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Ind. Eng. Chem. Res. 2008, 47, 1784-1792
Kinetic Study of S-Alkylation of 2-Mercaptobenzimidazole with Allyl Bromide in a Two-Phase Medium Biing-Lang Liu and Maw-Ling Wang*
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Department of Chemical Engineering, Wu Feng Institute of Technology, Ming-Hsiung, Chiayi County, Taiwan, 621, ROC, and Department of EnVironmental Engineering, Hungkuang UniVersity, Shalu, Taichung County, Taiwan, 433, ROC
The S-alkylation (substitution on sulfur atom) of 2-mercaptobenzimidazole (MBI) by allyl bromide (RBr) catalyzed by quaternary ammonium salts was successfully carried out in an aqueous solution of KOH/organic solvent two-phase medium. No product was obtained from N-alkylation (substitution on the nitrogen atom) during or after the reaction period using a limited quantity of allyl bromide at a low alkaline concentration of KOH. The reaction is greatly enhanced by adding a small amount of potassium hydroxide. The conversion of allyl bromide for the substitution of MBI either by cetyltrimethylammonium bromide (CTMAB), benzyltriethylammonium chloride (BTEAC), polyethylene glycol-400 (PEG-400), or tetrabutylammonium hydroxide (TABOH) catalyst in the presence of KOH is larger than that using KOH only. However, the conversion of allyl bromide for the alkylation of MBI by tetrabutylammonium bromide (TBAB), tetrabutylammonium hydrogensulfate (TBAHS) and tetrabutylammonium iodide (TBAI) catalyst in the presence of KOH is less than that using KOH only. On the basis of the experimental evidence, a rational reaction mechanism is proposed and a kinetic model is derived, from which a pseudo-steady-state hypothesis (PSSH) is applied to the reaction system. The kinetic behaviors and the characteristics of the reaction are sufficiently described by the pseudo-first-order rate law. This investigation covers the effects on the conversion of the reactant and the apparent rate constants (kapp) due to the reaction conditions, including the agitation speed, amount of TBAOH (or QOH) catalyst, amount of KOH, quaternary ammonium salts, volume of water, volume of dichloromethane, amount of 2-mercaptobenzimidazole, amount of allyl bromide, organic solvents, and temperature. Introduction Molecular collisions are the essential requirement for a chemical reaction. Therefore, the reaction of two reactants that remain separated in two immiscible phases is slow due to limited molecular contact area and their mutual solubility. The conventional methods to overcome this problem are to carry out the reaction at high temperature, at high agitation speed, or using a solvent or cosolvent with both lipophilic and hydrophilic properties. However, much energy is consumed to bring the reaction to a high temperature. Furthermore, byproducts usually accompany the generation of the main products. In using a cosolvent to dissolve both reactants, the reactivity of the nucleophilic reagent is decreased for the solvation and hydrogen bonding with the protic solvent. Hence, the reaction rate is low. Although the solvation of the aprotic solvent and the nucleophilic reagent is minimized when the reaction is carried out in an anhydrous condition, nevertheless the high boiling-point aprotic solvent is difficult to recover. Therefore, these improvements in the reaction of two immiscible reactants are still limited. This problem of two immiscible reactants was not resolved until the development of phase-transfer catalysis (PTC) for the synthesis of organic chemicals (Dehmlow and Dehmlow, 1993; Freedman, 1986; Keller, 1986 and 1987; Starks, 1985; Starks, Liotta, and Halpern, 1994; Weber and Gokel, 1977) from two immiscible reactants. In this method, quaternary salts are conventionally employed as the phase-transfer catalysts, dra* To whom correspondence should be addressed. Tel.: +886-42631-8652, ext. 4175. Fax: +886-4-2652-9226. E-mail: chmmlw@ sunrise.hk.edu.tw.
