Kinetic study of S-alkylation of 2-mercaptobenzimidazole by allyl bromide in the presence of potassium hydroxide

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Journal of the Chinese Institute of Chemical Engineers 39 (2008) 587–595 www.elsevier.com/locate/jcice

Kinetic study of S-alkylation of 2-mercaptobenzimidazole by allyl bromide in the presence of potassium hydroxide Maw-Ling Wang a,*, Yen-Chun Liu b b

a Department of Environmental Engineering, Hungkuang University, Shalu, Taichung County 433, Taiwan Department of Chemical Engineering, Wu-Feng Institute of Technology, Ming-Hsiung, Chiayi County 621, Taiwan

Received 15 January 2008; received in revised form 24 May 2008; accepted 28 May 2008

Abstract The S-alkylation (substitution on sulfur atom) of 2-mercaptobenzimidazole (MBI or ArSH) by allyl bromide (RBr) was successfully carried out in an aqueous solution of KOH/organic solvent two-phase medium. The reaction, which may take place either in the organic phase or on the interface, is greatly enhanced in the presence of KOH without the aid of quaternary salts as the catalyst to promote the reaction. No product was obtained from N-alkylation (substitution on nitrogen atom) during or after the reaction period by using a limited amount of allyl bromide (RBr) relative to that of MBI. Based on the experimental evidence, the kinetic behaviors and the characteristics of the reaction are sufficiently described by the pseudo-first-order rate law. The effects of the reaction conditions, including the agitation speed, the amount of KOH, volume of water, volume of dichloromethane, amount of allyl bromide, amount of 2-mercaptobenzimidazole, organic solvents and temperature on the conversion of allyl bromide and the apparent rate constants (kapp) were investigated in detail. Peculiar result is obtained in studying the effect of the volume of water on the conversion (or the reaction rate) in this work. Rational explanations are provided for the observed phenomena from experimental results. # 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: S-alkylation; 2-Mercaptobenzimidazole (MBI) derivative; Kinetics; KOH effect

1. Introduction Molecular collisions are necessary for reactions to occur. The reaction of two reactants which are separated in two immiscible phases is slow because of their limited contact area and their mutual solubility. To overcome this problem, the conventional methods including raising the temperature, high agitation speed or using a co-solvent have been employed to increase the rate of reaction of two immiscible reactants. However, the resulting improvements are still limited. Much energy is consumed to bring the reaction at high temperature. Furthermore, byproducts usually accompany generation of the main products. This difficulty was overcome by the development of phase-transfer catalysis (PTC) for the synthesis of organic chemicals from two immiscible reactants (Dehmlow and Dehmlow, 1993; Freedman, 1986; Keller, 1986, 1987, 1992; Starks, 1985; Starks et al., 1994; Weber and Gokel,

* Corresponding author. Tel.: +886 4 2631 8652; fax: +886 4 2652 9226. E-mail address: [email protected] (M.-L. Wang).

1977). Quaternary salts are conventionally employed as the phase-transfer catalysts, dramatically increasing the conversion and the reaction rate. Today, PTC has been extensively applied as an effective tool for synthesizing organic chemical via alkylation, displacement, oxidation and reduction, or dichlorocarbenation and polymerization (Wang and Wu, 1991; Wang and Yang, 1991; Wang and Chen, 2006; Wang and Lee, 2007; Wang and Rajendran, 2007). 2-Mercaptobenzimidazole (MBI) and its derivatives, which are important industrial inhibitors, antioxidants, antiseptics, and adsorbents (Moreira et al., 1990; Saxena et al., 1982; Thomas, 1953; Van Allan and Deacon, 1963; Xue et al., 1991) have been synthesized by various methods. In principle, two reaction steps are required to synthesize the derivatives of MBI. First, the most commonly used one is the reaction of ophenylene diamine and carbon disulfide to synthesize MBI by tertiary amine (Wang and Liu, 1995, 1996a,b, 1998a, 2006) and tetraalkylammonium salt (Wang and Liu, 1998b, 2005). The synthesis of MBI through the reactions of carbon disulfide and o-phenylene diamine either in a homogeneous or two-phase solution is thus carried out in the presence of KOH and the

