has been investigated by expanding DCE droplets containing CH3(CH2)nCOCl into an aqueous buffer receptor phase to promote hydrolysis. By determining the resulting Cl- distribution adjacent to the droplet in the aqueous phase, using a potentiometric Ag/AgCl UME, the rate constants for hydrolysis can be determined. A model is developed to explain the data, in which adsorption of CH3(CH2)nCOCl at the interface follows the Langmuir isotherm, with interfacial hydrolysis as the rate-limiting step. Experimental Section Chemicals. The chemicals CH3(CH2)nCOCl (n ) 2, 3, 4, and 8) (g98%), CH3(CH2)nCOOH (n ) 2, 3, and 4) (g99%), and CH3(CH2)8COOH (96%) were from Sigma-Aldrich and were used as received. Dried DCE solvent, obtained by adding molecular sieves (4 Å, Rose Chemicals, Gillingham, U.K.) to HPLC grade DCE (Sigma-Aldrich) overnight, was used throughout. Aqueous buffer solutions were prepared according to literature procedures,20 using reagents that were at least analytical grade and Milli-Q water (Millipore Corp.). Apparatus and Procedure. The basic apparatus used for MEMED experiments has been described previously.17-19 In this study, DCE droplets containing either 0.050-0.40 M CH3(CH2)nCOCl (n ) 2 and 3), 0.050-0.50 M CH3(CH2)4COCl, or 0.050-1.0 (19) (a) Slevin, C. J.; Unwin, P. R. Langmuir 1999, 15, 7361. (b) Zhang, J.; Slevin, C. J.; Unwin, P. R. Chem. Commun. 1999, 1501. (c) Zhang, J.; Unwin, P. R. J. Phys. Chem. B 2000, 104, 2341. (d) Zhang, J.; Barker A. L.; Unwin, P. R. J. Electroanal. Chem. 2000, 483, 95. (e) Zhang, J.; Unwin, P. R. Phys. Chem. Chem. Phys. 2000, 2, 1267. (f) Zhang, J.; Slevin, C. J.; Murtoma¨ki, L.; Kontturi, K.; Williams, D. E.; Unwin, P. R. Langmuir 2001, 17, 821. (g) Zhang, J.; Unwin, P. R. Anal. Sci. 2001, 17, i223. (h) Slevin, C. J.; Zhang, J.; Unwin, P. R. J. Phys. Chem. B 2002, 106, 3019. (i) Zhang, J.; Unwin, P. R. Phys. Chem. Chem. Phys. 2002, 4, 3820. (j) Zhang, J.; Chapman, D.; Slevin, C. J.; Unwin, P. R. J. Electroanal. Chem. 2002, 538-539, 277. (20) Handbook of Chemistry and Physics, 80th ed.; CRC Press: Boca Raton, FL, 1999.

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Figure 1. Schematic of the application of MEMED to the measurement of acid chloride hydrolysis kinetics. M CH3(CH2)8COCl were grown periodically from a tapered glass capillary with an internal diameter of ca. 200 µm (Figure 1). An aqueous buffer solution with pH in the range 4-11 served as the receptor phase. The Cl- concentration profile adjacent to the expanding droplet in the aqueous phase, due to the hydrolysis process (eq 1), was determined using potentiometric detection. Electrodes were fabricated by sealing a 25 µm diameter silver wire in glass, exposing a disk-shaped electrode, and coating anodically with a AgCl film in a 0.1 M KCl (Analytical Reagent, Fisher Scientific) solution. The electrode was calibrated (Nernstian response) over the range of concentrations measured.18 In MEMED experiments, the potential of the tip electrode versus a saturated calomel electrode (SCE) was recorded as a function of time, as the droplet grew toward the electrode. Since the droplet expanded spherically in a well-defined manner (as confirmed by optical microscopy17), it was possible to relate the time-dependent potentiometric electrode response to a corresponding distance response. The time at which the advancing droplet surface made contact with the electrode resulted in a dramatic rise in the level of Cl- detected, presumably because a thin layer of aqueous solution wets the probe which results in the trapping and accumulation of Cl- adjacent to the electrode. This allowed the zero distance between the electrode and probe to be defined for the concentration profiles that were constructed from the potentiometric data, as described elsewhere.18 Pinned top and bottom by the capillary and the electrode, the DCE solution continues to flow beyond this point, resulting in the increasing distortion of the droplet shape and its eventual detachment from the capillary and electrode (the droplet falls to the bottom of the cell). This is again marked by a sudden change in the electrode response (back to bulk solution levels) and corresponds to the birth of a new droplet from the capillary. Data acquired during the period when the droplet is pinned and distorted are not used. With this procedure, the uncertainty in the measurement of Cl- fluxes is better than (5%. A relatively high DCE flow rate of 800 µL/h was used for all measurements, so that the interface was quickly refreshed. A high-precision syringe pump was used to control the flow of DCE solution, as described previously.19 The distance between the end of the capillary and the surface of the electrode, which determines the final diameter of the droplets, was 1.34 mm, corresponding to a drop time of 5.7 s. All measurements were made at ambient temperature in an airconditioned room (23 ( 0.5 °C).

