3688
Ind. Eng. Chem. Res. 1995,34, 3688-3695
Reaction of Carbon Disulfide and o-Phenylenediamine Catalyzed by Tertiary Amines in a Homogeneous Solution Maw-Ling Wane and Biing-Lang Liu Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China
Mercaptobenzimidazole (MBI) was first synthesized from the reaction of carbon disulfide and o-phenylenediamine catalyzed by tertiary amines in a homogeneous solution. A kinetic model was also proposed, which considered the reaction of carbon disulfide and tertiary amine to form a n active intermediate (R3N-CS21, and further reacted with o-phenylenediamine to produce the desired product. Thus, a n equilibrium reaction of carbon disulfide and tertiary amine was established in the synthesis of MBI. By appropriating choice of the organic solvents, a large conversion of o-phenylenediamine was obtained within a relatively short time interval of reaction. Experimental results indicated that a pseudo-first-order rate law sufficiently expresses the reaction for using carbon disulfide in a large excess amount. Furthermore, a mixture of organic solvents with different polarities was utilized in the reaction to examine the effect of the polarity of solvent on the conversion.
Introduction
Experimental Section
Synthesis of 2-mercaptobenzimidazole (MBI) and its derivatives have been reported. The procedure involves the reaction of o-phenylenediamine and the reactants in a cosolvent of methanol and water catalyzed by expensive quaternary ammonium salts or active carbon (Norit) (Van Allan and Deacon, 1963). However, the reaction was carried out a t a higher temperature t o obtain the desired product for a longer time period. Blocher et al. (1966), Fournier (19711, and Goodman (1975) employed quaternary ammonium hydroxide as a catalyst in the reaction of carbon disulfide and o-phenylenediamine. A 90% yield of product was obtained at a relatively high temperature after 8 h of twophase reaction. However, the kinetics and mechanism of the reaction were not discussed. Yashinori et al. (1990) recently found that MBI was a byproduct in the reaction of enaminones with carbon disulfide to produce 1,2-dithiole-3-thiones(trithiones). Additionally, Broada and Dehmlow (1983) reported that MBI could be synthesized by the reaction of RNH2, CC4, and Na2S catalyzed by quaternary ammonium salts. The spectroscopic identification of MBI and its derivatives was carried out by Saxena et al. (1982)and Suri et al. (1983). The primary objective of this work is to synthesize 2-mercaptobenzimidazole(MBI),which is an important inhibitor, antioxidant, antiseptic, and adsorbent (Moreira et al., 1990; Saxena, 1982; Thomas, 1953; Xue, et al., 1991) in industries, from the reaction of carbon disulfide and o-phenylenediamine. In this work, we first employed tertiary amines as the catalysts to synthesize MBI in a homogeneous solution by appropriating choice of the organic solvents. The polar solvents, e.g., MeCN, MeOH, EtOH, DMSO, DMF, and THF, functioned as the cosolvents t o create a homogeneous solution. The advantage of this approach is that a large reaction rate (or conversion of o-phenylenediamine) was obtained when the reaction was carried out in a polar organic solvent. The effects of the operating conditions on the reaction rate were also investigated. The mechanism of the reaction is also identified from the experimental data.