matically increasing the conversion and the reaction rate. Today, PTC is extensively applied as an effective tool for synthesizing organic chemicals via alkylation, displacement, oxidation and reduction, or dichlorocarbenation and polymerization (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. Basically, two reaction steps participate in synthesizing MBI derivatives. First, the most attractive step is the reaction of o-phenylene diamine and carbon disulfide to synthesize MBI by tetraalkylammonium salt (Wang and Liu, 1998, 2005) and tertiary amines (Wang and Liu, 1995, 1996, and 1998). The synthesis of MBI through the reactions of carbon disulfide and o-phenylene diamine either in a homogeneous or two-phase solution were thus carried out in the presence of KOH and the quaternary ammonium salts (Wang and Liu, 2005, 2006). Second, the derivatives of MBI can then be obtained by S-alkylation or N-alkylation of MBI with a nucleophilic alkylation reagent. This current work investigates the S-alkylation of MBI to synthesize the MBI derivatives in an aqueous solution of KOH/ organic solvent two-phase medium via phase-transfer catalysis (PTC). The nuclophile allyl bromide is employed as the alkylation reagent. It is found that only the product by S-alkylation was obtained in using a limited amount of allyl bromide in the present reaction system, and no product was obtained from N-alkylation. The reaction is greatly enhanced both in the presence of potassium hydroxide and phase-transfer catalysts. On the basis of the experimental data, a rational mechanism is proposed to account for the observed reaction. A
10.1021/ie0709537 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/20/2008
Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008 1785
kinetic model is derived, from which the pseudo-steady-state hypothesis (PSSH) is applied. A simplified pseudo-first-order rate equation is derived to describe the experimental data. Effects of the reaction conditions on the conversion and the apparent rate constants (kapp) were investigated in detail. Experimental Section Materials. 2-Mercaptobenzimidazole (MBI, C6H4(N)(NH)CSH), allyl bromide, potassium hydroxide, phase-transfer catalyst including cetyltrimethylammonium bromide (CTMAB), benzyltriethylammonium chloride (BTEAC), tetrabutylammonium bromide (TBAB), tetrabutylammonium iodide (TBAI), tetrabutylammonium hydrogensulfate (TBAHS), tetrabutylammonium hydroxide (TBAOH) and polyethylene glycol (PEG400), organic solvents including benzene, toluene, chloroform, chlorobenzene and dichloromethane, and other reagents used were all GR-grade chemicals for synthesis. Procedures. A. Synthesis of the Active Intermediate (ArSQ, C6H4(N)(NH)CS-N(C4H9)4). MBI is soluble in the alkaline solution of KOH to produce the potassium salt of MBI (ArSK). Known quantities of TBAOH and ArSK are mixed in the aqueous solution to produce the product in the form of a solid precipitate (ArSQ). The product was then identified by NMR spectrum (1H, DMSO D-solvent) in which four peaks appear on the chemical shift between 1.0 and 3.5 ppm shown in the NMR spectrum. The ratio of hydrogen a/b/c/d is 3:2:2:2. This confirms that the reaction of ArSK and the TBAOH (or QOH) catalyst takes place in the aqueous solution to produce the active intermediate ArSQ. B. Identification of the Alkylation of the MBI Product (ArSR). Normally, S-alkylation and N-alkylation may take place both from the reaction of MBI and the alkylation agent catalyzed by the phase-transfer catalyst. However, only the product of MBI derivatives by S-alkylation is produced when using a limited quantity of allyl bromide under mild reaction conditions. In this work, the product was identified both by NMR and GCMASS. In the NMR analysis, the chemical shift is 2.2, 4.5, and 6.9-7.7 ppm, shown in the NMR spectrum where the chemical shift of 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. C. Kinetics of Synthesizing the Derivatives of MBI. The reactor was a 150 mL four-necked Pyrex flask capable of agitating the solution, inserting the thermometer, taking samples, 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 °C. To start an experimental run, known quantities of MBI, caffeine (internal standard), and potassium hydroxide (KOH) were dissolved in the organic-phase solution (dichloromethane and water mixture) and introduced into the reactor. The liquid solution was stirred mechanically by a two-bladed paddle (5.5 cm) at 1000 rpm. Then, a mixture of allyl bromide, TBAOH, and dichloromethane in the liquid phase was introduced to the reactor to initiate the reaction. During the reaction, an aliquot sample of 0.1 mL was withdrawn from the solution at a chosen time. The sample was immediately poured into methanol at 4 °C for dilution and retardation of the reaction and then analyzed by HPLC. The product MBI derivative for identification was purified from the reaction solution by vacuum evaporation to strip off
Scheme 1
the organic solvent. It was then recrystallized from ethanol as white crystals. The product of the MBI derivative (ArSR) and the reactants (allyl bromide RBr and 2-mercaptobenzimidazole ArSH) were all 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 reactants and product. The column used was Merck RP-8 (5 µm), λ ) 254 nm. The eluent was CH3CN/H2O ) 1/1 with a flow rate of 1.2 mL/min.
