Solubilization of Benzyl Chloride and Rate Enhancement of Its

Alkaline hydrolysis of benzyl chloride (BzCl) in aqueous solutions of sodium cumenesulfonate (NaCS) is investigated for the hydrotropic effect. The pr...
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Ind. Eng. Chem. Res. 2005, 44, 434-441

Solubilization of Benzyl Chloride and Rate Enhancement of Its Hydrolysis Reaction in Aqueous Sodium Cumenesulfonate Solutions Daliya S. Mathew and V. G. Gaikar* Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai 400 019, India

Alkaline hydrolysis of benzyl chloride (BzCl) in aqueous solutions of sodium cumenesulfonate (NaCS) is investigated for the hydrotropic effect. The presence of the hydrotrope, NaCS, increased the solubility of BzCl in aqueous solutions by an order of magnitude and consequently the rates of the reaction by 6-fold as compared to those in water. In addition, the hydrotropic medium increased the selectivity toward the formation of dibenzyl ether. Both of these effects have been attributed to the increased solubilization of BzCl, the microenvironment experienced by the reactants in the hydrotrope aggregates and proximity of the reactants for the subsequent condensation reaction. Introduction In recent years, water as a reaction medium has been gaining increasing attention because of its easy availability, low cost, safe operation, and in some cases higher rates.1-4 However, the major obstacle in developing water-based reactions is the poor solubility of many organic compounds in aqueous solutions. If the solubility of organic reactants can be increased in aqueous solutions, then higher reaction rates are expected. Hydrotropy refers to the enhanced solubilization of sparingly water-soluble or water-insoluble organic compounds in aqueous solutions of hydrotropes.5,6 The solubility of an organic compound increases almost exponentially with the hydrotrope concentration beyond a characteristic concentration called minimum hydrotrope concentration (MHC) but generally approaches a constant value at very high hydrotrope concentration. The major advantage of the hydrotrope solutions for solubilizing an organic solute is the ease with which the solute can be recovered. A simple dilution with water, bringing the hydrotrope concentration below its MHC, is sufficient to separate out the dissolved solute as a separate liquid-solid phase. The hydrotrope, being itself highly water-soluble, does not contaminate such a precipitated product. The use of hydrotropes in alkaline hydrolysis of esters,7 oximation of cyclododecane,8 and Cannizzaro’s cross-reaction of benzaldehyde9 reportedly increased the reaction rates by 2-3 orders of magnitude. In the synthesis of chalcones10 and dihydropyridines,11 the organic reactants were completely solubilized in the hydrotrope solutions, and with the progress of the reaction, the product precipitated out in each case, providing easy recovery. In all of these reactions, the hydrotropic effect was attributed to the increased solubility of the reactants, and no attention was given either to the mechanism by which the hydrotrope functions or to the modification of the reaction kinetics in the presence of a hydrotrope at such high concentrations. The hydrotrope selected for the present investigations, i.e., sodium cumenesulfonate (NaCS), is expected to form a stacklike, but somewhat open, aggregate * To whom correspondence should be addressed. Fax: 91-022-24145614. E-mail: [email protected].