Results and Discussion Kinetic Model and Coupled Mass Transport. With a large concentration of ROCl in the DCE phase, which acted as a continuous source, the hydrolysis reaction could

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be controlled by either interfacial transfer of ROCl (across the DCE/water boundary), interfacial hydrolysis of adsorbed ROCl, or a homogeneous reaction in the aqueous phase (after the nonlimiting transfer of ROCl). Based on the experimental results that follow, a model for ROCl hydrolysis is considered in which interfacial hydrolysis is rate limiting, preceded by rapid adsorption of ROCl at the interface (so that this process is at equilibrium). The hydrolysis product (acid) may either transfer to the DCE phase, dissolve in the aqueous phase, or remain adsorbed at the interface. Based on this model, we may write the following steps for the process:

Step 1:

(ROCl)o / (ROCl)i

fast

Step 2: (ROCl)i + H2O (or OH-) f ROOH + HClw (or Cl-) slow where the subscripts, o, i, and w denote the organic phase, the interface, and the aqueous phase, respectively. With this mechanism, the rate law for the hydrolysis reaction is simply

j ) k′[(ROCl)i] ) kθΓmax

(2)

where j is the interfacial Cl- flux (mol cm-2 s-1) into the aqueous phase, k′ (cm s-1) is a first-order heterogeneous rate constant, θ is the fractional coverage of the liquid/ liquid interface by ROCl, Γmax (mol cm-2) is the maximum surface excess concentration, and k (s-1) is the frequency of the hydrolysis process. Assuming a Langmuir isotherm for the adsorption of ROCl, the hydrolysis rate law can be written as

K[(ROCl)o]

j ) kΓmax 1 + K[(ROCl)o]

Figure 2. Dependence of the CH3(CH2)2COCl hydrolysis flux on pH in the aqueous phase. The DCE phase contained 0.100 M CH3(CH2)2COCl. (a) Chloride concentration profiles, shown as solid experimental curves, are (from bottom to top) for pH ) 4.0 to 11.0 in steps of one pH unit. The corresponding dashed theoretical curves (bottom to top) are for j/10-8 mol cm-2 s-1 ) 1.5, 1.6, 1.7, 1.8, 2.0, 2.2, 2.5, and 3.0. (b) j-pH plot of the data in part a.

the drop radius at any time, t, less than the total drop time, td. The latter relates to the period that elapses from the point where the drop starts to grow to the point where the drop contacts the electrode.

r0 ) (3)

3q (4π )

1/3

t1/3

(6)

where c is the concentration of Cl phase, D is the diffusion coefficient of Cl- (1.8 × 10-5 cm2 s-1 under the conditions of the present experiments21), and r is the coordinate defining the spherical geometry starting at the center of the droplet. The parameter r0 is

where q is the volume flow rate. The mass transport problem to be solved relates to the domain r > r0, since depletion and hence diffusional effects within the droplet are negligible under the experimental conditions. The convective-diffusion equation for mass transport close to the interface of the expanding droplet can be solved numerically using the simple explicit method,22 as described fully in our previous studies.19a,f This model treats mass transport to a symmetrically expanding sphere. The modeling produced chloride concentration versus radial distance profiles, as a function of time, from which the time-dependent concentration versus distance profile, observed at the probe electrode, could be evaluated. Dependence of Hydrolysis Rates on Aqueous pH. Experiments were carried out with 0.10 M CH3(CH2)nCOCl (n ) 2, 3, 4, and 8) in the DCE phase, with an aqueous buffer solution, pH 4-11, as the receptor phase. The tip potential was recorded as a function of distance between the tip and the advancing drop surface, as described above, and converted to an effective chloride concentration profile using a potentiometric calibration curve. Note that this profile is both space- and time-dependent but is presented as the former.17-19 Typical results for each of the ROCl compounds of interest are shown in Figures 2-5. In each of these figures, part a shows typical Cl- profiles recorded at the pH values defined, fitted to simulation to obtain the optimum flux, j, at each pH. The corresponding fluxes