Reaction Mechanism and Kinetic Model In this work, quaternary ammonium salt (e.g., tetrabutylammonium bromide, QBr, or tetrabutylammonium hydroxide, TBAOH) is served as the phase transfer catalyst. The overall reaction in the two-phase medium is expressed as Scheme 1 (R1). Basically, 2-mercaptobenzimidazole (MBI or ArSH) does not dissolve in pure water even in the presence of tetrabutylammonium hydroxide (TBAOH). However, it dissolves in the aqueous solution of KOH and reacts with KOH to produce the potassium salt of MBI (ArSK). Then, ArSK further reacts with the catalyst (QBr or TBAOH) in the aqueous phase to produce an active organic-soluble ArSQ, which prepares for reacting with the organic substrate (RBr) in the organic phase. The desired product (ArSR) is thus produced from the two-phase reaction. The regenerated catalyst QBr is produced from the organicphase reaction and then transfers back to the aqueous-phase solution for further reaction. By appropriately using the limited amount of allyl bromide (RBr), only S-substitution at low aqueous concentration of KOH occurs in the reaction. On the basis of the experimental evidence, the mechanism of the catalyzed reaction of MBI and allyl bromide (RBr) in an aqueous solution of KOH/organic solvent two-phase medium can be expressed as,
In this work, the amount of MBI (ArSH) is in large excess relative to that of allyl bromide (RBr). From the experimental observation, no product from the N-alkylation of MBI was
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observed in using a limited quantity of allyl bromide, and only the product of MBI derivatives (ArSR) was obtained by S-alkylation in the organic-phase solution. Furthermore, the reaction of MBI and KOH to produce ArSK quickly reaches an equilibrium state. Therefore, the main reaction includes four steps, that is, the aqueous-phase reaction of ArSK and QBr to produce the active intermediate (ArSQ), the mass transfer of the active intermediate (ArSQ) to the organic-phase solution from the aqueous-phase solution, the reaction of the active intermediate (ArSQ) and allyl bromide (RBr) in the organic phase, and the mass transfer of the regenerated catalyst (QBr) back to the aqueous-phase solution from the organic-phase solution. The four steps of reactions can be expressed as,
where f is the volume ratio of organic phase over aqueous phase, and mArSQ and mQBr are the distribution coefficients of ArSQ and QBr between two phases, respectively. A PSSH is employed to simplify the rate equation, that is, the rate of change of the active intermediate ArSQ either in the aqueous and organic phases and the rate of change of catalyst QBr either in the aqueous and organic phases are zero. Thus, for [ArSQ](o) and [ArSQ](a), we have,
d[ArSQ](a)
d[ArSQ](o)
)
dt
dt
(
k2[ArSK](a)[QBr](a) ) KArSQAf [ArSQ](a) -
k2
ArSK(a) + QBr(a) 98 ArSQ(a) + KBr(a)
(
)
[ArSQ](o) mArSQ
)
[ArSQ](o)
k3[ArSQ](o) ) KArSQA [ArSQ](a) -
KArSQ
ArSQ(a) 98 ArSQ(o)
(7)
From eqs 2-(3) and (7), we have
k1
ArSH(s) + KOH(a) 98 ArSK(a) + H2O
)0
mArSQ
(8)
(9)
k3
ArSQ(o) + RBr(o) 98 ArSR(o) + QBr(o) KQBr
QBr(o) 98 QBr(a)
(R3)
where ArSH, ArSK, ArSQ, and ArSR denote MBI, the potassium salt of 2-mercaptobenzimidazole, the active intermediate, and the final product, respectively. In addition, RBr denotes the species allyl bromide. The subscripts, s, a, and o represent the species in the solid phase, in the aqueous phase, and in the organic phase, respectively, whereas k1 and k2 are the intrinsic rate constants in the aqueous phase and k3 is the intrinsic rate constant in the organic phase. KQBr and KArSQ are the mass transfer coefficients of QBr and ArSQ, respectively. In this work, a simple kinetic rate law and a two-film theory are used to model the reaction system. The rates of the reacting species shown in R2 can be expressed as,