0368-1653/$ – see front matter # 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jcice.2008.05.013

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Nomenclature A ArSH ArSK kapp kL RBr t Vi X X

interfacial area per unit volume of organic-phase solution between two phases (m2/m3) 2-mercaptobenzimidazole (MBI) potassium salt of 2-mercaptobenzimidazole apparent rate constant (1/min) mass transfer coefficient allyl bromide time i = a or o conversion of allyl bromide, defined by Eq. (4) chloride ion

Subscripts a aqueous phase int interface o organic phase p o and int

quaternary ammonium salts (Wang and Liu, 1998b, 2005). Second, the derivatives of MBI can then be obtained under phase-transfer catalysis via S-alkylation or N-alkylation of MBI with a nucleophilic alkylation reagent (Liu and Wang, 2008). Nevertheless, we found that the synthesis of MBI derivatives can also be carried out only in the presence of potassium hydroxide. No quaternary ammonium salts are required to promote the reactions. This current work investigates the S-alkylation of 2mercaptobenzimidazole (MBI) in an aqueous solution of KOH/organic solvent two-phase medium. The reaction is greatly enhanced by the addition of potassium hydroxide even in the absence of quaternary ammonium salts (Liu and Wang, 2008). The nuclophile allyl bromide (RBr) is employed as the alkylation reagent. It is found that only the product by Salkylation was obtained by using a limited amount of allyl bromide (RBr) in the present reaction conditions and no product was obtained from N-alkylation in a low concentration of potassium hydroxide. Based on the experimental data, the kinetic behaviors and the characteristics of the reaction are sufficiently described by the pseudo-first-order rate law. The effects of the reaction conditions, including the agitation speed, the amount of KOH, volume of water, volume of dichloromethane, amount of 2-mercaptobenzimidazole, amount of allyl bromide, organic solvents and temperature on the conversion of allyl bromide and the apparent rate constants (kapp) were investigated in detail. Rational explanations are provided for the observed phenomena from experimental results. 2. Experimental 2.1. Materials 2-Mercaptobenzimidazole (MBI or ArSH, C6H4(N)(NH)CSH)), allyl bromide (RBr), potassium hydroxide, tetrabutylammonium bromide (TBAB or QBr), tetrabutylammonium

hydroxide (TBAOH or QOH), potassium hydroxide (KOH), organic solvents including benzene, toluene, chloroform, chlorobenzene and dichloromethane, and other reagents used were all GR-grade chemicals for synthesis. 2.2. Procedures 2.2.1. Identification of the alkylation of MBI product (ArSR) Normally, S-Alkylation and N-alkylation may take place from the reaction of MBI (or ArSH) and the alkylation agent catalyzed by the phase-transfer catalyst in the presence of potassium hydroxide. In this work, only the product of MBI derivative by S-alkylation is produced when using a limited quantity of allyl bromide (RBr) at low aqueous concentration of potassium hydroxide and in the absence of quaternary ammonium salt. The MBI derivative product was identified both by NMR and GC–Mass. In the NMR analysis, the chemical shift (d) is 2.2, 4.5 and 6.9–7.7 ppm, as shown in the NMR spectrum, where the chemical shift 6.9–7.7 ppm indicates the hydrogen attached to the benzene ring. The ratio of hydrogen a:b:c:d:e is 2:1:2:2:2. In the GC–Mass analysis, the main charge/mass ratio of the chemical formula is 245, which appears as the M + 1 peak on the spectrum. The main charge/ mass ratios after fragmentation are 221, 149, 122, 105 and 77, respectively. 2.2.2. Kinetics of synthesizing the derivatives 2mercaptobenzimidazole (MBI) The reactor was a 150-mL four-necked Pyrex flask, permitting agitation of the solution, taking samples, inserting the thermometer, and feeding the reactants. A reflux condenser was attached to the port of the reactor to recover the species from the gas phase. The reactor was submerged in a constant temperature water bath with the temperature controlled to 0.1 8C. To start an experimental run, known quantities of 2mercaptobenzimidazole (MBI), caffeine (internal standard), and potassium hydroxide (KOH) were dissolved in the organicphase solution (dichloromethane and water mixture) and were introduced into the reactor. The liquid solution was stirred mechanically by a two-bladed paddle (5.5 cm) at 700 rpm. Then, a mixture of allyl bromide, and dichloromethane in liquid phase was introduced to the reactor to initiate the reaction. During the reaction, an aliquot of 0.1 mL was withdrawn from the solution at a chosen time. The sample was immediately poured into methanol at 4 8C for dilution and retardation of the reaction, and then analyzed by HPLC. The MBI derivative product for identification was purified from the reaction solution by vacuum evaporation to strip off the organic solvent. It was then recrystallized from ethanol as white crystals. Furthermore, the product of MBI derivative (ArSR), and the reactants (allyl bromide (RBr) and 2mercaptobenzimidazole (MBI)) were identified by NMR and IR analyses. The results obtained from the instrumental analysis are consistent with those of the literature reports. An HPLC Model (Shimadzu) with an absorbance detector (254 nm, SPD-6A) was employed to measure the amounts of

