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Synthesis of 2-mercaptobenzimidazole from the reaction of o-phenylene diamine and carbon disulfide in the presence of potassium hydroxide
Journal of the Chinese Institute of Chemical Engineers 38 (2007) 161–167 www.elsevier.com/locate/jcice
Synthesis of 2-mercaptobenzimidazole from the reaction of o-phenylene diamine and carbon disulfide in the presence of potassium hydroxide Maw-Ling Wang a,1,*, Biing-Lang Liu b,2 b
a Department of Environmental Engineering, Hung Kuang University, Shalu, Taichung County 433, Taiwan Department of Chemical Engineering, Wu Feng Institute of Technology, Min Hsiung, Chiayi County 621, Taiwan
Received 29 September 2006; accepted 5 January 2007
Abstract The reaction of o-phenylene diamine and carbon disulfide to synthesize 2-mercaptobenzimidazole (MBI) enhanced by potassium hydroxide was carried out in a homogeneous solution. No catalysts are required in the reaction. In addition to the reaction of carbon disulfide and hydrogen sulfide to produce S2 and CS32, potassium hydroxide is also acted as the enhancer to promote the reaction of synthesizing MBI. A mixture of organic solvent (protic or aprotic solvent) and water is used as the reaction solution in order to obtain a homogeneous phase. The effects of the reaction conditions, including the amount of o-phenylene diamine, amount of carbon disulfide, organic solvents, volume ratio of organic solvent to water, and temperature, on the conversion of o-phenylene diamine were investigated in detail. An appropriate amount of KOH is recommended to produce a high yield of MBI and a high reaction rate in using protic solvent/water as the mixed solution. Nevertheless, the conversion is increased with the increase in the amount of KOH using aprotic solvent. This behavior is different from that of the KOH effect on the conversion of ophenylene diamine using protic solvent/water as the mixed solution. # 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Synthesis of 2-mercaptobenzimidazole; Potassium hydroxide; Kinetics; Protic and aprotic solvent
1. Introduction The necessary condition for a reaction of two or more reactants to occur is the contact of molecules. The traditional methods, such as: using cosolvents, increasing agitation speed and elevating the temperature, have been suggested in order to produce a high yield or to obtain a high rate from a two-phase reaction in the past decade. Therefore, the reaction in a homogeneous solution is usually faster than that in a heterogeneous solution. Nevertheless, many products or chemicals are still produced from two immiscible reactants. The reason for the small rate of a two-phase reaction in which the reactants exist in different phases is clearly the limited contact of the reactants by an interface due to their mutual solubility. The conventional method to overcome this difficulty is to use the protic or aprotic solvents as the cosolvents to
* Corresponding author. Tel.: +886 4 2631 8652x4160; fax: +886 4 2652 9226. E-mail address: [email protected] (M.-L. Wang). 1 2
dissolve all the reactants. Therefore, many reactions were carried out under anhydrous conditions using aprotic solvents. The nucleophilic agent solvates with the protic solvent in decreasing the activity. Both protic and aprotic solvents lead to less availability of using such cosolvents. In addition, the byproducts accompanied by the side reactions at high temperature were produced. Therefore, there has been limited success in enhancing the reaction rate. This low-rate problem was not solved until the development of the phase-transfer catalysis (PTC) by adding a catalytic amount of quaternary salt. The main advantage of PTC is that the reactions of synthesizing organic chemicals are the increased product yield, large reaction rate and high selectivity of the desired product at moderate conditions. Currently, quaternary ammonium salts are recognized as the preferred enhancer to promote the reaction in both homogeneous and two-phase solutions (Dehmlow and Dehmlow, 1983; Starks et al., 1994; Weber and Gokel, 1977). 2-Mercaptobenzimidazole (MBI) is an important chemical which has industrial applications as an antioxidant, inhibitor, antiseptic and adsorbent (Goodman, 1975; Saxena et al., 1982; Scherhag et al., 1974; Van Allan and Deacon, 1963). Several processes, including the reaction of o-phenylene diamine and
0368-1653/$ – see front matter # 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jcice.2007.01.003
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reactants at high temperature have been used to produce the desired products (Goodman, 1975; Saxena et al., 1982; Scherhag et al., 1974; Van Allan and Deacon, 1963). In Yoshinori et al. (1990) also synthesized MBI from the reaction of enaminones and carbon disulfide with success. Our preliminary results (Wang and Liu, 1995, 1996a,b, 1998a,b,c, 2005a,b, 2006a,b, 2007; Wang and Lam, 2004, 2006; Wang and Hsu, 2006; Wang et al., 2004) indicate that the synthesis of MBI from the reaction of o-phenylene diamine and carbon disulfide can be carried out in the presence of quaternary ammonium hydroxide or tertiary amine. No catalytic effect was observed when using tetrabutylammonium bromide (TBAB) as the catalyst. In this work, we found that the reaction of ophenylene diamine and carbon disulfide to synthesize MBI can also be greatly enhanced by using only potassium hydroxide in the absence of phase-transfer catalysts in a homogeneous solution. The most advantage of the present process is that no phase-transfer catalysts are required in the reaction. In order to achieve this reaction, a mixture of protic (or aprotic) solvent with water in an appropriate volume ratio served as the reaction solution. Therefore, the reaction of o-phenylene diamine and carbon disulfide was carried out in a homogeneous solution. The effects of the reaction conditions, such as: the amount of KOH, organic solvents, volume ratio of organic solvent to water, amounts of C6H4(NH2)2, amount of CS2, and temperature, on the conversion of o-phenylene diamine were investigated in detail. Rational explanations for the effect of KOH on the conversion of o-phenylene diamine in using protic or aprotic solvent was achieved. 2. Experimental