(21) Turq, P.; Lantela, F.; Chemla, M. Electrochim. Acta 1969, 14, 1081.

(22) Britz, D. Digital Simulation in Electrochemistry, 2nd ed.; Springer-Verlag: New York, 1988.

where K is the adsorption constant of ROCl. Equation 3 forms the boundary condition at the expanding droplet surface. Under the experimental conditions described herein, [(ROCl)o] is in sufficient excess to be maintained at bulk concentration values throughout the space region of interest, even in the part of the organic phase adjacent to the interface. For a given [(ROCl)o], it is therefore appropriate to describe the interfacial flux of Cl- from the droplet into the aqueous phase, due to the hydrolysis reaction, in terms of a pseudo-zero-order process and determine how the flux, j, depends on [(ROCl)o] and other factors. We show later in this paper how the measurement of j at a range of [(ROCl)o] can be used to estimate k and K, in particular. In the spherically symmetric geometry of MEMED, the following boundary conditions thus apply:

r ) r0:

∂c -D )j ∂r

r f ∞:

c)0

(4) (5) - in the aqueous receptor

Acid Chloride Hydrolysis in an Oil/Water System

Figure 3. Dependence of the CH3(CH2)3COCl hydrolysis flux on pH in the aqueous phase. The DCE phase contained 0.100 M CH3(CH2)3COCl. (a) Chloride concentration profiles. From bottom to top, the solid experimental curves are for pH ) 4; 5 and 11 (two lines coincident); 6, 7, and 10 (three lines coincident); 8 and 9 (two lines coincident). The corresponding dashed theoretical curves (bottom to top) are for j/10-8 mol cm-2 s-1 ) 1.0, 1.1, 1.2, and 1.3. (b) j-pH plot of the data in part a.

Figure 4. Dependence of the CH3(CH2)4COCl hydrolysis flux on pH in the aqueous phase. The DCE phase contained 0.100 M CH3(CH2)4COCl. (a) Chloride concentration profiles. From bottom to top, the solid experimental curves are for pH ) 4; 5 and 11 (two lines coincident); 6; 7 and 10 (two lines coincident); 8 and 9 (two lines coincident). The corresponding dashed theoretical curves (bottom to top) are for j/10-9 mol cm-2 s-1 ) 6.0, 7.0, 8.0, 9.0, and 10. (b) j-pH plot of the data in part a.

are plotted against pH in part b of each of Figures 2-5, which clearly identifies the pH dependence of the reaction rate for the four compounds of interest. These results demonstrate, in each case, that the flux of chloride due to hydrolysis of ROCl increases only marginally in the range pH ) 4-9. At pH 10 and 11, there is an enhanced Cl- flux for CH3(CH2)2COCl and CH3(CH2)8COCl but a slight reduction in the hydrolysis rate for CH3(CH2)3COCl and CH3(CH2)4COCl. Acid-base catalytic effects23,24 which are common for homogeneous hydrolysis reactions do not

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Figure 5. Dependence of the CH3(CH2)8COCl hydrolysis flux on pH in the aqueous phase. The DCE phase contained 0.100 M CH3(CH2)8COCl. (a) Chloride concentration profiles. From bottom to top, the solid experimental curves are for pH ) 4 and 5 (two lines coincident), 6, 7, 8, 9, 10, and 11. The corresponding dashed theoretical curves (bottom to top) are for j/10-9 mol cm-2 s-1 ) 0.9 (effectively coincident with the experimental curves), 1.0, 1.2, 1.5, 1.8, 2.2, and 3.0. (b) j-pH plot of the data in part a.

