Ind. Eng. Chem. Res. 1998, 37, 4625-4630
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Kinetic Studies on the Esterification of a Substituted Phenylacetic Acid by Phase-Transfer Catalysis Ruey-Shin Juang* and Shing-Chaur Liu Department of Chemical Engineering, Yuan Ze University, Chung-Li, Taiwan 320, Republic of China
The reaction rates for the esterification of a substituted phenylacetic acid by phase-transfer catalysis were measured using a constant interfacial area cell. Experiments were performed at different concentrations of aqueous-phase reactant, phase-transfer catalyst, NaOH, and organicphase reactant. It was shown that the stirring significantly affected the reaction rates. An attempt was made to express the reaction rate as a function of the concentrations of reactive species. The effect of temperature on the reaction was also studied, and the activation energies were evaluated. A possible mechanism was proposed and discussed based on a mixed Makosza and modified interfacial mechanisms. Introduction Phase-transfer catalysis is a well-established technique on the synthesis of organic chemicals in the past 3 decades. Its scope and mechanistic features have been the subjects of numerous studies and appear to be recognized and understood. The addition of a small amount of phase-transfer catalysts (PTCs) makes extremely slow reactions between the components existing in two immiscible phases fast enough. In addition, the high product yield and selectivity are usually obtained.1-3 The kinetics and mechanism in liquid-liquid phasetransfer catalysis under vigorous agitation conditions have been recently reviewed.3 It was shown that under restricted concentration ranges the kinetics of such reaction systems can be followed by a pseudo-first-order model. A common feature involved in these previous studies was that the resistance of mass-transfer steps is negligibly small compared to chemical reactions themselves, whether it was assumed or experimentally verified.4 In fact, many widely used PTCs including quaternary ammonium and phosphonium salts and their reaction intermediates show surface activity near the interface between the two media.5-7 Several phasetransfer-catalyzed reactions such as alkylations and carbene formation were considered to occur by the “interfacial” mechanism.8 Furthermore, Lele et al.9 indicated that the volumetric rate of extraction depends on the interfacial area for two-phase hydrolysis of the esters. Regardless of the zones where chemical reactions occur (near the interface, in the bulk phase, or both), it remains always unclear what role of “interface” to use for a particular phase-transfer-catalyzed process.5,10 The aim of this work was to study the effect of species concentrations on reaction rates for esterification of (4methoxyphenyl)acetic acid (RH, the aqueous-phase reactant) by phase-transfer catalysis using a Lewis-type cell. Because the contact areas of the two immiscible phases was known, the reaction rate per unit area could be calculated. The organic-phase reactant was 1-bromobutane. Tetrabutylammonium bromide (QBr) and toluene were used as the PTC and organic solvent, * To whom correspondence should be addressed. E-mail:
[email protected].
respectively. All experiments were performed as a function of the concentrations of aqueous-phase reactant, PTC, NaOH, and organic-phase reactant. An attempt was made to obtain an empirical rate equation of the reactions, allowing us to give a deep insight into the reaction scheme for the present catalyzed synthesis process. Experimental Section Reagents and Solutions. The PTC QBr was offered from Merck Co. It had a purity greater than 99% and was used without further purification. The solvent toluene was supplied by Mallinckrodt Chemical Co. as HPLC grade and used after double distillation. The aqueous-phase reactant RH had a purity above 99% (Aldrich Co.). Sodium hydroxide (Merck Co.) was obtained as guaranteed reagent grade. The organicphase reactant 1-bromobutane (R′X) had a purity greater than 98% (Merck Co.). The aqueous phase was prepared by dissolving different amounts of RH, QBr, and NaOH in deionized water (Millipore, Milli-Q). The initial concentrations of RH, QBr, and NaOH were in the ranges of 0.1-0.5, 0.012-0.124, and 0.75-2.5 M, respectively. The organic phase was prepared by diluting R′X in toluene, and the initial concentration of R′X ranged from 0.1 to 1.0 M. Apparatus and Experimental Procedure. Kinetic studies were carried out in this work using a stirred Lewis cell, as shown in Figure 1. It was made of Pyrex glass and had a contact area of about 43.2 cm2. The two stirrer blades were symmetrically located with respect to the organic-aqueous interface, and the stirrers were driven in opposite directions but at the same rate in the range of 20-120 rpm using a ColeParmer Servodyne motor. The time profile of concentration of the product in the organic phase was measured by the following procedures. An aqueous phase (255 cm3) was first placed in the lower cell, and an equal volume of the organic phase was carefully poured into the “upper” cell in order to minimize any disturbance at the interface. The timing was started upon the addition of the organic phase. Samples (5 cm3) were taken from the organic phase at certain time intervals, and the concentration of the product, 4-methoxyphenylacetic acid butyl ester, was
10.1021/ie980226z CCC: $15.00 © 1998 American Chemical Society Published on Web 10/30/1998
4626 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998
Figure 3. Variation of the organic-phase concentration of the product with contact time.
Results and Discussion Figure 1. Stirred cell used in this work: 1, organic-aqueous interface; 2, sampling and feeding ports; 3, water jacket; 4, twopaddle stirrer; 5, PTFE gasket (dimensions is given in millimeters).
Determination of Initial Reaction Rates. Figure 3 shows typical time profiles of amounts of the product (R′R) in the organic phase at a stirring speed of 70 rpm. If Figure 3 is plotted in Cartesian coordinates (not shown), all data points well lie on straight lines passing through the origin. This is likely because the concentration of the product in the organic phase is far less than those of both reactants (