600 rpm. During the reaction, an aliquot sample of 0.2 mL was withdrawn from the solution at a chosen time. The sample was immediately introduced into the same organic solvent at 4 8C for dilution to retard the reaction, and then analyzed by HPLC instrument. The product 2-mercaptobenzimidazole (MBI) for identification was synthesized from the reaction of o-phenylene diamine in a limited quantity and carbon disulfide in an organic solvent. After completing the reaction, the solution was purified by vacuum evaporation to strip off organic solvent and carbon disulfide. Then, MBI in solid form was re-dissolved into ethanol for re-crystallization. A white crystal form of MBI was obtained by cooling the solution. The MBI product and the reactant (o-phenylene diamine) were also identified by NMR and IR. The content of MBI and reactants were analyzed by HPLC instrument. The results obtained from NMR and IR are consistent with the published data. An HPLC model LC9A (Shimadzu) with an absorbance detector (254 nm, SPD-6A) was employed to measure the contents of reactant and product. The column used was ShimPack CLC-ODS RP-18 (5 mm). The eluent was CH3CN/ H2O = 20/80 (with 5 nm KH2PO4 + 0.1% H3PO4) (volume ratio) with a flow rate 1.0 mL/min. 3. Reaction mechanism and the kinetics In the absence of catalyst and alkaline compound, the reaction of o-phenylene diamine and carbon disulfide to produce 2-mercaptobenzimidazole (MBI) is slow. However, the reaction is greatly enhanced simply by adding an appropriate amount of potassium hydroxide in this work. The overall reaction in the presence of potassium hydroxide is expressed as:
2.1. Materials Carbon disulfide (CS2), o-phenylene diamine (C6H4(NH2)2), potassium hydroxide (KOH), organic solvents, including, N,Ndimethylformamide (DMF), dimethylsulfoxide (DMSO), methyl acetontrile (MeCN), ethyl alcohol (EtOH), methyl alcohol (MeOH) and tetrahydrofuran (THF), tetrabutylammonium hydroxide (TBAOH or QOH), tetrabutylammonium bromide (TBAB), tripropylamine (TPA) and other reagents are all G.R. grade chemicals for synthesis.
(R1) The reason for enhancing reaction by KOH is that the potassium salt of 2-mercaptobenzimidazole (MBI) is formed during the reaction. In such as way, the reaction is shifted toward to the right hand side of Reaction (R1). For this, the mechanism of the reaction in a homogeneous-phase solution is proposed as:
2.2. Procedures Kinetics of synthesizing 2-mercaptobenzimidazole (MBI). The reactor is a 125-mL four-necked Pyrex flask able to serve the purposes of agitating the solution, inserting the thermometer, taking samples, and feeding the reactants. A reflux condenser is attached to the port of the reactor to recover carbon disulfide and organic solvent. The reactor is submerged into a water bath in which the temperature is controlled to 0.1 8C. To start a kinetic run, measured quantities of o-phenylene diamine, carbon disulfide, potassium hydroxide, and caffeine (external standard) were dissolved in a mixture of organic solvent/water and introduced into the reactor. The mixture was stirred mechanically by a two-blade paddle (5.5 cm) at
(R2)
(R3)
H2 S þ 2KOH ! K2 S þ 2H2 O
(R4)
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As shown in Reaction (R2), the reaction of o-phenylene diamine and carbon disulfide to synthesize 2-mercaptobenzimidazole (MBI) is slow, i.e. at the beginning of reaction, the rate is low. However, Reactions (R3) and (R4) take place immediately even a trace amount of MBI was produced from Reaction (R2). Therefore, the total reaction shown in (R1) is greatly enhanced toward to the right hand side of Reaction (R2). Hence, the reaction is enhanced in the presence of potassium hydroxide. In addition, carbon disulfide can further react with potassium hydroxide to form CS32 and CO32, i.e.: 3CS2 þ 6KOH ! 2K2 CS3 þ K2 CO3 þ 3H2 O
(R5)
As shown in Reactions (R2)–(R5), potassium hydroxide does not directly participate in the production of 2-mercaptobenzimidazole (MBI) which is shown in Reaction (R2). In Reactions (R3) and (R4), KOH only participates in the reaction with MBI to produce the potassium salt of MBI, and in the reaction with H2S to produce K2S, respectively. These two reactions, which shift the total reaction toward to the right hand side of Reaction (R1), both enhance the reaction of o-phenylene diamine and carbon disulfide according to LeChatelier’s principle. The formation of potassium salt of MBI can be easily converted to MBI simply by adding acid compound to the solution of potassium salt of MBI. 4. Results and discussion
Fig. 2. Effect of the amount of potassium hydroxide on the conversion of o-phenylene diamine in MeOH/H2O solution; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 50 mL of MeOH/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
QOH), etc. These results exhibit the potential of potassium hydroxide in the reaction of o-phenylene diamine and carbon disulfide. 4.1. Effect of the KOH amount
In this work, the inexpensive potassium hydroxide is used to enhance the reaction, avoiding the use of high priced quaternary ammonium salt. Fig. 1 shows the effect of various base compounds on the conversion of o-phenylene diamine. It is clear that potassium hydroxide has high reactivity to enhance the reaction of o-phenylene diamine and carbon disulfide even in the absence of other base compounds, including tripropylamine (TPA), tetrabutylammonium bromide (TBAB), tetrabutylammonium hydroxide (TBAOH or
As stated, KOH is the key compound to promote the reaction. The effect of the amount of KOH on the conversion of o-phenylene diamine in MeOH/H2O (volume ratio: 4/1; protic solvent) solution is shown in Fig. 2. This conversion is increased with the increase in the amount of KOH up to 1.0 g approximately. Further increasing the amount of KOH (>1.0 g) leads to decrease the conversion of o-phenylene diamine and decrease the reaction rate. A plot of the conversion (X) versus the amount of KOH at various reaction times is shown in Fig. 3. A maximum conversion is obtained at an appropriate amount of KOH. This peculiar result can be
Fig. 1. Effect of the various base compounds on the conversion of o-phenylene diaminein MeOH/H2O solution; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 50 mL of MeOH/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
Fig. 3. A plot of the conversion of o-phenylene diamine vs. amount of KOH at various reaction times; Same reaction conditions as given in Fig. 2.