d[ArSK](a)
dt
dt
) k2[ArSK](a)[QBr](a) -
(
d[ArSQ](o)
(
d[QBr](o) dt
)
(2)
)
(3)
[ArSQ](o) mArSQ
) -k3[ArSQ](o)[RBr](o) + KArSQA [ArSQ](a) -
dt
(
k3[RBr](o)
1 mArSQ
KArSQA
)
+ 1 [ArSQ](o)
(10)
Substituting eq 10 into eq 8, we obtain
[QBr](a) )
fk3[RBr](o) k2[ArSK](a)
[ArSQ](o)
(11)
Similarly, for [QBr](o) and [QBr](a) we have
d[QBr](a) dt
) k1[ArSH](a)[KOH](a) - k2[ArSK](a)[QBr](a) (1)
KArSQAf [ArSQ](a) -
d[QBr](a)
[ArSQ](a) )
)
d[QBr](o) dt
)0
(12)
From eqs 4 and 12, we obtain
d[ArSQ](a)
dt
From eq 9, the concentration of ArSQ in the aqueous phase can be expressed in terms of the concentration of ArSQ in the organic phase, that is
[ArSQ](o) mArSQ
) k3[ArSQ](o)[RBr](o) -
k3[RBr](o) k2[ArSK](a)
}
[ArSQ](o) (13)
Q(o) ) V(o)([ArSQ](o) + [QBr](o)) + V(a)([ArSQ](a) + [QBr](a)) (14)
[ArSQ](o) )
Q(o) V(o)
{
1+
1 fmArSQ
k3[RBr](o)
KQBrA([QBr](a) - mQBr[QBr](a)) (5) ) - k3[ArSQ](o)[RBr](o)
KQBrA
+ mQBr
Substituting eqs 8, 10, and 12 into eq 13, we obtain
KQBrAf([QBr](o) - mQBr[QBr](a)) (4)
dt
k3[RBr](o)
The total amount of catalyst QBr added to the reaction solution is Qo, that is,
) -k2[ArSK](a)[QBr](a) +
d[RBr](o)
{
[QBr](o) ) f
(6)
k2[ArSK](a)
[
+
k3[RBr](o) fKArSQA
1 + fmQBr +
+
]}
k2[ArSK](a) KQBrA
-1
(15)
Define the following Damkohler numbers (DaArSQ) and DaQBr) for QBr and ArSQ, respectively, and the Ra number as
Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008 1787
DaArSQ )
k3[RBr](o) KArSQA DaQBr ) Ra )
)
k3(1 - X)[RBr](o),0 KArSQA
k2[ArSK](a) KQBrA
k3[RBr](o) k2[ArSK](a)
where X is the conversion of RBr, that is
X)1-
[RBr](o)
(16)
[RBr](o),i
where subscript i denotes the initial condition of the species. Thus, eq 15 can be reduced to the following dimensionless form, that is
[ArSQ](o) )
Q(o) V(o)
{
1+
DaArSQ 1 + + fmQBr f
}
Ra[1 + fmQbr + DaQBr]
Figure 1. Effect of the amount of KOH, TBAOH, and TBAB + KOH catalyst on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole (MBI); 2.00 g of MBI, 1.00 g of allyl bromide, 0.4 g of KOH, volume ratio of CH2Cl2/H2O ) 50 mL/50 mL, 1000 rpm, 30 °C (note, blank denotes no catalyst and KOH participating in the reaction).
-1
(17)
For most phase transfer catalytic reactions, the mass transfer of ArSQ and QBr between two phases is fast. In addition, the aqueous-phase reaction rate is larger than that in the organic phase. For this, the Damkholer numbers DaArSQ and DaQBr, and Ra are all small. Thus, eq 17 is reduced to
[ArSQ](o) )
Q(o) V(o)
[
1+
1 fmQBr
]
(18)
Furthermore, the distribution coefficients of ArSQ and QBr are all larger than unity. For this, the concentration of ArSQ in the organic phase is inversely proportional to the volume of the organic solvent, as expected.
[ArSQ](o) )
Q(o) V(o)
(19)
The concentration of ArSQ in the organic phase can be considered as constant for small values of DaArSQ and DaQBr at early stages of the reaction. Thus, the concentration of RBr in the organic phase follows a pseudo-first-order rate law. Therefore, eq 6 can be simplified as,
d[RBr](o) dt
) -kapp[RBr](o)
(20)
where the apparent rate constant kapp is given as
kapp ) k3[ArSQ](o)
(21)
Equation 20 is rewritten as,
- ln(1 - X) ) kappt
(22)
by plotting the experimental data -ln(1 - X) versus t to get a straight line with slope kapp. Results and Discussion As shown in the reaction mechanism, the ionic reaction of 2-mercaptobenzimidazole (MBI or ArSH) and potassium hy-
Figure 2. Effect of the amount of TBAOH catalyst on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole (MBI); 2.00 g of MBI, 1.00 g of allyl bromide, 0.4 g of KOH, volume ratio of CH2Cl2/H2O ) 50 mL/50 mL, 1000 rpm, 30 °C (note, blank denotes no catalyst and KOH participating in the reaction).