structure.12 Intercalation of an organic molecule between two hydrotrope molecules in an aggregate seems to be an appropriate mechanism of the hydrotropic solubilization. In this paper, we report for the first time the hydrolytic reaction of benzyl chloride (BzCl) to benzyl alcohol (BzOH) in a hydrotropic medium and their subsequent condensation to form dibenzyl ether (DBE). The reaction is considered not only for the enhancement in the reaction rates but also for the modulation of the selectivity by the catalytic or inhibitory effect of the hydrotrope. The relevant reactions are shown in Scheme 1. Under alkaline conditions, the reaction has been reported to yield mainly BzOH.13,14 The use of a phase-transfer catalyst (PTC), however, has been claimed to give complete selectivity toward DBE.15 We have also attempted to delineate the solubilization effect of hydrotrope from activation/inhibition of the reaction by estimating the reaction rate constants in the interfacial region of the hydrotrope assemblies with the aqueous phase and in the bulk aqueous phase. The lipophilic reactant is expected to embed itself within the hydrotrope aggregates, but it is not completely shielded from the aqueous environment because there is no significant hydrocarbonaeous region within the hydrotrope aggregates. Therefore, the reactants should still be accessible to the ionic reactant in the surrounding aqueous phase. The kinetic information is further used under two-phase conditions to predict the rates of the reactions and selectivity toward DBE. Experimental Section Materials. NaCS was obtained from Navdeep Chemicals, Mumbai, India, and was used as received. BzCl and BzOH were obtained from S.D. Fine Chemicals, Mumbai, India, with purities of 99% and 98%, respectively, as verified by gas-liquid chromatography (GLC). The GLC analysis was conducted with a 2.0-m-long, 0.3-cm-i.d., 30% SE-30 column. Experimental Procedures (a) Solubility Measurements. The solubility of BzCl was measured at different temperatures and for

10.1021/ie030601+ CCC: $30.25 © 2005 American Chemical Society Published on Web 01/05/2005

Ind. Eng. Chem. Res., Vol. 44, No. 3, 2005 435 Scheme 1

a range of hydrotrope concentrations. For each solubility run, an excess amount of BzCl (∼10 cm3) was stirred, in a fully baffled glass vessel, into the hydrotrope solution (100 cm3) of known concentration in a thermostated bath for 4 h to ensure equilibrium. The solubilization of BzCl in plain water and hydrotrope solutions at neutral pH did not show any significant hydrolysis. Only 0.3-0.5% of the solubilized BzCl hydrolyzed to BzOH in the hydrotrope solutions at neutral pH. After the equilibration, the dispersion was allowed to settle into two phases. A known volume of the aqueous phase, saturated with BzCl, was extracted thrice with toluene to transfer BzCl and BzOH, if any, to the toluene phase. For the experiments involving higher concentrations of hydrotrope (>1.0 mol‚dm-3), the complete extraction of BzCl and BzOH to the toluene phase was confirmed by dilution of the aqueous hydrotropic phase below the MHC of the hydrotrope (∼0.15 mol‚dm-3), followed by extraction with toluene twice. At higher hydrotrope concentrations, this was necessary because both BzCl and BzOH showed a greater affinity toward the hydrotrope solutions. The dilution step was not necessary for the BzCl recovery at lower hydrotrope concentrations. The toluene phase was analyzed by GLC. The solubility of BzCl was also measured separately for a range of hydrotrope concentrations in the presence of BzOH. (b) Hydrolytic Reaction under Homogeneous Conditions. An excess of BzCl was added to a 200-cm3 hydrotrope solution of known concentration at neutral pH. The mixture was then stirred in a 500-cm3 fully baffled glass vessel in a constant-temperature water bath for 4 h to achieve equilibrium. The BzCl-saturated aqueous hydrotrope solution, after separation from the excess of BzCl, was charged into a 500-cm3 fully baffled cylindrical glass reactor with a six-bladed pitched impeller, followed by continual addition of a sufficient amount of 3.0 mol‚dm-3 aqueous sodium hydroxide to maintain the pH between 12.5 and 12.8. Intermittent samples were withdrawn from the reaction mixture for GLC analysis. The reaction was repeated at three different temperatures. The consumption of NaOH during the reaction period was monitored by titration of the mixture with a 0.01 mol‚dm-3 saturated HCl solution at regular intervals of time, which gives the net utilization of BzCl. GLC analysis gives the amounts of BzOH and DBE formed over the reaction period. These two analytical methods gave closely matched results with less than 3% error. After a predetermined reaction time, the aqueous phase was diluted with water, below the MHC of the hydrotrope, to form a separate organic phase containing the unreacted BzCl and reaction products, i.e., BzOH and DBE. (c) Hydrolytic Reaction under Heterogeneous Conditions. An aqueous solution of the hydrotrope, an excess amount of BzCl, and a sufficient amount of sodium hydroxide, to keep the reaction mixture pH between 12.5 and 12.8, were charged into a 500-cm3