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explained by the complicate reaction of CS2, KOH and C6H4(NH2)2. When the amount of KOH in stoichiometric quantity is less than that of CS2, CS2 in a relatively large quantity dominantly reacts with C6H4(NH2)2 to produce MBI. Meanwhile, KOH at this stage also continuously reacts with H2S to produce K2CS3 and K2CO3 for ionic neutralization. Therefore, the conversion is increased with the increase in the KOH amount. Nevertheless, CS2 will also continuously react with KOH, when the amount of KOH in stoichiometric quantity is larger than that of CS2 (e.g., the amount of KOH is larger than 1.0 g). In fact, carbon disulfide reacts both with C6H4(NH2)2, H2S and KOH in the reaction solution. At this situation, only a small amount of CS2 reacts with C6H4(NH2)2 to produce MBI, i.e. not all amount of CS2 participate in reacting with o-phenylene diamine. Under this circumstance, the conversion is suppressed at a higher concentration of KOH. Therefore, the conversion of o-phenylene diamine is decreased with further increase in the amount of KOH above 1.0 g. In contrast, the results obtained from the reactions using various amounts of KOH carried out in a DMF/H2O (volume ratio: 4/1; aprotic solvent) are shown in Fig. 4. The conversion is increased monotonously with the increase in the amount of KOH. These aprotic solvents, which do not act as hydrogen-bond donors since their C–H bonds are not strongly enough polarized, have large dielectric constants (er > 15), sizeable dipole moments and low ETN -values (as empirical parameter of solvent polarity) (Ebel, 1990). Those chemicals, such as: dimethylsulfoxide (DMSO), N,Ndimethylformamide (DMF), acetonitrile (MeCN), tetrahydrofuran (THF) and cyclic carbonates (propylene carbonate) are the most important aprotic solvents. However, those aprotic solvents contain hydrogen atoms bound to the electronegative elements, such as: F–H, –O–H, –N–H, etc. In contrast, the protic solvents are hydrogen bond donors, the dielectric constants are usually larger, and the ETN -values lie between 0.5 and 1.0; indicating that these solvents are
strongly polar. Typical chemicals are water, alcohols, formamides, etc. The present result using DMF (aprotic solvent) conflicts with the results obtained from the reaction using protic solvents, in which the reaction is inhibited by a large amount of KOH (Fig. 2). This is probably due to the competition in Reactions (R2), (R3), (R4) and (R5) when the aprotic solvent, including DMF, DMSO, THF and MeCN mixed with water as the organic solution. The reactivity of the reactants, which are surrounded by the organic solvent, are affected by the interaction with the organic solvent via solvation and hydration. Therefore, the reactivity of the species in different organic solvents does not correspond to the order of dielectric constant or the Dimroth-Reichardt parameter of organic solvents ETN . In principle, Reactions (R2)–(R4), which are favorable for synthesizing MBI, are all faster than the reaction (R5). As shown in Fig. 4, the conversion reaches 90% within 15 min.
Fig. 4. Effect of the volume ratio of DMF to H2O on the conversion of ophenylene diamine in DMF/H2O solution; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 50 mL of DMF/ H2O (volume ratio: 4/1), 600 rpm, 40 8C.
Fig. 5. Effect of the organic solvents on the conversion of o-phenylene diamine in various organic solvents; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 40 mL of organic solvent, 10 mL of H2O, 600 rpm, 40 8C.
4.2. Effect of the organic solvents In this work, aprotic solvent (DMF, DMSO, THF and MeCN), and protic solvent (EtOH and MeOH) were used as the organic solvent to dissolve o-phenylene diamine. Therefore, a mixture of organic solvent and water is used to dissolve KOH. The effects of the organic solvents on the conversion of ophenylene diamine are shown in Fig. 5. The order of the reactivity for these organic solvents are: DMSO DMF > MeCN > THF > MeOH EtOH. It is clear that these aprotic solvents have high reactivity. The order of the dielectric constants for these organic solvents are: DMSO (46.5) > DMF (36.7) > MeCN (35.94) > MeOH (32.66) > EtOH (24.55) > THF (7.58). The conversion is not directly related to the dielectric constants of the organic solvents, i.e. an organic solvent with high dielectric constants do not lead to a high conversion of o-phenylene diamine. In addition, the Bronsted acid–base behaviors probably are the main factor influencing the reaction. This is because the reaction has to be carried out in
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Fig. 6. Effect of the volume ratio of DMSO to H2O on the conversion of o-phenylene diamine; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 7.14 103 mol of KOH, 50 mL of DMSO/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
Fig. 8. Effect of the volume ratio of MeCN to H2O on the conversion of o-phenylene diamine; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 7.14 103 mol of KOH, 50 mL of MeCN/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
a basic environment. For example, MeCN, EtOH and MeOH are relatively neutral compounds. Therefore, the conversion is relatively low in using MeCN, EtOH and MeOH as the organic solvents. In fact, water has high dielectric constants (78.3) of polarity compound. The order of the polarity of these solvents are: H2O (1.00) > formamide (0.799) > MeOH (0.762) > N-methyl formamide (0.722) > EtOH (0.654) > propylene carbonates (0.491) > MeCN (0.463) > DMSO (0.444) > DMF (0.404) > THF (0.207). The conversion of o-phenylene diamine is also not related to the polarity of the solvent.