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reactants and product. The column used was Merck RP-8 (5 mm), l = 254 nm. The eluent was CH3CN/H2O = 1/1 with a flow rate of 1.2 mL/min. 3. Reaction mechanism and kinetic model In this work, only potassium hydroxide is introduced in the two-phase reaction of 2-mercaptobenzimidazole (MBI) and allyl bromide (RBr). No quaternary ammonium salts are required to catalyze the reaction. The overall reaction in the two-phase medium is expressed as follows

ðR1Þ From the experimental observation, 2-mercaptobenzimidazole (MBI or ArSH) does not dissolve in pure water. However, it dissolves in the aqueous solution of KOH and reacts with KOH to produce the water-soluble and organic-soluble potassium salt of MBI (ArSK). Then, ArSK further reacts with the organicphase reactant allyl bromide (RBr) in the organic phase or on the interface between two phases to produce the desired product ArSR. No quaternary ammonium salts are required to enhance the reaction. By appropriately using a limited amount of allyl bromide, only S-substitution occurs in the low aqueous concentration of KOH solution. Based on the experimental evidence, the mechanism of the reaction of 2-mercaptobenzimidazole (MBI or ArSH) and allyl bromide (RBr) in an aqueous solution of KOH/organic solvent two-phase medium can be expressed as, interface

(R2-1)

ArSKðoÞ @ ArSKðintÞ

(R2-2)

organic

ArSKðoÞ þ RBrðoÞ ! ArSRðoÞ þ KBrðoÞ

the product of the MBI derivative (ArSR) by S-alkylation was obtained in the solution. Furthermore, the reaction of MBI and KOH to produce ArSK quickly reaches an equilibrium state. In this work, it is difficult to identify the occurrence of the reaction between potassium salt of MBI (ArSK) and allyl bromide (RBr) on the interface or in the organic phase. In fact, the two-phase reaction of ArSK and RBr may take place on the interface between two phases or in the organic phase because of the water-soluble and organic-soluble ArSK and the water-insoluble RBr. Therefore, a simple kinetic rate law is used to model the reaction system in this work. Based on the experimental data, it is observed that the concentration of RBr in the organic phase follows a pseudo-first-order rate law. Based on the experimental results as shown in Figs. 3 and 9, the reaction is dominated by the mass transfer and the interfacial transport steps. Therefore, we have d½RBro ¼ kapp ½RBro dt

(R2-3)

or

(1)

where the apparent rate constant kapp is given as, kapp ¼ kL a½ArSKp ;

p ¼ o and int

(2)

where kL is the mass transfer coefficient, and a is the interfacial area (m2/m3 of the organic-phase solution). Eq. (1) is rewritten as lnð1Þ ¼ kapp t