Figure 1. Solubility of BzCl in aqueous NaCS solutions (inset: at concentrations below MHC of NaCS, BzCl is salted out of hydrotrope solutions): ], 333 K; 4, 343 K; 0, 353 K; O, 363 K.

fully baffled reactor with a six-bladed pitched impeller, in that sequence. The reaction was carried out at three different temperatures in a constant-temperature water bath. The progress of the reaction was monitored by titration for alkali consumption and GLC as described above. The experiments when repeated twice gave 95+% reproducibility. Separate experimental studies of the hydrolysis of BzCl in water at three different temperatures were also carried out in a similar manner. Results and Discussion Figure 1 shows the solubility of BzCl in aqueous solutions of NaCS at different temperatures. The solubility of BzCl increased significantly only beyond ∼0.15 mol‚dm-3 NaCS, which is the MHC of the hydrotrope, and increased up to 180 mmol‚dm-3 in 2.5 mol‚dm-3 aqueous NaCS solutions from as low as 5 mmol‚dm-3 in water at 60 °C. Usually below the MHC, the solubility of an organic compound is taken to be the same as that in water, but detailed solubility measurements showed rather a decrease in the solubility with increasing hydrotrope concentration up to the MHC of NaCS. The strong influence of ionic headgroups of the hydrotrope can salt-out BzCl at concentrations below the MHC, before the hydrotropic solubilization sets in through the aggregation.16 Although no detailed information about the hydrotrope aggregates is available, particularly for the shape and size, the coaggregation of the aromatic solutes, such as BzCl, with the aromatic hydrotrope seems to be responsible for its solubilization. A recently proposed association model of hydrotropic solubilization16 characterizes the hydrotrope-hydrotrope and solute-hydrotrope interactions through a step aggregation and coaggregation. The model relates hydrotrope dimerization constant (K2) to monomer concentration ([H1]) and total concentration Cs under the assumption that self-aggregation of the hydrotrope

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Table 1. Solute-Hydrotrope and Hydrotrope-Hydrotrope Interaction Parameters of the Association Model K2 (mol‚dm-3)

temp (K)

Ks (mol‚dm-3)

Hydrotrope: NaCS 0.10 0.12 0.15 0.19

333 343 353 363

11.01 7.01 2.88 1.54

molecules becomes weaker with an increase in the aggregation number (eq 1).16

Cs ) [H1]{2 exp(K2[H1]) - 1}

(1)

The model also relates the increase in the solubility (∆S) to the solute solubility in water (S1) through the hydrotrope-solute interaction constant (Ks) (eq 2).16

()

∆St ) 2

Ks S {exp(K2[H1]) - (1 + K2[H1])} K2 t

(2)