o-phenylene diamine For these three systems, the conversions are all increased with the increase in the amount of organic solvent even the dielectric constant of water is 78.3. However, the reaction solution should contain sufficient water at least to dissolve KOH for further reaction. 4.4. Effect of the C6H4(NH2)2 amount
Figs. 6–8 show the effect of the volume ratio of the organic solvent to water for typical aprotic solvents, such as: DMSO, DMF and MeCN, respectively, on the conversion of
In this work, an excess large amount of CS2 is used in the reaction, so o-phenylene diamine is thus the limited compound. The effect of the amount of C6H4(NH2)2 on the conversion is shown in Fig. 9. The conversion is decreased with the increase in the amount of o-phenylene diamine. This result is the same as that of obtaining from the liquid–liquid two-phase catalyzed reaction by tertiary amine and quaternary ammonium salts. This phenomenon is probably due to the fact that the reaction system contains more than one basic reaction. The reason for
Fig. 7. Effect of the volume ratio of DMF to H2O on the conversion of o-phenylene diamine; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 7.14 103 mol of KOH, 50 mL of DMF/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
Fig. 9. Effect of the amount of o-phenylene diamine on the conversion o-phenylene diamine in MeOH/H2O solution; 2.70 102 mol of carbon disulfide, 7.14 103 mol of KOH, 50 mL of MeOH/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
4.3. Effect of the volume ratio of organic solvent to water
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Fig. 10. Effect of the amount of carbon disulfide on the conversion o-phenylene diamine in MeOH/H2O solution; 3.18 103 mol of o-phenylene diamine, 7.14 103 mol of KOH, 50 mL of MeOH/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
Fig. 11. Effect of temperature on the conversion of o-phenylene diamine in DMF/H2O solution; 3.70 103 mol of o-phenylene diamine, 2.70 102 mol of carbon disulfide to o-phenylene diamine, 7.14 103 mol of KOH, 50 mL of DMSO/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
the occurrence is accounted as follows:
5. Conclusion
R3 N þ CS2 ! R3 N CS2
The synthesis of 2-mercaptobenzimidazole (MBI) from the reaction of o-phenylene diamine and carbon disulfide was carried out in a homogeneous solution in the presence of KOH successfully. No catalysts are required to promote the reaction in this work. Potassium hydroxide does not directly participate in the reaction of o-phenylene diamine and carbon disulfide. However, the reaction of MBI and KOH to form the potassium salt of MBI, and the reaction of hydrogen sulfide and KOH to produce the inert compound K2S, both enhance the total reaction of synthesizing MBI. Using protic solvent, an appropriate amount of KOH is recommended to obtain a high yield of MBI. In contrast, the conversion of o-phenylene diamine is increased with the increase in the amount of KOH when the reaction is carried out in an aprotic solvent/water mixed solution. This peculiar phenomenon is effectively explained by the property of the aprotic or protic solvents with water. The conversion is increased with the increase in the amount of carbon disulfide, in the volume ratio of organic solvent to water, and in temperature. However, the conversion of o-phenylene diamine is decreased with the increase in the amount of o-phenylene diamine.
C6 H4 ðNH2 Þ2 þ R3 N CS2 ! C6 H4 ðNH2 ÞCS2 R3 N þ 2Hþ ! C6 H4 ðNH2 ÞCþ S þ H2 S þ R3 N ! C6 H4 ðNÞðNHÞCSH þ H2 S þ R3 N 4.5. Effect of the CS2 amount In this work, carbon disulfide is used in large excess. The effect of the amount of CS2 on the conversion of o-phenylene diamine is shown in Fig. 10. The conversion is still increased with the increase in the amount of CS2. In addition, CS2 also reacts with KOH to form inert salt, as shown in Reaction (R3). 4.6. Effect of temperature A typical reaction in the DMF/H2O for investigating the temperature effect was carried out. The results are depicted in Fig. 11. As expected, the conversion is increased with the increase in temperature. In this work, the kinetic model in describing the reactions of o-phenylene diamine and carbon disulfide by potassium hydroxide in a mixture of organic solvent (protic or aprotic solvents) and water is not available. The reason is that neither the first-order nor the second-order rate law can be applied to model the kinetic behaviors of the reaction. Only the method of the initial rates is employed to account for the experimental data at early reaction time. An Arrhenius plot of ln(1 X) versus 1/T by the method of initial rates is obtained to get the activated energy of the reaction near 28.91 kcal/mol.