Materials. Carbon disulfide (CSz), o-phenylenediamine (C&(NH2)2), tertiary amines including (C2H&,N, (C3H7)3N, and (C4H9)3N, and other reagents used were all GR-grade chemicals for synthesis. Procedures. (A) Kinetics of the Reaction of Tributylamine and Carbon Disulfide. Tributylamine ((C4Hg)3Nor Bu3N) (3.5 mL, 0.1 M in MeOH) was put in a W cell, which was immersed in a constant temperature water bath to reach a thermoequilibrium state. Next, carbon disulfide (1pL, 4.74 x lov3 M in MeOH) was injected into the W cell to start the reaction. The absorbance (At) at 364 nm vs time then was studied, and the absorbance (A,) for 10 half-life periods was determined. A plot of ln[AJ(A, - At)] vs time led to a straight line. (B)Kinetics of Synthesizing 2-Mercaptobenzimidazole (MBI). The reactor is a 125-mLfour-necked Pyrex flask, capable of agitating the solution, inserting the thermometer, taking samples, and feeding the reactants. A reflux condenser is attached t o the port of the reactor t o recover carbon disulfide. The reactor is submerged into a constant temperature water bath in which the temperature can be controlled to fO.l "C. To start an experimental run, known quantities of ophenylenediamine, carbon disulfide, caffeine (internal standard), and tertiary amine were dissolved in the organic solvent and introduced into the reactor. The mixture was stirred mechanically by a two-blade paddle (5.5 cm) at 600 rpm. 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 "C for dilution and retardation of the reaction and then analyzed by HPLC. The product MBI for identification was purified from the reaction solution without containing tertiary amine by vacuum evaporation to strip off the organic solvent and carbon disulfide. Next, it was recrystallized from ethanol as white crystals. The product (MBI)and the reactants (carbon disulfide and o-phenylenediamine) were identified by NMR and
* To whom all correspondence should be sent. 0888-5885I95l2634-3688$09.00/0
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 11, 1995 3689 IR analyses. An HPLC Model LCSA (Shimadzu) with an absorbance detector (254 nm, SPD-6A)was employed to measure the amounts of reactants and product. The column used was Shim-Pack CLC-ODS RP-18 (5 pm). The eluent was CH&N/HzO = 20/80 (with 5 mM K H 2 PO4 0.1% H3PO4) (volume ratio) with a flow rate of 1.0 mumin.
+
Kinetic Model of Reaction
(A) Kinetics of the Reaction of CSz and BUN. In this work, carbon disulfide is the limiting reactant while tributylamine is used in a large excess. The reason is that the absorbance of CSz is much greater than that of Bu3N. The reaction of CS2 and Bu3N (R3N) reaches an equilibrium state after 10 min or more, i.e.
R3N
+ CS
2
kl k-1
R3N-CSz
RSN-CS,
+ CS, -R3N-CS2 kl
(R2)
The rate of reaction R2 was expressed as
k, + R3N e RSN-CS, k-i
+ C,H4(",),
k2 +
MBI
+ H,S + R3N
rl = kl[CS21[R3Nl- k-,[R3N-CS21 r2 = k,[R3N-CS21[C6H4(NH,)21
(R5)
(6) (7)
A pseudo-steady-state hypothesis is assumed to apply to the active intermediate R~N-CSZ, i.e. d[R,N-CS,] =O dt No byproducts were obtained. Therefore, it is reasonable to assume that the consumption rate of R3N-CS2 in R5 equals the production rate of R~N-CSZ in R4, i.e. k1[CS2I[R3Nl - k-l[R3N-CS,l = k,[R,N-CS,I[C,H,(",),I
For a constant concentration of R3N, the above equation is integrated:
034)
Reaction R4 is very fast and reaches an equilibrium state within 10 min. Obtaining an 80% conversion of o-phenylenediamine at moderate reaction conditions normally took approximately 2 h. Therefore, we can conclude that the reaction of carbon disulfide and tertiary amine (reaction R4) is in an equilibrium state relative to reaction R5. The reaction rates of R4 and R5 are expressed as
(R1)
where k l and k-1 are the intrinsic rate constants of the forward and backward reactions R1, respectively. The reaction was considered as a simple forward reaction at early time (e.g. 4 min), i.e.
R3N
CS,
(9)
Rearranging eq 9, we obtain
in which K is the equilibrium constant
where
K = k-Jk1 The subscript "0" represents the species at initial condition. Since no other byproducts were produced, eq 2 was rewritten as (4)
In
Applying Beer's law, the concentration of R~N-CSZ was expressed by W absorbance At, i.e.
(11)
Substituting eq 10 into eq 7, we get
It is reasonable to assume k-l[R3N-CSzl>>kdR3N-CSzI[C6H4(NH2)21 at low concentration of o-phenylenediamine or low rate of reaction R3, i.e.