The dimerization constant (K2) of the hydrotrope and the interaction constant (Ks) between BzCl and the hydrotrope aggregate were estimated by fitting the solubility data in eqs 1 and 2. The lines in Figure 1 are the best fit lines. S1 was taken the same as the lowest solubility of BzCl observed in the hydrotrope solutions. There was no significant difference in the values of Ks and K2 if the pure water solubility of BzCl was taken for fitting the data. At all temperatures, Ks is much higher than K2 (Table 1).The association model does not consider the change in the aggregation behavior of the hydrotrope in the presence of a solute. As compared with conventional surfactants, NaCS has very poor hydrophobic character because of its three-carbon isopropyl chain and an aromatic ring, which itself can be treated as equivalent to a chain of three to four carbon atoms in length. Still this very weak hydrophobic region is responsible for the aggregation behavior of amphiphilic hydrotrope molecules in the aqueous solutions. The hydrotropic solubilization is a consequence of the hydrotrope’s aggregative behavior, but a solute with strong hydrophobic nature, when dissolved in such a hydrotropic medium, may force the growth of the hydrotrope assemblies. The aggregation of the hydrotrope molecules not only is driven by its own structure but also could be decided by the nature of the solute. The solubility of BzCl increased considerably at high temperatures, but the relative increase in the solubility was higher at lower temperatures considering the ratio of the solubility in the hydrotrope solutions to that in plain water (the relative increase, i.e., S/S1 was 28.30 in 2.5 mol‚dm-3 NaCS solutions at 333 K and 5.83 in 2.5 mol‚dm-3 solutions at 363 K). In the earlier reports, the rate enhancements using hydrotrope were related to the increased solubility in the hydrotropic media. When a hydrotrope is present at very high concentrations in aqueous solutions, the nature of the aqueous solutions can be substantially changed from that of water. Because the solute is also intercalated between two strongly ionic hydrotrope molecules, the effective medium polarity experienced by the solute molecules can be very different from that of either water or salt solutions and even from that of a hydrocarbon solvent17 and thus can have an effect on the reaction rates.

The hydrolysis of BzCl is a nucleophilic substitution reaction that follows the SN2 mechanism, where the rate depends on the concentrations of both haloalkanes and hydroxide ion. The product of the hydrolysis reaction, BzOH, also reacts with BzCl in the presence of sodium hydroxide, which activates the alcohol to form DBE. In an SN2 mechanism, if the substrate is neutral and the nucleophile is charged, dispersal of the charge occurs at the transition state. Hence, decreasing the solvent polarity favors the reaction more. Because our objective was to investigate the effect of a hydrotropic medium on kinetic parameters apart from the solubility of the solute, we decided to consider the reaction(s) in two steps: first, the estimation of intrinsic kinetic factors under homogeneous conditions using a hydrotropic medium saturated with BzCl and, second, the reaction with an excess of BzCl in a heterogeneous two-phase system. The kinetic parameters were used to predict the reaction rates and the selectivity toward DBE. Kinetics of a Homogeneous Reaction in Hydrotropic Media. The reactions are assumed to take place in two phases, the bulk water phase and the hydrotrope aggregates, but to different extents. The volume fraction of the hydrotropic phase (β) was calculated, for each hydrotrope concentration, assuming additivity of volumes of water and the hydrotrope. The effective concentrations of the reacting species were calculated in each phase by considering the corresponding volumes and the increased solubilities. The subscripts w and h indicate the concentrations in the bulk water phase and in the hydrotrope phase, respectively. The effective hydroxide ion concentration ([OH-]h) in the vicinity of the hydrotrope aggregates, which should be lower than the bulk phase concentration ([OH-]w) because of electrostatic repulsion from negatively charged hydrotrope aggregates, was calculated by using eq 3,18 where ψr is

[OH]h ) [OH]w exp(-eψr/κT)

(3)

the electrostatic potential at distance r from the surface of the hydrotrope aggregate because of charge e on it, κ is the Boltzmann constant, and T is the temperature. [OH]h at a distance of 5 Å from the hydrotrope aggregate was estimated by assuming a charge of 10 on the aggregate. Recent small-angle neutron scattering studies on a similar hydrotrope sodium n-butylbenzenesulfonate have indicated an aggregation number of 35-40 with an almost 35% fractional charge.19 We expect NaCS to show similar characteristics because of the similarity in the structures of both of these hydrotropes. The electrostatic potential ψr at this distance was not very sensitive to the charge on the aggregate. We also do not expect a larger aggregation number for the NaCS aggregates. The aggregation number may increase in the presence of BzCl, but the charge per aggregate may decrease because of the compacting effect of such a growth and because of increased counterion association with the aggregate. The charge on the hydrotrope aggregate may otherwise be used as a parameter to fit the kinetic data. The rate constant, k1w, for the hydrolysis of BzCl in water at 353 K was estimated to be 3.2 × 10-3 s-1 as opposed to the reported value20 of 3.4 × 10-3 s-1, which are in close agreement. When the rate constants of the two reactions in water were known (k1w and k2w; Table 2), the rate constants in the hydrotropic pseudophase (k1h and k2h) were estimated from eq 4 using a least-squares method. The first two terms of eq