Acknowledgement The authors thank NSC of R.O.C. for financial support (NSC 82-0402-E-007-005) for this work. References Dehmlow, E. V. and S. S. Dehmlow, Phase Transfer Catalysis, Verlag Chemie, Weinheim, Germany (1983). Ebel, H. F., Solvents and Solvent Effects in Organic Chemistry, Second revised and enlarged edition, p. 408, VCH Verlagsgesellschaft mbH Press (1990). Goodman, A. L., ‘‘2-Mercaptobenzimidazole by Reacting o-Phenylene Diamine and Carbon Disulphide,’’ U.S. Patent 3,895,025 (1975).
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Synthesis of 2-mercaptobenzimidazole from the reaction of o-phenylene diamine and carbon disulfide in the presence of potassium hydroxide Maw-Ling Wang a,1,*, Biing-Lang Liu b,2 b
a Department of Environmental Engineering, Hung Kuang University, Shalu, Taichung County 433, Taiwan Department of Chemical Engineering, Wu Feng Institute of Technology, Min Hsiung, Chiayi County 621, Taiwan
Received 29 September 2006; accepted 5 January 2007
Abstract The reaction of o-phenylene diamine and carbon disulfide to synthesize 2-mercaptobenzimidazole (MBI) enhanced by potassium hydroxide was carried out in a homogeneous solution. No catalysts are required in the reaction. In addition to the reaction of carbon disulfide and hydrogen sulfide to produce S2 and CS32, potassium hydroxide is also acted as the enhancer to promote the reaction of synthesizing MBI. A mixture of organic solvent (protic or aprotic solvent) and water is used as the reaction solution in order to obtain a homogeneous phase. The effects of the reaction conditions, including the amount of o-phenylene diamine, amount of carbon disulfide, organic solvents, volume ratio of organic solvent to water, and temperature, on the conversion of o-phenylene diamine were investigated in detail. An appropriate amount of KOH is recommended to produce a high yield of MBI and a high reaction rate in using protic solvent/water as the mixed solution. Nevertheless, the conversion is increased with the increase in the amount of KOH using aprotic solvent. This behavior is different from that of the KOH effect on the conversion of ophenylene diamine using protic solvent/water as the mixed solution. # 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Synthesis of 2-mercaptobenzimidazole; Potassium hydroxide; Kinetics; Protic and aprotic solvent
1. Introduction The necessary condition for a reaction of two or more reactants to occur is the contact of molecules. The traditional methods, such as: using cosolvents, increasing agitation speed and elevating the temperature, have been suggested in order to produce a high yield or to obtain a high rate from a two-phase reaction in the past decade. Therefore, the reaction in a homogeneous solution is usually faster than that in a heterogeneous solution. Nevertheless, many products or chemicals are still produced from two immiscible reactants. The reason for the small rate of a two-phase reaction in which the reactants exist in different phases is clearly the limited contact of the reactants by an interface due to their mutual solubility. The conventional method to overcome this difficulty is to use the protic or aprotic solvents as the cosolvents to
* Corresponding author. Tel.: +886 4 2631 8652x4160; fax: +886 4 2652 9226. E-mail address: [email protected] (M.-L. Wang). 1 2
dissolve all the reactants. Therefore, many reactions were carried out under anhydrous conditions using aprotic solvents. The nucleophilic agent solvates with the protic solvent in decreasing the activity. Both protic and aprotic solvents lead to less availability of using such cosolvents. In addition, the byproducts accompanied by the side reactions at high temperature were produced. Therefore, there has been limited success in enhancing the reaction rate. This low-rate problem was not solved until the development of the phase-transfer catalysis (PTC) by adding a catalytic amount of quaternary salt. The main advantage of PTC is that the reactions of synthesizing organic chemicals are the increased product yield, large reaction rate and high selectivity of the desired product at moderate conditions. Currently, quaternary ammonium salts are recognized as the preferred enhancer to promote the reaction in both homogeneous and two-phase solutions (Dehmlow and Dehmlow, 1983; Starks et al., 1994; Weber and Gokel, 1977). 2-Mercaptobenzimidazole (MBI) is an important chemical which has industrial applications as an antioxidant, inhibitor, antiseptic and adsorbent (Goodman, 1975; Saxena et al., 1982; Scherhag et al., 1974; Van Allan and Deacon, 1963). Several processes, including the reaction of o-phenylene diamine and
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reactants at high temperature have been used to produce the desired products (Goodman, 1975; Saxena et al., 1982; Scherhag et al., 1974; Van Allan and Deacon, 1963). In Yoshinori et al. (1990) also synthesized MBI from the reaction of enaminones and carbon disulfide with success. Our preliminary results (Wang and Liu, 1995, 1996a,b, 1998a,b,c, 2005a,b, 2006a,b, 2007; Wang and Lam, 2004, 2006; Wang and Hsu, 2006; Wang et al., 2004) indicate that the synthesis of MBI from the reaction of o-phenylene diamine and carbon disulfide can be carried out in the presence of quaternary ammonium hydroxide or tertiary amine. No catalytic effect was observed when using tetrabutylammonium bromide (TBAB) as the catalyst. In this work, we found that the reaction of ophenylene diamine and carbon disulfide to synthesize MBI can also be greatly enhanced by using only potassium hydroxide in the absence of phase-transfer catalysts in a homogeneous solution. The most advantage of the present process is that no phase-transfer catalysts are required in the reaction. In order to achieve this reaction, a mixture of protic (or aprotic) solvent with water in an appropriate volume ratio served as the reaction solution. Therefore, the reaction of o-phenylene diamine and carbon disulfide was carried out in a homogeneous solution. The effects of the reaction conditions, such as: the amount of KOH, organic solvents, volume ratio of organic solvent to water, amounts of C6H4(NH2)2, amount of CS2, and temperature, on the conversion of o-phenylene diamine were investigated in detail. Rational explanations for the effect of KOH on the conversion of o-phenylene diamine in using protic or aprotic solvent was achieved. 2. Experimental