appear significant over the pH range considered, although the increase in rate with pH in Figures 2 and 5, for CH3(CH2)2COCl and CH3(CH2)8COCl, respectively, suggests some effect of hydroxide ion concentration on the rate of reaction. Examining the data at neutral pH in each of Figures 2-5, it appears that there is a systematic decrease in the rate of reaction of CH3(CH2)nCOCl, as n increases from 2 to 4, with j ) 18.0 nmol cm-2 s-1 (n ) 2), 11.5 nmol cm-2 s-1 (n ) 3), and 9.0 nmol cm-2 s-1 (n ) 3). For n ) 8, the flux decreases significantly to j ) 1.3 nmol cm-2 s-1. Reasons for this trend in reaction rates will be considered later. The decrease in the flux of chloride at the higher pH values for CH3(CH2)3COCl and CH3(CH2)4COCl may be due to slight adsorption of the ionized acid product at the interface. The pKa value of short-chain aliphatic carboxylic acids is ca. 5,25 so for a pH higher than 9, the hydrolysis products will effectively be in the ionized form, CH3(CH2)nCOO-, in aqueous solution. Such effects appear to be important for CH3(CH2)3COO- and CH3(CH2)4COO-. In contrast, CH3(CH2)2COO- will have a higher water solubility and so the interfacial activity would be negligible. One might expect significant adsorption effects for CH3(CH2)8COO-, but the concentration of this species will be much lower, due to the slower reaction rate for CH3(CH2)2COOCl. To prove these arguments, further experiments were carried out in buffer solutions with different pHs (4, 7, and 11) to examine the extent of inhibition of the hydrolysis reaction byproduct adsorption. Blocking effects on the hydrolysis reactions of CH3(CH2)3COCl and CH3(CH2)4COCl were detected when 5 mM of the corresponding acid was added to the aqueous phase, buffered to either pH 7 or 11. However, no effect of these products was observed (23) Connors, K. A. Chemical Kinetics; VCH: New York, 1990. (24) Price, N. C.; Dwek, R. A. Principles and Problems in Physical Chemistry for Biochemists, 2nd ed.; Clarendon Press: Oxford, 1979. (25) Zhang, J. PhD Thesis, University of Warwick, UK, 2001.

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Figure 6. Effect of CH3(CH2)3COOH on CH3(CH2)3COCl hydrolysis. [(CH3(CH2)3COCl)o] ) 0.100 M, [(CH3(CH2)3COOH)w] ) 0 or 5.00 mM. (a) Chloride concentration profiles recorded at pH ) 4. The coincident solid experimental lines are for the system with and without CH3(CH2)3COOH in the aqueous phase. The corresponding dashed line is for j ) 1.0 × 10-8 mol cm-2 s-1. (b) Chloride concentration profiles recorded at pH ) 7. The solid experimental lines are for the system with (bottom) and without (top) CH3(CH2)3COOH in the aqueous phase. The corresponding dashed lines are for j ) 1.1 × 10-8 mol cm-2 s-1 (bottom) and 1.2 × 10-8 mol cm-2 s-1 (top). (c) Chloride concentration profiles recorded at pH ) 11. The solid experimental lines are for the system with (bottom) and without (top) CH3(CH2)3COOH in the aqueous phase. The corresponding dashed lines are for j ) 7.0 × 10-9 mol cm-2 s-1 (bottom) and 1.1 × 10-8 mol cm-2 s-1 (top).

at pH ) 4. The results of these experiments are summarized in Figures 6 and 7. The data in these figures clearly show that at pH 11, the chloride flux into the aqueous phase may be decreased by almost 50% in the presence of 5 mM CH3(CH2)3COOH or CH3(CH2)4COOH. Dependence of Hydrolysis Rates on [(CH3(CH2)nCOCl)o]. These experiments were carried out with aqueous receptor phase buffer solution at different pHs (4, 7, or 11). The concentration of (CH3(CH2)nCOCl)o was varied over the range 0.05-0.40 M for CH3(CH2)2COCl and CH3(CH2)3COCl, 0.05-0.50 M for CH3(CH2)4COCl, and 0.05-1.0 M for CH3(CH2)8COCl. Kinetic data obtained from many experiments are summarized in Figure 8. This figure plots the measured chloride fluxes as a function of bulk ROCl concentration, c*. The lines through the data points are simply a guide to the eye in these plots. At pH 4, where complications from product adsorption, identified above, are negligible, it can be seen that for a given value of c* there is a slight concomitant decrease in j for the hydrolysis of CH3(CH2)nCOCl when n increases from 2 to

Zhang et al.