(3)

where X is the conversion of allyl bromide (RBr), i.e., X ¼1

ArSHðoÞ þ KOHðaÞ ! ArSKðintÞ þ H2 O

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½RBro ½RBro;int

(4)

where ‘‘int’’ is the initial condition of the species. By plotting the experimental data ln(1  X) vs. t to get a straight line with slope apparent rate constant (kapp). 4. Results and discussion

interface

ArSKðintÞ þ RBrðoÞ ! ArSRðoÞ þ KBrðoÞ

(R2-4)

KBrðoÞ @ KBrðaÞ

(R2-5)

The subscripts ‘‘a’’, ‘‘o’’ and ‘‘int’’ represent the species in the aqueous phase, in the organic phase, and on the interface, respectively. It is clear that 2-mercaptobenzimidazole (MBI or ArSH) first reacts with potassium hydroxide either in the aqueous solution or on the interface between organic and aqueous phases to produce the potassium salt of MBI (ArSK). The potassium salt of MBI (ArSK) may react with the organicphase reactant allyl bromide (RBr) to produce the MBI derivative product (ArSR) either on the interface or in the organic phase. In the reaction system, a pseudo-first-order rate law is applied to the case for the amount of MBI (ArSH) larger relatively than that of allyl bromide (RBr). From the experiments, no product from N-alkylation of MBI was observed using a limited quantity of allyl bromide, and only

As shown in the reaction mechanism, the reaction of 2mercaptobenzimidazole (MBI) and potassium hydroxide in the aqueous phase to produce the potassium salt of 2-mercaptobenzimizadole (ArSK) is fast compared to the organic-phase reaction or the interface reaction. It is clear that the organicphase reaction or the interface reaction is the rate-controlling step for the whole reaction, as shown in reactions (R2-3) and (R2-4). As stated, the S-alkylation and N-alkylation of 2-mercaptobenzimidazole may take place either in the phase-transfer catalytic conditions or in the absence of phase-transfer catalytic conditions. However, selective alkylation can be achieved by appropriate specific reaction conditions and parameters. In this work, allyl bromide (RBr), which acts as the alkylation reagent, participates in the reaction in a limited quantity. Therefore, only S-alkylation occurs for the whole reaction and no products were obtained from N-alkylation, if the molar quantity of allyl bromide is less than that of 2-mercaptobenzimidazole at a

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Fig. 1. Effect of the active reagents (QOH, TBAB or KOH) on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole; 2.00 g of MBI, 1.00 g of allyl bromide, 0.4 g of KOH, 0.002 mol of the active reagent, volume ratio of CH2Cl2/H2O = 50 mL/50 mL, 1000 rpm, 30 8C. (Note: Blank denotes no catalyst and KOH participating in the reaction.)

Fig. 2. Effect of the amount of potassium hydroxide on the reaction rate for the S-alkylation of 2-mercaptobenzimidazole in the absence of TBAB catalyst; 2.00 g of MBI, 1.00 g of allyl bromide, volume ratio of CH2Cl2/H2O = 50 mL/ 50 mL, 1000 rpm, 30 8C. (Note: Blank denotes no catalyst and KOH participating in the reaction.)

relatively low aqueous concentration of KOH. Based on this evidence, the kinetics of the phase-transfer catalysis are discussed below.

hydroxide alone without further adding quaternary ammonium salts as the phase-transfer catalysts. 4.2. Effect of the amount of potassium hydroxide

4.1. Comparison of the reactivity of KOH with quaternary salts Fig. 1 shows the conversion of allyl bromide for the reactions using KOH, QOH and TBAB + KOH. It is seen that there is almost no reaction in the absence of potassium hydroxide and quaternary salts. The reaction is enhanced with the addition of quaternary ammonium hydroxide (QOH), tetrabutylammonium bromide (TBAB) and potassium hydroxide, or potassium hydroxide. As indicated by Liu and Wang (2008) in studying the primary effect of KOH, the reaction of 2mercaptobenzimidazole and allyl bromide can be enhanced by only adding potassium hydroxide in the absence of quaternary salts catalyst. A largest conversion is obtained only using potassium hydroxide among these reagents. This evidence verifies that the reaction is greatly enhanced by potassium