600 rpm. During the reaction, an aliquot sample of 0.2 mL was withdrawn from the solution at a chosen time. The sample was immediately introduced into the same organic solvent at 4 8C for dilution to retard the reaction, and then analyzed by HPLC instrument. The product 2-mercaptobenzimidazole (MBI) for identification was synthesized from the reaction of o-phenylene diamine in a limited quantity and carbon disulfide in an organic solvent. After completing the reaction, the solution was purified by vacuum evaporation to strip off organic solvent and carbon disulfide. Then, MBI in solid form was re-dissolved into ethanol for re-crystallization. A white crystal form of MBI was obtained by cooling the solution. The MBI product and the reactant (o-phenylene diamine) were also identified by NMR and IR. The content of MBI and reactants were analyzed by HPLC instrument. The results obtained from NMR and IR are consistent with the published data. An HPLC model LC9A (Shimadzu) with an absorbance detector (254 nm, SPD-6A) was employed to measure the contents of reactant and product. The column used was ShimPack CLC-ODS RP-18 (5 mm). The eluent was CH3CN/ H2O = 20/80 (with 5 nm KH2PO4 + 0.1% H3PO4) (volume ratio) with a flow rate 1.0 mL/min. 3. Reaction mechanism and the kinetics In the absence of catalyst and alkaline compound, the reaction of o-phenylene diamine and carbon disulfide to produce 2-mercaptobenzimidazole (MBI) is slow. However, the reaction is greatly enhanced simply by adding an appropriate amount of potassium hydroxide in this work. The overall reaction in the presence of potassium hydroxide is expressed as:
2.1. Materials Carbon disulfide (CS2), o-phenylene diamine (C6H4(NH2)2), potassium hydroxide (KOH), organic solvents, including, N,Ndimethylformamide (DMF), dimethylsulfoxide (DMSO), methyl acetontrile (MeCN), ethyl alcohol (EtOH), methyl alcohol (MeOH) and tetrahydrofuran (THF), tetrabutylammonium hydroxide (TBAOH or QOH), tetrabutylammonium bromide (TBAB), tripropylamine (TPA) and other reagents are all G.R. grade chemicals for synthesis.
(R1) The reason for enhancing reaction by KOH is that the potassium salt of 2-mercaptobenzimidazole (MBI) is formed during the reaction. In such as way, the reaction is shifted toward to the right hand side of Reaction (R1). For this, the mechanism of the reaction in a homogeneous-phase solution is proposed as:
2.2. Procedures Kinetics of synthesizing 2-mercaptobenzimidazole (MBI). The reactor is a 125-mL four-necked Pyrex flask able to serve the purposes of agitating the solution, inserting the thermometer, taking samples, and feeding the reactants. A reflux condenser is attached to the port of the reactor to recover carbon disulfide and organic solvent. The reactor is submerged into a water bath in which the temperature is controlled to 0.1 8C. To start a kinetic run, measured quantities of o-phenylene diamine, carbon disulfide, potassium hydroxide, and caffeine (external standard) were dissolved in a mixture of organic solvent/water and introduced into the reactor. The mixture was stirred mechanically by a two-blade paddle (5.5 cm) at
(R2)
(R3)
H2 S þ 2KOH ! K2 S þ 2H2 O
(R4)
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As shown in Reaction (R2), the reaction of o-phenylene diamine and carbon disulfide to synthesize 2-mercaptobenzimidazole (MBI) is slow, i.e. at the beginning of reaction, the rate is low. However, Reactions (R3) and (R4) take place immediately even a trace amount of MBI was produced from Reaction (R2). Therefore, the total reaction shown in (R1) is greatly enhanced toward to the right hand side of Reaction (R2). Hence, the reaction is enhanced in the presence of potassium hydroxide. In addition, carbon disulfide can further react with potassium hydroxide to form CS32 and CO32, i.e.: 3CS2 þ 6KOH ! 2K2 CS3 þ K2 CO3 þ 3H2 O
(R5)
As shown in Reactions (R2)–(R5), potassium hydroxide does not directly participate in the production of 2-mercaptobenzimidazole (MBI) which is shown in Reaction (R2). In Reactions (R3) and (R4), KOH only participates in the reaction with MBI to produce the potassium salt of MBI, and in the reaction with H2S to produce K2S, respectively. These two reactions, which shift the total reaction toward to the right hand side of Reaction (R1), both enhance the reaction of o-phenylene diamine and carbon disulfide according to LeChatelier’s principle. The formation of potassium salt of MBI can be easily converted to MBI simply by adding acid compound to the solution of potassium salt of MBI. 4. Results and discussion
Fig. 2. Effect of the amount of potassium hydroxide on the conversion of o-phenylene diamine in MeOH/H2O solution; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 50 mL of MeOH/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
QOH), etc. These results exhibit the potential of potassium hydroxide in the reaction of o-phenylene diamine and carbon disulfide. 4.1. Effect of the KOH amount
In this work, the inexpensive potassium hydroxide is used to enhance the reaction, avoiding the use of high priced quaternary ammonium salt. Fig. 1 shows the effect of various base compounds on the conversion of o-phenylene diamine. It is clear that potassium hydroxide has high reactivity to enhance the reaction of o-phenylene diamine and carbon disulfide even in the absence of other base compounds, including tripropylamine (TPA), tetrabutylammonium bromide (TBAB), tetrabutylammonium hydroxide (TBAOH or
As stated, KOH is the key compound to promote the reaction. The effect of the amount of KOH on the conversion of o-phenylene diamine in MeOH/H2O (volume ratio: 4/1; protic solvent) solution is shown in Fig. 2. This conversion is increased with the increase in the amount of KOH up to 1.0 g approximately. Further increasing the amount of KOH (>1.0 g) leads to decrease the conversion of o-phenylene diamine and decrease the reaction rate. A plot of the conversion (X) versus the amount of KOH at various reaction times is shown in Fig. 3. A maximum conversion is obtained at an appropriate amount of KOH. This peculiar result can be
Fig. 1. Effect of the various base compounds on the conversion of o-phenylene diaminein MeOH/H2O solution; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 50 mL of MeOH/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
Fig. 3. A plot of the conversion of o-phenylene diamine vs. amount of KOH at various reaction times; Same reaction conditions as given in Fig. 2.