Figure 7. Effect of CH3(CH2)4COOH on CH3(CH2)4COCl hydrolysis. [(CH3(CH2)4COCl)o] ) 0.100 M, [(CH3(CH2)4COOH)w] ) 0 or 5.00 mM. (a) Chloride concentration profiles recorded at pH ) 4. The coincident solid experimental lines are for the system with or without CH3(CH2)4COOH in the aqueous phase. The corresponding dashed line is for j ) 6 × 10-9 mol cm-2 s-1. (b) Chloride concentration profiles recorded at pH ) 7. Solid experimental lines are for the system with (bottom) or without (top) CH3(CH2)4COOH in the aqueous phase. The corresponding dashed lines are for j ) 8 × 10-9 mol cm-2 s-1 (bottom) and 9 × 10-9 mol cm-2 s-1 (top). (c) Chloride concentration profiles recorded at pH ) 11. Solid experimental lines are for the system with (bottom) or without (top) CH3(CH2)4COOH in the aqueous phase. The corresponding dashed lines are for j ) 4 × 10-9 mol cm-2 s-1 (bottom) and 7 × 10-9 mol cm-2 s-1 (top).

4 (Figure 8a-c), with a very large drop in reaction rate for n ) 8 (Figure 8d). This trend is broadly seen at the other pH values, but it is important to note that there are some complications from product adsorption, particularly at pH 11, as already discussed. On the other hand, the notable effect of hydroxide ion in promoting hydrolysis for CH3(CH2)2COCl (Figure 8a) and CH3(CH2)8COCl (Figure 8d) is also clear. A key feature of the results in Figure 8 at all pH values and for all compounds studied is that j has a dependence on [(ROCl)o], but the order is less than unity and decreases with increasing [(ROCl)o]. This behavior is typical of a surface process where the interface tends toward saturation by adsorption of the reactant, as the bulk concentration increases. Analysis of the results in Figure 8 in terms of the simple Langmuir model (eq 3) was found to provide a good description of the data. Equation 3 can be written as

1 1 1 1 ) + j kKΓmax c* kΓmax

( )

(7)

Acid Chloride Hydrolysis in an Oil/Water System

Figure 8. Plots of j vs c* for the hydrolysis of (a) CH3(CH2)2COCl, (b) CH3(CH2)3COCl, (c) CH3(CH2)4COCl, and (d) CH3(CH2)8COCl under different conditions: pH 4 (0), pH 7 (O), and pH 11 (4). The solid lines through the data are a guide to the eye.

The corresponding analysis, in terms of plots of j-1 versus [(ROCl)o]-1, is shown in Figure 9. In principle, the intercept of the plots in Figure 9 provides a value of (kΓmax)-1 while the slope yields (kKΓmax)-1. However, there is a sizable uncertainty in the value of kΓmax from this analysis, since the intercept, which is close to zero, provides a reciprocal value. Nonetheless, it is striking that all of the plots tend to a similar value of j-1 in the limit c* f 0. On the basis of the dimensions of RCOCl and with knowledge that adsorption at the liquid/ liquid interface will occur with the polar acid chloride group oriented toward the aqueous phase, we can estimate Γmax ≈ 5 × 10-10 mol cm-2. It is then found from the intercepts on the j-1 axis that k g 100 s-1. Specifically, for n ) 2, k ) 300 ( 150 s-1; for n ) 3 and 4, k ) 200 ( 100 s-1; and for n ) 8, k ) 120 ( 60 s-1. At first sight, it appears that the smaller the value of n, the larger the rate constant, but the necessity to estimate Γmax and assume that it is similar for all compounds introduces a

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Figure 9. Plots of j-1 vs c*-1 for the data from Figure 8: pH 4 (0), pH 7 (O), and pH 11 (4).