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Figure 2. Enhancement factor for the formation of BzOH and of DBE in a homogeneous system. The bars indicate variation in the repeated runs, while the lines are the fitted behavior using eq 4. Table 2. Rate Constants (k1w and k2w) for the Hydrolysis of BzCl in Water temp (K)

k1w (mol‚dm-3‚s-1)

k2w (mol‚dm-3‚s-1)

333 343 353

5.5 × 10-4 9.1 × 10-4 3.2 × 10-3

1.5 × 10-6 2.1 × 10-6 5.6 × 10-6

4 consider the formation of BzOH by the first reaction (Scheme 1), while the latter two terms consider the formation of DBE (Scheme 1) in each phase of the reaction medium where BzOH is consumed.

d[BzOH]h ) βk1h[BzCl]h[OH]h + (1 dt β)k1w[BzCl]w[OH]w - βk2h[BzCl]h[OH]h[BzOH]h (1 - β)k2w[BzCl]w[OH]w[BzOH]w (4) The conversion of BzCl to BzOH was found to be pseudo-first-order with respect to BzCl because the concentration of OH- ions in the bulk phase was kept constant throughout the reaction. The enhancement factor(s) in the rates of formation of BzOH and of DBE in hydrotrope solutions are shown with respect to the NaCS concentration in Figure 2 (continuous lines are the predictions using eq 4). The enhancement factor is defined as the ratio of the rate in the hydrotrope solution to that in water. The lines in Figure 2 indicate the best fit obtained for the kinetic data, and the bars indicate the spread of repeated runs. At the hydrotrope concentrations, very close to MHC, the hydrolysis rate jumped suddenly, giving an enhance-

ment factor of 5-6. A further increase in the hydrotrope concentration, however, showed a gradual decline in the rate. Because the hydrotrope aggregates carry negative charges on their surface, the approach of a hydroxide ion to BzCl embedded within hydrotrope aggregates is severely affected by electrostatic repulsions, particularly at higher hydrotrope concentrations.21 Despite the decreasing trends in the reaction rates with the hydrotrope concentration, the overall rates of the reaction in the hydrotrope solutions were still higher than those in water. The highest rate of the reactions was observed, however, at the MHC of the hydrotrope, unlike previously reported continuously increasing rate enhancements in the hydrotropic media.7-9 The rate of formation of DBE also increased up to 1.0 mol‚dm-3 NaCS concentration and decreased with a further increase in the hydrotrope concentration. The enhancement in the DBE formation rate was at least twice that of the BzOH formation showing increased kinetic selectivity toward the condensation reaction. The initial rates of the BzCl hydrolysis in water and in 2.0 mol‚dm-3 NaCS aqueous solutions at 353 K were 0.1 and 0.35 mmol‚dm-3‚min-1, respectively. The solubility values of BzCl in their respective solutions are 53 and 600 mmol‚dm-3, respectively, at the same temperature, i.e., an 11-fold increase in the solubility. If the hydrotropic rate enhancement was strictly due to the solubility increase, one expects a similar rate enhancement in the hydrolysis reaction. However, there was only a 4-fold rate enhancement. Also, the observed reaction rates decreased with a further increase in the NaCS concentration after the initial increase in the rate up to 1.0 mol‚dm-3. For example, at a NaCS concentration of 0.15 mol‚dm-3 (at 353 K), the rate enhancement was about 6 times, which was reduced to 3.5 times at a 2.0 mol‚dm-3 NaCS concentration (Figure 2). Similar observations were also made at other temperatures too (not shown). This decrease in the rate enhancement seems to be driven by the electrostatic repulsion between the hydrotrope aggregates and the OH- ion.20 The presence of salt, i.e., NaCl, generated in the hydrolysis reaction can have a beneficial effect on the electrostatic interactions because of counterion association by the common ion effect with the charged surface of the hydrotrope aggregates. However, when one compares the concentration of NaCl generated because of the chemical reaction with the concentration of hydrotrope itself, the effect will not be substantial. If there is any effect at all, then it is expected that it would reduce the electrostatic repulsion and the hydrolysis rate will increase. The temperature has a marked effect on the rate constants of the reactions. Table 2 gives the rate constants in water at different temperatures, while Figures 3 and 4 give the variations of k1h and k2h at different hydrotrope concentrations. With an increase in the temperature from 333 to 353 K at 0.15 mol‚dm-3 NaCS concentration, the rate constant (k1h) also increased from 0.04 to 0.5 min-1. The rate constant for the second step (k2h) increased from 5 mmol‚dm-3‚min-1 in water to 60 mmol‚dm-3‚min-1 in 0.15 mol‚dm-3 hydrotrope solutions at 353 K. There is also a considerable enhancement in the rate constant (k2h) for the formation of DBE as a function of the hydrotrope concentration. However, it also showed a decreased value at concentrations beyond 1 mol‚dm-3 NaCS (Figure 4). Below 1.0 mol‚dm-3 NaCS, sufficient amounts of BzCl