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explained by the complicate reaction of CS2, KOH and C6H4(NH2)2. When the amount of KOH in stoichiometric quantity is less than that of CS2, CS2 in a relatively large quantity dominantly reacts with C6H4(NH2)2 to produce MBI. Meanwhile, KOH at this stage also continuously reacts with H2S to produce K2CS3 and K2CO3 for ionic neutralization. Therefore, the conversion is increased with the increase in the KOH amount. Nevertheless, CS2 will also continuously react with KOH, when the amount of KOH in stoichiometric quantity is larger than that of CS2 (e.g., the amount of KOH is larger than 1.0 g). In fact, carbon disulfide reacts both with C6H4(NH2)2, H2S and KOH in the reaction solution. At this situation, only a small amount of CS2 reacts with C6H4(NH2)2 to produce MBI, i.e. not all amount of CS2 participate in reacting with o-phenylene diamine. Under this circumstance, the conversion is suppressed at a higher concentration of KOH. Therefore, the conversion of o-phenylene diamine is decreased with further increase in the amount of KOH above 1.0 g. In contrast, the results obtained from the reactions using various amounts of KOH carried out in a DMF/H2O (volume ratio: 4/1; aprotic solvent) are shown in Fig. 4. The conversion is increased monotonously with the increase in the amount of KOH. These aprotic solvents, which do not act as hydrogen-bond donors since their C–H bonds are not strongly enough polarized, have large dielectric constants (er > 15), sizeable dipole moments and low ETN -values (as empirical parameter of solvent polarity) (Ebel, 1990). Those chemicals, such as: dimethylsulfoxide (DMSO), N,Ndimethylformamide (DMF), acetonitrile (MeCN), tetrahydrofuran (THF) and cyclic carbonates (propylene carbonate) are the most important aprotic solvents. However, those aprotic solvents contain hydrogen atoms bound to the electronegative elements, such as: F–H, –O–H, –N–H, etc. In contrast, the protic solvents are hydrogen bond donors, the dielectric constants are usually larger, and the ETN -values lie between 0.5 and 1.0; indicating that these solvents are
strongly polar. Typical chemicals are water, alcohols, formamides, etc. The present result using DMF (aprotic solvent) conflicts with the results obtained from the reaction using protic solvents, in which the reaction is inhibited by a large amount of KOH (Fig. 2). This is probably due to the competition in Reactions (R2), (R3), (R4) and (R5) when the aprotic solvent, including DMF, DMSO, THF and MeCN mixed with water as the organic solution. The reactivity of the reactants, which are surrounded by the organic solvent, are affected by the interaction with the organic solvent via solvation and hydration. Therefore, the reactivity of the species in different organic solvents does not correspond to the order of dielectric constant or the Dimroth-Reichardt parameter of organic solvents ETN . In principle, Reactions (R2)–(R4), which are favorable for synthesizing MBI, are all faster than the reaction (R5). As shown in Fig. 4, the conversion reaches 90% within 15 min.
Fig. 4. Effect of the volume ratio of DMF to H2O on the conversion of ophenylene diamine in DMF/H2O solution; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 50 mL of DMF/ H2O (volume ratio: 4/1), 600 rpm, 40 8C.
Fig. 5. Effect of the organic solvents on the conversion of o-phenylene diamine in various organic solvents; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 40 mL of organic solvent, 10 mL of H2O, 600 rpm, 40 8C.
4.2. Effect of the organic solvents In this work, aprotic solvent (DMF, DMSO, THF and MeCN), and protic solvent (EtOH and MeOH) were used as the organic solvent to dissolve o-phenylene diamine. Therefore, a mixture of organic solvent and water is used to dissolve KOH. The effects of the organic solvents on the conversion of ophenylene diamine are shown in Fig. 5. The order of the reactivity for these organic solvents are: DMSO DMF > MeCN > THF > MeOH EtOH. It is clear that these aprotic solvents have high reactivity. The order of the dielectric constants for these organic solvents are: DMSO (46.5) > DMF (36.7) > MeCN (35.94) > MeOH (32.66) > EtOH (24.55) > THF (7.58). The conversion is not directly related to the dielectric constants of the organic solvents, i.e. an organic solvent with high dielectric constants do not lead to a high conversion of o-phenylene diamine. In addition, the Bronsted acid–base behaviors probably are the main factor influencing the reaction. This is because the reaction has to be carried out in
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Fig. 6. Effect of the volume ratio of DMSO to H2O on the conversion of o-phenylene diamine; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 7.14 103 mol of KOH, 50 mL of DMSO/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
Fig. 8. Effect of the volume ratio of MeCN to H2O on the conversion of o-phenylene diamine; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 7.14 103 mol of KOH, 50 mL of MeCN/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
a basic environment. For example, MeCN, EtOH and MeOH are relatively neutral compounds. Therefore, the conversion is relatively low in using MeCN, EtOH and MeOH as the organic solvents. In fact, water has high dielectric constants (78.3) of polarity compound. The order of the polarity of these solvents are: H2O (1.00) > formamide (0.799) > MeOH (0.762) > N-methyl formamide (0.722) > EtOH (0.654) > propylene carbonates (0.491) > MeCN (0.463) > DMSO (0.444) > DMF (0.404) > THF (0.207). The conversion of o-phenylene diamine is also not related to the polarity of the solvent.