considerable uncertainty, which makes an intercomparison of the results difficult. On the other hand, we can conclude that the order of reactivity, defined as j, decreases from n ) 2 to 8. Most significantly, the magnitude of the rate constant, k, for the interfacial hydrolysis process is of the same order as that measured for the homogeneous reaction.10 This can be rationalized because, as highlighted above, ROCl is likely to adsorb at the liquid/liquid interface with the polar acid chloride functionality exposed to the aqueous phase. The slopes of the plots provide information on K once kΓmax is known. As competition from product adsorption has been identified as a complication at high pH for two of the acid chlorides, it is most useful to compare the different acid chlorides at pH 4, where such problems are minimal. Moreover, base hydrolysis effects are also negligible under these conditions. The slopes of the plots in Figure 9 indicate that the adsorption constants are similar for n ) 2, 3, and 4 (K ) 1 ( 0.5 M-1), with a lower value for n ) 8 (K ) 0.2 ( 0.1 M-1). Although the adsorption constants are broadly of the same order, the lower value

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for n ) 8 most likely reflects the higher affinity of this species for the DCE phase compared to the other compounds. Location of the Hydrolysis Reaction. Further information on the hydrolysis process can be obtained by calculating the flux that would be expected for the alternative process involving transfer of ROCl across the DCE/aqueous interface followed by rapid (first-order) hydrolysis in the aqueous phase. The maximum rate of this process, jhom, results when there is no resistance to the interfacial transfer:

jhom )

xDRk[(ROCl)o] P

(8)

where P is the partition coefficient, [(ROCl)o]/[(ROCl)w]. P values of ca. 106, 103.5, 103, and 102.5 can be estimated for decanoyl chloride (n ) 8), hexanoyl chloride (n ) 4), valeryl chloride (n ) 3), and butyryl chloride (n ) 2), respectively.26 DR is the diffusion coefficient for the reactant, ROCl, in the aqueous phase, which may be calculated from27

DR )

13.3 × 10-9 η1.14VL0.589

(9)

where η (g cm-1 s-1) is the viscosity of the solvent and VL (cm3 mol-1) is the LeBas molar volume. Diffusion coefficients of (2.6, 2.4, 2.2, and 1.7) × 10-6 cm2 s-1 were obtained for CH3(CH2)nCOCl (n ) 2, 3, 4, and 8, respectively). Using k ) 200 s-1, as found for homogeneous hydrolysis,10 we estimate jhom for the example case of [(ROCl)o] ) 0.1 M as 7.2 × 10-9 mol cm-2 s-1 (n ) 2), 2.2 × 10-9 mol cm-2 s-1 (n ) 3), 6.6 × 10-10 mol cm-2 s-1 (n ) 4), and 1.8 × 10-12 mol cm-2 s-1 (n ) 8). These values (which represent the maximum possible homogeneous rates) are much smaller than those measured experi(26) Hansch, C.; Quinlan, J. E.; Lawrence, G. L. J. Org. Chem. 1968, 33, 347. (27) Hayduk, W.; Laudie, H. AIChE J. 1974, 20, 611.

mentally (particularly at larger n). Even for n ) 2, the measured rates are more than twice the maximum possible homogeneous rate, so that the contribution of any homogeneous reaction to the measured flux will be minor. The results of this analysis, together with the observation of saturation effects on the flux at high [(ROCl)o] (Figure 8), indicate clearly that the hydrolysis reactions occur interfacially. Conclusions ROCl hydrolysis kinetics has been studied over a wide range of conditions using MEMED. The rate of hydrolysis increased slightly with an increase in pH in the range 4-9. For decanoyl or butyryl chloride hydrolysis, the rate increased more significantly at pH 10 and 11, whereas the hexanoyl or valeryl chloride hydrolysis reactions showed a reduction in reaction rate, due to product adsorption. A kinetic model involving rapid adsorption of (ROCl)o at the liquid/liquid interface followed by rate-limiting hydrolysis accounted for the observed rates. Under conditions where there were no complications from product adsorption, the reaction flux of chloride decreased slightly with increasing size of the acid chloride, from butyryl chloride to hexanoyl chloride. There was a significant decrease in the reaction flux for decanoyl chloride. On the other hand, when the surface coverage of adsorbed ROCl was taken into account, the hydrolysis rate constant was of the same order for all acid chlorides studied and of a similar magnitude to that measured homogeneously.10 Acknowledgment. J.Z. gratefully acknowledges scholarships from the ORS scheme, Avecia, and the University of Warwick. We thank Professor T. W. Bentley (University of Wales, Swansea) for helpful discussions. LA0355951