Fig. 2 shows the effect of the amount of KOH on the conversion of MBI in the absence of quaternary ammonium salt. There is almost no reaction when no KOH is added. The experimental data follows a pseudo-first-order rate law. It is believed that KOH affects the environment of the reaction. The distribution of the potassium salt of MBI (ArSK) in the organic phase is influenced by the concentration of KOH. The hydration of the nucleophile (ArSK) is also affected by the concentration of KOH. Therefore, the reactivity of the potassium salt of MBI (ArSK) in reacting with allyl bromide is also affected by the amount of KOH in the aqueous solution. As shown in Fig. 2, the reaction rate is increased with larger amount of KOH. The effect of the amount of KOH on the apparent rate constant (kapp) is also shown in Table 1. It is clear that KOH promotes the Salkylation in synthesizing the MBI derivative even though the

Table 1 Effects of the reactions conditions on the apparent rate constants (kapp, 1/min) catalyzed by quaternary ammonium salts; 2.00 g of ArSH, 0.6 g of KOH, 1.00 g of allyl bromide, 50 mL/50 mL of CH2Cl2/H2O, 30 8C, 1000 rpm KOH (g) kapp  10 3 ArSH (g) kapp  10 3 KBr kapp  10 3 Agitation (rpm) kapp  10 3 CH2Cl2 (mL) kapp  10 3 H2O (mL) kapp  10 3 Temperature (8C) kapp  10 3 Solvent kapp  10 3

0.60 11.67 1.00 11.36 0 18.93 0 0.89 20 41.38 10 9.53 5 3.33 CH2Cl2 18.07

0.80 18.07 1.50 15.43 0.30 18.80 200 3.89 30 23.71 20 10.67 10 4.92 CHCl3 16.22

1.00 33.70 2.00 18.07 0.60 19.06 300 7.94 40 19.60 30 12.73 20 10.01 C6H5Cl 10.33

3.00 18.28 0.90 18.33 400 10.33 50 18.06 40 17.33 30 18.07 C6H5CH3 6.32

4.00 18.43 1.40 19.26 600 11.11 60 12.59 50 18.07 35 28.53 C6H6 7.89

5.00 19.85 2.00 19.40 800 14.22

60 18.93

1000 18.07

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reaction was carried out in the absence of quaternary ammonium salt. In principle, potassium hydroxide (KOH), which is a basic compound, affects the distribution of the active intermediate (ArSK) between two phases, and the solubility of MBI (ArSH) in the aqueous phase also. The content of ArSK either in the aqueous phase or in the organic phase will be affected by the amount of KOH. Therefore, as expected, it affects the reaction rate and the conversion of RBr. In such situation, the chain effects are complicated. Although Eqs. (1) and (2) do not explicitly affect the dependence of the amount of KOH on the concentration of ArSK in the organic phase. However, the concentration of ArSK in the organic phase is dependent on the amount of KOH implicitly. 4.3. Effect of agitation speed The effect of agitation speed on the conversion of allyl bromide were investigated for 2.00 g of 2-mercaptobenzimidazole (MBI), and 1.00 g of allyl bromide, 0.8 g of KOH, 30 8C, at 50 mL/50 mL of CH2Cl2/H2O volume ratio. The results are shown in Fig. 3. It is clear that the reaction follows the pseudo-first-order rate law. In general, the conversion is increased with increased agitation speed up to a certain value from the view of Starks extraction model (Starks et al., 1994). Further increase in the agitation speed does not increase the conversion any more. However, as shown in Fig. 3, the conversion is still increased with increased agitation speed even at 1000 rpm. This result appears that the interface reaction of ArSK and RBr plays an important role on the whole reaction system. The experimental data follows a pseudo-first-order rate law. In principle, the conversion of allyl bromide is increased with the increase in the agitation speed when an interfacial reaction takes place. The reason is that the interfacial area between two phases which directly influences the reaction rate is increased with the increase in the agitation speed. The effect of the agitation speed on the apparent rate constant (kapp) is shown in Table 1. It is obvious that the kapp-value is increased with increased agitation speed.