droxide to produce the potassium salt of mercaptobenzimizadole (ArSK) and the subsequent reaction with tetrabutylammonium salt to produce the active intermediate (ArSQ) in the aqueous phase are fast compared to the organic-phase reaction. Furthermore, the mass transfer of the species (ArSQ and QBr) between two phases is also fast (Wang and Wu, 1991; Wang and Yang, 1991). It is thus clear that the organic-phase reaction is the ratecontrolling step for the whole reaction. Basically, the S- and N-alkylation of 2-mercaptobenzimidazole may take place under 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 period and no products are obtained from N-alkylation, if the stoichiometric molar quantity of allyl bromide is less than that of 2-mercaptobenzimidazole at a relatively low concentration of alkaline solution. On the basis of this evidence, the kinetics of the phase-transfer catalysis is discussed below. (a) Effect of the Amount of TBAOH Catalyst. The basic quaternary ammonium salts reacted with the potassium salt of MBI (ArSK) to produce an active intermediate ArSQ, which can react with the organic-phase reactant to produce the desired product (ArSR). Figure 1 shows the effect of the amount of
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Table 1. Effects of the Reactions’ Conditions on the Apparent Rate Constants (kapp, min-1) Catalyzed by Quaternary Ammonium Salts; 2.00 g of ArSH, 0.4 g of KOH, 0.002 mol of of TBAOH Catalyst, 1.00 g of Allyl Bromide, 50 mL/50 mL of CH2Cl2/H2O, 30 °C, 1000 rpma TBAB, g kapp × 103
blank 0.13
0 14.08
0.16 13.80
0.30 12.63
0.50 10.88
0.75 8.89
1.00 7.13
catalysts kapp × 103
blank 0.13
TBAHS 4.05
TBAI 7.57
TBAB 8.24
only KOH 9.68
PEG-400 11.35
TBAOH 14.18
BTEAC 30.44
KOH, g kapp × 103
blank 0.13
0 1.08
0.10 2.17
0.20 5.00
0.40 14.18
0.60 20.75
0.80 27.33
1.00 28.42
agitation, rpm kapp × 103
0 4.58
200 8.33
400 12.08
600 12.50
800 13.58
1000 14.18
ArSH, g kapp × 103
1.50 14.00
2.00 14.18
3.00 15.40
4.00 15.80
5.00 15.70
RBr, g kapp × 103
1.50 12.83
2.00 14.18
3.00 24.83
4.00 28.50
5.00 30.17
H2O, mL kapp × 103
20 13.67
30 14.69
40 14.80
50 14.18
60 14.56
CH2Cl2, mL kapp × 103
30 16.00
40 14.20
50 14.18
60 11.70
70 10.80
solvent kapp × 103
CH2Cl2 14.18
CHCl3 7.10
C6H5Cl 9.60
C6H5CH3 4.20
C6H6 6.00
temp., °C kapp × 103
10 3.00
20 6.75
25 10.33
30 14.18
35 20.25
80 10.30
a Note, the conditions for those experiment sets are the same except changing for investigating the effect of the reaction condition on the reaction rate in each item.
TBAOH catalyst on the conversion of allyl bromide. There is almost no reaction between allyl bromide and 2-mercaptobenzimidazole in the absence of both KOH and phase-transfer agent (blank). The conversion is increased with the increase in the amount of TBAOH catalyst. However, it is still low only in the presence of TBAOH catalyst at a low amount of 0.001 and 0.002 mol, respectively, and in the absence of KOH. The conversions of allyl bromide for using 0.004 mol of KOH, TBAOH, and TBAB + KOH compounds are also shown in Figure 1. The order of the enhancement of the reaction for these three compounds is: KOH > TBAOH > TBAB + KOH. As pointed out by Starks, Liotta, and Halpern (1994), a portion of TBAB will react with KOH to produce TBAOH. Therefore, the reactivity of TBAOH is different from that of TBAB + KOH even when these two-compound sets are all 0.004 mol. In this work, the reactivity of TBAOH is obviously greater than that of TBAB + KOH. Figure 2 also shows the effect of TBAOH on the conversion of allyl bromide. In the absence of KOH, the conversion is increased with the increase in the amount of TBAOH from 0.001 to 0.002 mol, but the conversion is also low and the change in the conversion due to the variation of the amount of TBAOH is small too. Nevertheless, the conversion is greatly increased by further adding a small amount of KOH in the presence of QOH. Typical results from studying the effect of the amount of the TBAB catalyst in the presence of KOH on the apparent rate constant (kapp) are shown in Table 1. The kapp values are decreased with the increase in the amount of TBAB catalysts. (b) Effect of the Quaternary Ammonium Salts. In this work, six quaternary ammonium salts, including CTMAB, BTEAC, TBAHS, TBAB, TABOH, and TBAI, were used to examine their reactivity. PEG-400 was also used to test its reactivity. These quaternary ammonium salts and PEG-400 exhibit different reactivity in the presence of KOH. Figure 3 shows the effects of the quaternary ammonium salts as the phase-transfer catalysts on the conversion of allyl bromide. It is obvious that those quaternary ammonium salts such as CTMAB, BTEAC, TBAOH, and PEG-400 act as effective catalysts in enhancing the reaction. The conversions of allyl bromide in using these catalysts are all larger than that of KOH
Figure 3. Effect of the quaternary ammonium salts on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole (MBI); 2.00 g of MBI, 1.00 g of allyl bromide, 0.4 g of KOH, 0.002 mol of the catalysts, volume ratio of CH2Cl2/H2O ) 50 mL/50 mL, 1000 rpm, 30 °C.