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Figure 5. Selectivity toward DBE with respect to the hydrotrope concentration in a saturated homogeneous system (time, 360 min): ], 333 K; 4, 343 K; 0, 353 K. Figure 3. Rate constant (k1h) of hydrolysis of BzCl as a function of the hydrotrope concentration: ], 333 K; 4, 343 K; 0, 353 K.

Figure 4. Rate constant (k2h) for the formation of DBE as a function of the hydrotrope concentration: ], 333 K; 4, 343 K; 0, 353 K.

and BzOH are available to form DBE, while OH- ions are required for activation of the molecules. Thus, the electrostatic repulsions do not have a marked effect on the formation of DBE, but beyond 1 mol‚dm-3, the rate constant (k2h) also decreased. Because the electrostatic

repulsions were taken into account by considering the effective concentration of OH- ions in the vicinity of the hydrotrope aggregates, the variation in the rate constant can be attributed to the microenvironment experienced by the reacting species. One of the striking features of the system was the increased selectivity of the condensation reaction. In water, the formation of BzOH was with almost complete selectivity. In the presence of hydrotropes, however, the selectivity toward DBE increased. A 1.0 mol‚dm-3 hydrotrope solution of NaCS, saturated with BzCl, gave 42% selectivity toward DBE as compared to ≈100% hydrolysis to alcohol in the absence of the hydrotrope. The effect of the hydrotrope (NaCS) concentration on the selectivity toward DBE is shown in Figure 5. The percent selectivity is defined as the percentage of reacted BzCl that led to the formation of DBE, during a reaction time of 7 h. The selectivity toward DBE increased because of the increase in the solubility of BzCl at higher hydrotrope concentrations. At 2.5 mol‚dm-3 and 353 K, the selectivity toward DBE was around 60%. At lower hydrotrope concentrations, this selectivity toward DBE was, however, low because of the poor solubility of BzCl. For the formation of DBE, a certain quantity of BzCl is required in the aqueous hydrotropic phase. The increased solubility of BzCl, at higher concentrations of NaCS, allows the consecutive reaction to proceed to DBE. The reaction rate constants followed the usual Arrhenius law dependence on the temperature. The yields of BzOH and DBE both increased in a given time with an increase in the temperature, with the effect being significant at the lower hydrotrope concentrations. The activation energies for both reactions are given in Table 3 at different hydrotrope concentrations. It is observed that the activation energies for both reactions were reduced in the presence of hydrotrope, which can be attributed to a certain catalytic effect of the microenvironment experienced by the reacting species. The