Fig. 3. Effect of the agitation speed on the reaction rate in the S-alkylation of 2mercaptobenzimidazole; 2.00 g of MBI, 1.00 g of allyl bromide, 0.8 g of KOH, volume ratio of CH2Cl2/H2O = 50 mL/50 mL, 30 8C.

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Fig. 4. Effect of the amount of MBI on the conversion of allyl bromide in the Salkylation of 2-mercaptobenzimidazole; 1.20 g of allyl bromide, 0.8 g of KOH, volume ratio of CH2Cl2/H2O = 50 mL/50 mL, 1000 rpm, 30 8C.

4.4. Effect of the amount of 2-mercaptobenzimidazole (MBI or ArSH) The effect of the amount of MBI on the conversion of allyl bromide (RBr) is shown in Fig. 4. The experimental data also follow a pseudo-first-order rate law. It can be seen that the conversion of allyl bromide (or the reaction rate) is increased with increased amount of MBI up to 2.0 g. The conversion is not significantly affected by further increasing the amount of MBI larger than 2.0 g. From experimental observation, an amount more than 2.0 g MBI appears as a suspension solid when its mole is larger than that of KOH. At this situation, the MBI suspension solid is insoluble in the solution. Therefore, the organic-phase reaction is affected by the limited amount of KOH. The effect of the amount of MBI on the apparent rate constant (kapp) is shown in Table 1. 4.5. Effect of the amount of allyl bromide (RBr) In this work, allyl bromide, which is an organic-soluble compound, is used in a limited quantity relative to the quantity of MBI. Allyl bromide only displaced the hydrogen on the sulfur atom of MBI (i.e., S-alkylation) rather than on the nitrogen atom (i.e., N-alkylation) when a limited amount of allyl bromide was used in this work. This is because the activity of the hydrogen on the sulfur atom is greater than that of the nitrogen atom in the MBI molecule when a limited amount of allyl bromide is used in the reaction. Fig. 5 shows the effect of the amount of allyl bromide on the yield of ArSR product. For the case of using 1.5–5.0 g of RBr, the yield of ArSR product is first increased and then decreased with reaction time (e.g., there is a maximum value of the yield). However, the yields are all increased with time for using less amount of RBr (e.g., 1.0 g of RBr). The reason is that the concentration of MBI derivative product is high using larger amount of RBr. Under this circumstance, the MBI derivative product will further react with potassium hydroxide to form the potassium salt by substituting the hydrogen on the nitrogen atom. Therefore, the yield of ArSR product (only counting the product obtained from S-alkylation) is then decreased with time. However, there is still

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Fig. 5. Effect of the amount of allyl bromide on the yield of ArSR product in the S-alkylation of 2-mercaptobenzimidazole; 2.00 g of MBI, 0.8 g of KOH, volume ratio of CH2Cl2/H2O = 50 mL/50 mL, 1000 rpm, 30 8C.

Fig. 6. Effect of the organic solvents on the conversion of allyl bromide in the Salkylation of 2-mercaptobenzimidazole; 2.00 g of MBI, 1.00 g of allyl bromide, 0.8 g of KOH, volume ratio of organic solvent/H2O = 50 mL/50 mL, 1000 rpm, 30 8C.

no product by N-alkylation found in the solution, i.e., only hydrogen atom was displaced by potassium to form the potassium salt. The effect of the amount of RBr on the apparent rate constant (kapp) is shown in Table 1.

product. The reaction rates for using these three reagents are shown in Fig. 7. There is no reaction between allyl alcohol and potassium salt of MBI at all. Also, the reactivity of allyl chloride is also low relative to that of allyl bromide. Similar results for the conversion of the nucleophiles are shown in Fig. 8. The conversion of the organic substrate using KOH only is larger than that of using TBAB + KOH as catalyst.