alone. This result shows that CTMAB, BTEAC, TBAOH, and PEG-400 are positive catalysts in enhancing the reaction. However, in the presence of KOH, the conversions of allyl bromide in using TBAB, TBAHS, and TBAI as the catalyst are all less than that of using KOH alone. Of course, the reaction is also enhanced only in the presence of KOH. Typical results for the conversion of allyl bromide using TBAHS (negative catalysis) and CTMAB (positive catalysis) as the phase transfer catalysts are shown in Figures 4 and 5, respectively. The reaction is completed within a couple of minutes when CTMAB is used as the catalyst. Therefore, the reaction rate constant (kapp) for CTMAB as the catalyst is difficult to obtain. The conversion is decreased with the increase in the amount of TBAHS, whereas the conversion is increased with the increase in the amount of CTMAB. Furthermore, the reaction rate using CTMAB is larger than that using TBAOH. (c) Effect of the Amount of KOH. Figure 6 shows the effect of KOH on the conversion of allyl bromide in the presence of 0.002 mol of TBAOH (or QOH) catalyst. The conversion is relatively low when no KOH is added, although the expression of the kinetic model shown in eqs 17 and 20 does not include KOH in directly affecting the conversion or the reaction rate.
Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008 1789 Table 2. The Alkalinity of Various Aqueous Solutions (pH values) at 25 °C pH-Values 0.01 M 0.02 M 0.05 M
Figure 4. Effect of the amount of TBAHS on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole (MBI); 2.00 g of MBI, 1.00 g of allyl bromide, volume ratio of CH2Cl2/H2O ) 50 mL/50 mL, 1000 rpm, 30 °C (note, blank denotes no catalyst and KOH participating in the reaction).
Figure 5. Effect of the amount of CTMAB catalyst on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole (MBI); 2.00 g of MBI, 1.00 g of allyl bromide, volume ratio of CH2Cl2/H2O ) 50 mL/ 50 mL, 1000 rpm, 30 °C (note, blank denotes no catalyst and KOH participating in the reaction).
Figure 6. Effect of the amount of potassium hydroxide on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole (MBI); 2.00 g of MBI, 1.00 g of allyl bromide, 0.002 mol of TBAOH (or QOH), volume ratio of CH2Cl2/H2O ) 50 mL/50 mL, 1000 rpm, 30 °C (note, blank denotes no catalyst and KOH participating in the reaction).