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[BzCl]BzOH ) [BzCl]0ek0[BzOH]

(5)

where [BzCl]0 is the solubility of BzCl in the absence of BzOH in the hydrotrope solution. The formation of BzCl and DBE can now be predicted from the rate constants obtained from homogeneous condition reactions and also by incorporation of the BzOH effect on the BzCl solubility in the hydrotropic medium. The volumetric coefficient of mass transfer (kLa ≈ 0.1 s-1) in an agitated system for BzCl transfer to the aqueous phase is much higher than the rate constant of the hydrolysis reaction (3.75 × 10-3 mol‚dm-3‚min-1) at 353 K. The hydrolysis reaction, therefore, belongs to regime 1 of heterogeneous reactions where the diffusional factors are unimportant.22 The hydrotropic phase, in which the reaction occurs, is saturated with BzCl at any moment under the two-phase conditions, and the rate of formation of the products will be determined by the kinetics of the chemical reaction. The rate equation for the formation of BzOH in a batch run can be written as

d[BzOH]/dt ) (βk1h[OH-]h[BzCl]hek0[BzOH]h + (1 - β)k1w[OH-]w[BzCl]wek0[BzOH]w) (βk2h[OH -]h[BzOH]h[BzCl]hek0[BzOH]h + (1 - β)k2w[OH-]w[BzOH]w[BzCl]wek0[BzOH]w) (6) Figure 6. Solubility of BzCl in the presence of BzOH (temp, 353 K): ], 1.0 mol‚dm-3 NaCS; 4, 1.5 mol‚dm-3 NaCS; 0, 2.0 mol‚dm-3 NaCS. Table 3. Energy of Activation for the Formation of BzOH and DBE at Different Hydrotrope Concentrations energy of activation (Ea, kcal‚mol-1) [NaCS]

(mol‚dm-3)

BzOH

DBE

0.11 0.15 0.5 1 2 2.5

24.05 24.53 24.36 20.86 20.50 20.34

19.94 20.91 18.54 18.57 18.32 18.12

effect on the hydrolysis reaction was, however, comparatively more than that on the condensation reaction, and a lower effect of the hydrotrope concentration on the activation energy also indicated that charged interactions do not have a significant effect on the condensation reaction. The hydrolytic reaction of BzCl in aqueous NaCS solutions is, therefore, characterized by the increased concentration of BzCl in the reaction medium, electrostatic repulsions between the hydrotrope aggregates and hydroxide ions, the microenvironment experienced by the reacting species, and the increased selectivity toward the formation of DBE. Kinetic Interpretation of Two-Phase Reaction. In the case of a heterogeneous reaction, with an excess of BzCl, the aqueous phase remains always saturated with BzCl and the selectivity toward the formation of DBE should increase in such a system. It was also observed that the increasing amount of BzOH, on hydrolysis of BzCl, could increase the BzCl solubility further. Figure 6 shows a further 5-6-fold increase in the solubility of BzCl in the presence of BzOH, which was fitted in the following equation:

and the rate equation for the formation of DBE is

d[DBE]/dt ) βk2h[BzOH]h[BzCl]hek0[BzOH]h + (1 - β)k2w[BzOH]w[BzCl]wek0[BzOH]w (7) Equations 6 and 7 were solved simultaneously by the Runge-Kutta fourth-order method to predict the formation of BzOH and DBE in the two-phase system. The rates of the reactions under the two-phase conditions also decreased with an increase in the hydrotrope concentration, but the overall enhancement factor for the formation of DBE increased to around 30-40-fold, as opposed to the 7-8-fold enhancement in the formation of BzOH (Figure 7). The lines in Figure 7 are the predicted values for the two species, while the bars indicate the experimental data of repeated runs. The match of the two is excellent within the experimental errors. Also the reaction rate decreased in both cases only beyond 1.0 mol‚dm-3 NaCS. The selectivity toward DBE increased to around 70% in 2.5 mol‚dm-3 NaCS at 353 K in the two-phase system as compared to 60% in the saturated homogeneous system because BzCl in the two-phase system was maintained at its saturation solubility the entire time and its availability to form DBE and in the second reaction is increased (Figure 8). The predicted values of [BzOH] and [DBE] and the experimental values at two different temperatures show good agreement with each other (Figure 9). These values were estimated with the assumption that the reaction was taking place completely in the aqueous phase. In the case of the two-phase system, the aqueous phase being the locus of the reaction was verified by changing the volume fraction of the organic phase in the dispersion, which showed no effect on the reaction rates. A considerable amount of DBE was finally present in the final organic phase in the case of a heterogeneous two-phase system. Because of its poor solubility, DBE