4.6. Effect of potassium bromide As shown in the reaction mechanism, one mole of potassium bromide is produced when one mole of the main product (ArSR) is produced from the solution. Therefore, it is interesting to understand the effect of potassium bromide (KBr), which is extra added to the reaction solution, on the conversion of allyl bromide or the reaction rate. The experimental data still follows a pseudo-first-order rate law even extra addition of potassium bromide. As shown in Table 1, it is seen that the kapp-values are insensitive to the amount of KBr which is extra added to the reaction solution. 4.7. Effect of the organic solvents In this work, five organic solvents including dichloromethane, chloroform, chlorobenzene, toluene and benzene were used to examine their influences on the S-alkylation of MBI, with the results as shown in Fig. 6. The experimental data also follows a pseudo-first-order rate law when the reactions were carried out in these five organic solvents, respectively. The order of the reactivity for these five organic solvents are: CH2Cl2 (dielectric constant 8.93) > C6H5Cl (5.62) > CHCl3 (4.81) > C6H6 (2.27) > C6H5CH3 (2.38) The conversion is affected by the dielectric constant of the organic solvent, so the conversion is generally increased with an organic solvent of larger dielectric constant. The effect of the organic solvents on the apparent rate constant (kapp) is shown in Table 1.

4.9. Effect of the volume of water In general, the volume of water directly affects both the concentration of KOH in the aqueous phase and the distribution of ArSK between two phases or its amount on the interface between two phases. Two factors including the dilution effect and the change in the interfacial contact area due to change of the volume of water affect the conversion of allyl bromide. In the conventional two-phase phase-transfer catalytic reaction (i.e., Starks extraction model), the catalyst and the active intermediate transfer across the interface rapidly and react with the reactant either in the organic phase or in the aqueous phase. Thus, the conversion of the reactant is usually decreased with

4.8. Effects of the organic-phase reactants (RX) In this work, three nucleophiles, including allyl bromide, allyl chloride and allyl alcohol were used as the organic-phase reactant in participating the reaction to synthesize the desired

Fig. 7. Effect of the organic-phase reactant (RX) on the reaction rate in the Salkylation of 2-mercaptobenzimidazole; 2.00 g of MBI, 0.0083 mol of organicphase reactant (RX), 0.8 g of KOH, volume ratio of organic solvent/ H2O = 50 mL/50 mL, 1000 rpm, 30 8C.

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Fig. 8. Effect of the organic-phase reagent (RX) on the conversion of organicphase reactant (RX) in the S-alkylation of 2-mercaptobenzimidazole; 2.00 g of MBI, 0.0083 mol of organic-phase reactant (RX), 0.8 g of KOH, volume ratio of organic solvent/H2O = 50 mL/50 mL, 1000 rpm, 30 8C.

Fig. 10. Effect of the volume of dichloromethane on the yield of ArSR product in the S-alkylation of 2-mercaptobenzimidazole; 2.00 g of MBI, 1.00 g of allyl bromide, 0.8 g of KOH, 50 mL of H2O, 1000 rpm, 30 8C.

the increase in the amount of water due to the effect of dilution of the compounds. However, the present reaction system was carried out only in the presence of KOH and in the absence of phase-transfer catalyst. As shown in Fig. 9, the conversion of allyl bromide (RBr) is increased with increased volume of water. Therefore, the change in the interfacial contact area dominates the conversion rather than the dilution effect. The experimental data follows a pseudo-first-order rate law. The generally accepted reason is that there is no catalyst or active intermediate transfer across the interface. The reaction takes place mostly on the interface from which the interfacial contact area plays an important role. The interfacial area between two phases is increased with increased amount of water. For this, the conversion of MBI is increased with the increase in the volume of water. The effect of the volume of water on the apparent rate constant (kapp) is shown in Table 1 also.