However, we believe that KOH affects the environment of the reaction. The distribution of the active intermediate (ArSQ) in the organic phase is highly influenced by the concentration of KOH. The hydration of the nucleophile (ArSK or ArSQ) is also affected by the concentration of KOH. Therefore, the reactivity of the active intermediate (ArSQ) is affected by the amount of
KOH
TBAOH
TBAB
TBAB + KOH
11.82 12.16 12.48
11.80 12.08 12.38
5.95 5.89 5.84
11.70 11.98 12.32
KOH in the aqueous solution. As shown in Figure 6, both the conversion and the reaction rate are increased with increasing the amount of KOH. The effect of the amount of KOH on the apparent rate constant (kapp) is also shown in Table 1. As shown in Figure 3, KOH itself still promotes the S-alkylation in synthesizing the derivative of MBI, even though the reaction was carried out in the absence of quaternary ammonium salt. Table 2 shows the pH values of the aqueous solutions that contain different quaternary ammonium salts and KOH at 25 °C. The pH values of KOH, TBAOH, and TBAB + KOH are about 11.70-12.48. In contrast, the pH values of TBAB are about 5.84-5.95. We conclude that the conversion (or the reaction rate) is highly dependent on the alkalinity of the solution (or pH value), that is the conversion is increased with higher pH value of the aqueous solution. In principle, potassium hydroxide (KOH), which is a basic compound, affects the distribution of the active intermediate (ArSQ) between two phases and the solubility of MBI (ArSH) in the aqueous phase also. The content of ArSQ 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 situations, the chain effects are complicated, although eq 15 (or eq 19) does not explicitly affect the dependence of the amount of KOH on the concentration of ArSQ in the organic phase. However, the concentration of ArSQ in the organic phase is dependent on the amount of KOH implicitly. Although the expression of the kinetic model for [ArSQ](o) shown in eqs 15 (or eq 19) does not include KOH in affecting the conversion or the reaction rate, we believe that KOH affects the environment of the reaction. The distribution of the active intermediate ArSQ in the organic phase is highly influenced by the concentration of KOH. The hydration of the nucleophile (ArSK or ArSQ) is also affected by the concentration of KOH. Therefore, the reactivity of the active intermediate ArSQ is affected by the amount of KOH in the aqueous solution. (d) Effect of the Agitation Speed. The effects of the agitation speed on the conversion of allyl bromide were investigated for 2.00 g of MBI, 1.00 g of allyl bromide, 0.4 g of KOH, 0.002 mol of TBAOH (or QOH) catalyst, at 30 °C, at a 50 mL/50 mL CH2Cl2/H2O volume ratio. The results are shown in Figure 7. It is clear that the reaction follows the pseudo first-order rate law. The conversion is increased significantly with increased agitation speed up to 400 rpm but there was only a minor improvement in the reaction after a further increase in the agitation speed from 400 to 1000 rpm. Therefore, the agitation speed was set at 1000 rpm to study the reaction phenomena from which the resistance of mass transfer stays at a constant value. The effect of the agitation speed on the apparent rate constant (kapp) is shown in Table 1. (e) Effect of the Amount of 2-Mercaptobenzimidazole (MBI or ArSH). The effect of the amount of MBI (or ArSH) on the conversion of allyl bromide (RBr) is shown in Figure 8. It can be seen that the conversion of allyl bromide (or the reaction rate) is not affected by the amount of MBI in which the amount of MBI is used in large excess. From experimental observation, an amount of more than 1.5 g MBI appears as a
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Figure 7. Effect of the agitation speed on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole (MBI); 2.00 g of MBI, 1.00 g of allyl bromide, 0.4 g of KOH, 0.2 g of QOH catalyst, volume ratio of CH2Cl2/H2O ) 50 mL/50 mL, 30 °C.
Figure 9. Effect of the amount of allyl bromide on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole (MBI); 2.00 g of MBI, 0.4 g of KOH, 0.002 mol of TBAOH (or QOH) catalyst, volume ratio of CH2Cl2/H2O ) 50 mL/50 mL, 1000 rpm, 30 °C.
Figure 8. Effect of the amount of MBI (or ArSH) on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole (MBI); 1.00 g of allyl bromide, 0.4 g of KOH, 0.002 mol of TBAOH (or QOH) catalyst, volume ratio of CH2Cl2/H2O ) 50 mL/50 mL, 1000 rpm, 30 °C.
Figure 10. Effect of the volume of water on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole (MBI); 2.00 g of MBI, 1.00 g of allyl bromide, 0.4 g of KOH, 0.002 mol of TBAOH (or QOH) catalyst, 50 mL of CH2Cl2, 1000 rpm, 30 °C.