o-phenylene diamine For these three systems, the conversions are all increased with the increase in the amount of organic solvent even the dielectric constant of water is 78.3. However, the reaction solution should contain sufficient water at least to dissolve KOH for further reaction. 4.4. Effect of the C6H4(NH2)2 amount
Figs. 6–8 show the effect of the volume ratio of the organic solvent to water for typical aprotic solvents, such as: DMSO, DMF and MeCN, respectively, on the conversion of
In this work, an excess large amount of CS2 is used in the reaction, so o-phenylene diamine is thus the limited compound. The effect of the amount of C6H4(NH2)2 on the conversion is shown in Fig. 9. The conversion is decreased with the increase in the amount of o-phenylene diamine. This result is the same as that of obtaining from the liquid–liquid two-phase catalyzed reaction by tertiary amine and quaternary ammonium salts. This phenomenon is probably due to the fact that the reaction system contains more than one basic reaction. The reason for
Fig. 7. Effect of the volume ratio of DMF to H2O on the conversion of o-phenylene diamine; 3.18 103 mol of o-phenylene diamine, 8 M ratio of carbon disulfide to o-phenylene diamine, 7.14 103 mol of KOH, 50 mL of DMF/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
Fig. 9. Effect of the amount of o-phenylene diamine on the conversion o-phenylene diamine in MeOH/H2O solution; 2.70 102 mol of carbon disulfide, 7.14 103 mol of KOH, 50 mL of MeOH/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
4.3. Effect of the volume ratio of organic solvent to water
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Fig. 10. Effect of the amount of carbon disulfide on the conversion o-phenylene diamine in MeOH/H2O solution; 3.18 103 mol of o-phenylene diamine, 7.14 103 mol of KOH, 50 mL of MeOH/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
Fig. 11. Effect of temperature on the conversion of o-phenylene diamine in DMF/H2O solution; 3.70 103 mol of o-phenylene diamine, 2.70 102 mol of carbon disulfide to o-phenylene diamine, 7.14 103 mol of KOH, 50 mL of DMSO/H2O (volume ratio: 4/1), 600 rpm, 40 8C.
the occurrence is accounted as follows:
5. Conclusion
R3 N þ CS2 ! R3 N CS2
The synthesis of 2-mercaptobenzimidazole (MBI) from the reaction of o-phenylene diamine and carbon disulfide was carried out in a homogeneous solution in the presence of KOH successfully. No catalysts are required to promote the reaction in this work. Potassium hydroxide does not directly participate in the reaction of o-phenylene diamine and carbon disulfide. However, the reaction of MBI and KOH to form the potassium salt of MBI, and the reaction of hydrogen sulfide and KOH to produce the inert compound K2S, both enhance the total reaction of synthesizing MBI. Using protic solvent, an appropriate amount of KOH is recommended to obtain a high yield of MBI. In contrast, the conversion of o-phenylene diamine is increased with the increase in the amount of KOH when the reaction is carried out in an aprotic solvent/water mixed solution. This peculiar phenomenon is effectively explained by the property of the aprotic or protic solvents with water. The conversion is increased with the increase in the amount of carbon disulfide, in the volume ratio of organic solvent to water, and in temperature. However, the conversion of o-phenylene diamine is decreased with the increase in the amount of o-phenylene diamine.
C6 H4 ðNH2 Þ2 þ R3 N CS2 ! C6 H4 ðNH2 ÞCS2 R3 N þ 2Hþ ! C6 H4 ðNH2 ÞCþ S þ H2 S þ R3 N ! C6 H4 ðNÞðNHÞCSH þ H2 S þ R3 N 4.5. Effect of the CS2 amount In this work, carbon disulfide is used in large excess. The effect of the amount of CS2 on the conversion of o-phenylene diamine is shown in Fig. 10. The conversion is still increased with the increase in the amount of CS2. In addition, CS2 also reacts with KOH to form inert salt, as shown in Reaction (R3). 4.6. Effect of temperature A typical reaction in the DMF/H2O for investigating the temperature effect was carried out. The results are depicted in Fig. 11. As expected, the conversion is increased with the increase in temperature. In this work, the kinetic model in describing the reactions of o-phenylene diamine and carbon disulfide by potassium hydroxide in a mixture of organic solvent (protic or aprotic solvents) and water is not available. The reason is that neither the first-order nor the second-order rate law can be applied to model the kinetic behaviors of the reaction. Only the method of the initial rates is employed to account for the experimental data at early reaction time. An Arrhenius plot of ln(1 X) versus 1/T by the method of initial rates is obtained to get the activated energy of the reaction near 28.91 kcal/mol.
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