fore, the concentration of ArSK and the concentration of RBr (allyl bromide) in the organic phase are both decreased with larger volume of CH2Cl2. Fig. 10 shows the effect of the volume of CH2Cl2 on the yield of ArSR product. The reaction also follows a pseudo-first-order rate law. For the case of using 20 and 30 mL of dichloromethane, the yield of ArSR product is first increased and then decreased with reaction time (e.g., there is a maximum at 20 min for using 20 mL of dichloromethane and at 60 min for using 30 mL of dichloromethane). The yields are all increased with time for using larger volume of dichloromethane (e.g., using 40 and 50 mL of dichloromethane). The reason is that the concentration of MBI derivative product is high using less dichloromethane volume (e.g., 20 and 30 mL of dichloromethane). Under this circumstance, the MBI derivative product will further react with potassium hydroxide to form the potassium salt by substituting the hydrogen on the nitrogen atom. Therefore, the yield of ArSR product (only counting the product obtained from S-alkylation) is then decreased with time. However, there is still no product by N-alkylation found in the solution, i.e., only hydrogen atom was displaced by potassium to form the potassium salt. The effect of the volume of CH2Cl2 on the apparent rate constant (kapp) is shown in Table 1.

4.10. Effect of the volume of dichloromethane Generally, as shown in reactions (R2-3) and (R2-4), the organic-phase concentration of ArSK and allyl bromide (RBr) directly affects the rate of the organic-phase reaction. There-

4.11. Effect of temperature

Fig. 9. Effect of the volume of water on the conversion of allyl bromide in the Salkylation of 2-mercaptobenzimidazole; 2.00 g of MBI, 1.00 g of allyl bromide, 0.8 g of KOH, 50 mL of CH2Cl2, 1000 rpm, 30 8C.

The effect of temperature on the conversion of allyl bromide for the reaction carried out in dichloromethane is shown in Fig. 11, it is shown that the reaction rate is enhanced and the conversion of MBI is increased at higher temperature. The reaction also follows the pseudo-first-order rate law. The effect of temperature on the apparent rate constant (kapp) is shown in Table 1. An Arrhenius plot for ln(kapp) vs. 1/T is shown in Fig. 12. The activation energy is 46.52 kJ/mol for the reaction carried out in CH2Cl2. The Arrhenius equation for the reaction in dichloromethane is:   5600 6 CH2 Cl2 : kapp ¼ 1:88  10 exp T

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provided in studying the peculiar effect of the amount of water on the conversion (or the reaction rate) which are obtained from the experimental observation. Acknowledgement The authors would like to thank the National Science Council of the ROC for the financial support of this manuscript under contract no. NSC 83-0402-E-007-004. References

Fig. 11. Effect of the temperature on the conversion of allyl bromide in the Salkylation of 2-mercaptobenzimidazole; 2.00 g of MBI, 1.00 g of allyl bromide, 0.8 g of KOH, volume ratio of CH2Cl2/H2O = 50 mL/50 mL, 1000 rpm.

Fig. 12. A plot of Arrhenius equation for the reaction in dichloromethane; same reaction conditions as given in Fig. 11.

5. Conclusion In this work, the S-alkylation of 2-mercaptobenzimidazole (MBI) by allyl bromide was successfully carried out in an aqueous solution of KOH/organic solvent two-phase medium. Using a limited amount of allyl bromide, only S-alkylation takes place in the two-phase solution. The reaction is enhanced by adding a small quantity of KOH. The reaction of the potassium salt of MBI and RBr takes place on the interface between two phases and in the organic phase. A reaction mechanism is proposed on the basis of the experimental evidence, and a pseudo-first-order rate equation is obtained to describe the kinetic behaviors. The apparent rate constant (kapp) obtained reflects the reaction rate. The conversion (or the reaction rate) is increased with increased agitation speed, amount of KOH, amount of allyl bromide, MBI, volume of water and the temperature; whereas it is decreased with increased volume of dichloromethane. Among the organic solvents, dichloromethane shows high reactivity in the Salkylation of synthesizing ArSR. Rational explanations are

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