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Figure 7. Enhancement factor for the formation of BzOH and DBE in a heterogeneous system: ], 333 K; 4, 343 K; 0, 353 K.

Figure 9. Comparison of experimental and predicted values of BzOH and DBE concentrations in a heterogeneous system (temp, 353 and 343 K; NaCS, 2.5 mol‚dm-3): ], BzOH; O, DBE.

its higher solubility, BzOH remains mostly (≈88%) in the aqueous hydrotropic phase at the end of the reaction. On the other hand, the organic phase contained ≈90% DBE. Unlike in the PTC-catalyzed reaction, the product DBE is not contaminated by the catalyst or the hydrotrope. The minor quantities of BzOH from DBE can be easily separated by distillation because of the large difference in their boiling points. Conclusions

Figure 8. Selectivity toward DBE in a heterogeneous system: ], 333 K; 4, 343 K; 0, 353 K.

gets transferred to the organic phase. This provides an additional advantage of the hydrolysis reaction in a hydrotropic medium, i.e., the simultaneous separation of the two products, viz., BzOH and DBE. Because of

The hydrolysis of BzCl when conducted in the aqueous hydrotrope solutions of NaCS led to a maximum 6-fold increase in the reaction rates and a 70% increase in the selectivity toward DBE. This increased reaction rate is because of the increased solubility of BzCl in the hydrotropic medium. The reaction in the hydrotropic medium is assumed to take place in two phases, the water phase and the hydrotropic phase. The reaction in the hydrotropic phase is responsible for the increased reaction rates. A higher concentration of BzCl in the hydrotropic phase leads to increased selectivity toward DBE. The formation of BzOH and DBE could be predicted under two-phase conditions from the kinetic and solubility parameters of a homogeneous system. The recovery of the products was much easier because of their preferential partitioning in two different phases. Acknowledgment We are thankful for the support of this work from the Department of Science & Technology (DST), Govern-

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ment of India (Swarnajayanti Fellowship Cell) (Ref. No. DST/SF/E-II/99-2000). Notations Cs ) concentration of the hydrotrope, mol‚dm-3 H1 ) concentration of a monomeric hydrotrope molecule, mol‚dm-3 k1h, k2h ) rate constants of hydrolysis reaction in the hydrotropic phase, min-1 k1w, k2w ) rate constants of hydrolysis reaction in the bulk water phase, min-1 K2 ) dimerization constant Kh ) solute-hydrotrope interaction constant KL ) liquid-side mass-transfer coefficient, m‚s-1 Kn ) association constant for an n-mer kw ) rate constant in the aqueous phase, min-1 l ) fractional liquid holdup r ) distance between the hydrotrope surface and the bulk phase, Å [OH-]h ) effective hydroxide ion concentration in the vicinity of hydrotrope aggregates, mol‚dm-3 [OH]w ) effective hydroxide ion concentration in the bulk water phase, mol‚dm-3 R ) universal gas constant, cal‚gmol-1‚K-1 Sw ) equilibrium solubility of a nonelectrolyte in pure water, mol‚dm-3 T ) temperature, K

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Received for review July 17, 2003 Revised manuscript received March 24, 2004 Accepted November 12, 2004 IE030601+