suspension solid when its mole is larger than that of KOH. In this situation, the MBI suspension solid is insoluble in the solution. 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. (f) 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 that of MBI. Allyl bromide alone displaced the hydrogen on the sulfur atom of MBI rather than on the nitrogen atom when using a limited amount of allyl bromide. This is because the activity of the hydrogen atom on the sulfur atom is greater than that of the nitrogen atom in the MBI molecule. Figure 9 shows the effect of the amount of allyl bromide on the conversion of allyl bromide. As expected, the conversion is increased with greater concentration of allyl bromide in the organic phase. The effect of the amount of MBI on the apparent rate constant (kapp) is shown in Table 1. (g) Effect of the Volume of Water. In principle, the volume of water directly affects the concentration of KOH in the aqueous phase and the distribution of ArSQ between two phases. Therefore, the conversion (or the reaction rate) should be affected by the volume of water. Figure 10 shows the effect of the amount of water on the conversion of allyl bromide (RBr). The conversion of RBr is almost unaffected by the volume of water. The generally accepted reason is that ArSQ stays almost completely in the organic phase. The moles or the concentration of ArSQ in the organic phase are almost constant when the
amount of KOH is fixed at a certain quantity with a large excess amount of MBI. The organic-phase reaction is the ratecontrolling step. Therefore, the conversion is not significantly influenced by the amount of water for the main organic-phase reaction. The effect of the volume of water on the apparent rate constant (kapp) is shown in Table 1. (h) Effect of the Volume of Dichloromethane. Generally, as shown in eq 6, the organic-phase concentration of ArSQ and allyl bromide (RBr) directly affect the rate of the organic-phase reaction. Therefore, the concentration of ArSQ and the concentration of RBr (allyl bromide) in the organic phase are both decreased with a greater volume of CH2Cl2. Figure 11 shows the effect of the volume of CH2Cl2 on the conversion of allyl bromide. It is clear that the conversion of RBr (or the reaction rate) is decreased with a greater volume of CH2Cl2, as expected. The effect of the volume of CH2Cl2 on the apparent rate constant (kapp) is shown in Table 1. (i) Effect of the Organic Solvents. In this work, five organic solvents including dichloromethane, chloroform, chlorobenzene, toluene, and benzene are used to examine their influences on the S-alkylation of MBI, with the results as shown in Figure 12. 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 increase in the
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Figure 11. Effect of the volume of dichloromethane on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole (MBI); 2.00 g of MBI, 1.00 g of allyl bromide, 0.4 g of KOH, 0.002 mol of TBAOH (or QOH) catalyst, 50 mL of H2O, 1000 rpm, 30 °C.
Figure 14. A plot of the Arrhenius equation for the reaction in various organic solvents with the same reaction conditions as given in Figure 12.
order rate law. Similar results were obtained using other organic solvents, such as chloroform, chlorobenzene, toluene, and benzene. The effect of temperature on the apparent rate constant (kapp) is shown in Table 1. An Arrhenius plot for -ln(kapp) versus 1/T is shown in Figure 14. The activation energy is 13.35 kcal/ mol for the reaction carried out in CH2Cl2. The Arrhenius equation for the reaction in dichloromethane is
CH2Cl2: kapp ) 6.196 × 107 exp(-6720/T) Conclusion
Figure 12. Effect of the organic solvents on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole (MBI); 2.00 g of MBI, 1.00 g of allyl bromide, 0.4 g of KOH, 0.002 mol of TBAOH (or QOH) catalyst, volume ratio of organic solvent/H2O ) 50 mL/50 mL, 1000 rpm, 30 °C.
Figure 13. Effect of the temperature on the conversion of allyl bromide in the S-alkylation of 2-mercaptobenzimidazole (MBI); 2.00 g of MBI, 1.00 g of allyl bromide, 0.4 g of KOH, 0.002 mol of TBAOH (or QOH) catalyst, volume ratio of CH2Cl2/H2O ) 50 mL/50 mL, 1000 rpm.
dielectric constant of the organic solvent. The effect of the organic solvents on the apparent rate constant (kapp) is shown in Table 1. (j) Effect of Temperature. The effect of temperature on the conversion of allyl bromide for the reaction carried out in dichloromethane is shown in Figure 13. The increase in temperature enhances both the reaction rate and the conversion of allyl bromide. The reaction also the follows pseudo-first-
In this work, the S-alkylation of 2-mercaptobenzimidazole (MBI) using allyl bromide catalyzed by quaternary ammonium salt was successfully carried out in an aqueous solution of a KOH/organic solvent two-phase medium. Using a limited amount of allyl bromide in a low alkaline concentration of KOH, only S-alkylation takes place in the two-phase solution. The reaction is greatly enhanced by adding a small amount of quaternary ammonium salt and KOH. Even in the absence of quaternary ammonium salt, the reaction synthesizing the product is promoted by the addition of KOH. 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 the CTMAB, BTEAC, TBAOH, and PEG-400 catalysts, amount of KOH, amount of allyl bromide, the alkalinity of the aqueous solution, and temperature; and the conversion is decreased with increased volume of dichloromethane in using TBAB, TBAHS, and TBAI catalysts. However, the conversion (or the reaction rate) is insensitive to the amount of MBI and the volume of water. Of the organic solvents, dichloromethane shows high reactivity in the S-alkylation of synthesizing ArSR. CTMAB and BTEAC catalysts display high reactivity in enhancing the reaction. Acknowledgment 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. Literature Cited (1) Dehmlow, E. V.; Dehmlow, S. S. Phase Transfer Catalysis, Third Edition; VCH: New York, 1993.
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ReceiVed for reView July 12, 2007 ReVised manuscript receiVed December 1, 2007 Accepted December 24, 2007 IE0709537