(k-llki) = K >> (kJk1)[C,H4(NH2),I
(13)
Thus, eq 12 is reduced to first-order kinetics with respect to the concentration of CeH4(NH&, i.e. A plot of ln[AJ(A, - At)] vs time leads to obtain a straight line with slope kapp. (B) Kinetics of Synthesizing Mercaptobenzimidazole (-1). The overall reaction is expressed as CS,
+ C,H4(",),
-
C,H,(N)(NH)(SH)
+ H2S
(R3)
In the homogeneous reaction for synthesizing MBI, it is assumed that carbon disulfide first reacts with tertiary amine to form an active intermediate (R3N-CSd. This active intermediate further reacts with o-phenylenediamine t o produce the desired product MBI, i.e.
r2 = (kJ~[CS,l[R3NI[C6H4(NH2),l
(14)
In this work, carbon disulfide was typically used in large excess relative to its stoichiometric quantity. Also, the concentration of R3N remains constant. Therefore, eq 14 is written as
r, = - d[C6H4(NH2)21 = k'a,p[C6H4(~H,),] (15) dt
where
3690 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
k'app = (k2/K)[CS2I[RJ'JI
(16) o 313.15 K
As indicated by eq 15, the reaction is expressed by a pseudo-first-order rate law -ln(l - x)= klappt
(17)
A
308.15 K
a
296.15 K
O
where X is the conversion of o-phenylenediamine and is defined as
0. IO
-
/
O
/i
(18) X = 1 - ([C,H~(NH~)~~[C,H~(NH~)~IO) At a higher [CsH4(NHz)zl, 122[CsH4(NH2)21[R3N-CSsl plays an important role in the expression of kinetics. Its value is probably larger than that of K-I[R~N-CSZI. Thus, eq 12 becomes a zero-order kinetics with respect to [CsH4(NHz)zl,i.e.
r2 = kl[CS21[R3Nl
(19)
For a constant concentration of CS2 and R3N, eq 19 is written as = k"app[C6H4(~H2)2]o(20) r2 = - d[C6H4(NHz)21 dt
where
The reaction is expressed by a zero-order rate law
x =k" aPPt
(22)
where X is given in eq 18. In general, eq 12 can be integrated t o obtain the conversion X of o-phenylenediamine, i.e.
Integration of eq 23 after appropriate rearrangement (k#1)([C,H,(NHz)zI
- [C,H4(NH2)2]0)
+
or in terms of the conversion of o-phenylenediamine X
The two terms in the left-hand side of eq 25 represent the characteristics of zero-order and first-order rate kinetics.
Results and Discussion (A) Kinetics of the Reaction of Carbon Disulfide and Tertiary Amines. Jensen and Nielsen (1963) carried out the reaction of carbon disulfide and tertiary phosphine t o produce a complex intermediate (R~P-CSZ) with a reddish color. The effects of the organic solvents on the reaction of CS2 and R3P were also studied (Fernando and Santos, 1980). The rate constants of the
0
3
2
I
4
Time ( M i n )
Figure 1. Reaction of carbon disulfide and tributylamine in a methanol solvent at various temperatures: [(CdH&N] = 0.1 M, M, I = 364 nm. [C&] = 4.74 x
reactions of CSZand (CzH&P (or (C2H5)2C&P) were measured by W (Trinidad et al., 1986). For the reaction system of CS2 and R3P, both kapp and K should be easily obtained. The reason is that the product R3PCS2 produced from the reaction of CS2 and R3P possesses a strong bond between these two reactant molecules. A reddish colored crystal of (R3P-CS2) was obtained. Thus, carrying out the backward reaction in reaction R4 is relatively easy. In this work, the active intermediate R3N-CS2 cannot be isolated and purified from solution. The reaction Z)~ very slow without rates of CS2 and C ~ H ~ ( N Hare adding tertiary amine and would be dramatically enhanced by adding a small amount of tertiary amine. The W absorbance of CS2 and R3N solution is increased during the reaction. Also, an orangish color, which indicates the product obtained from the reaction of CS2 and tertiary amine, appeared in the solution. However, the bond between CS2 and R3N is relatively weak, and the product R~N-CSZ could not be obtained on pure form. The intrinsic rate constant of reaction R5 was not determined in the conventional way in this work. In this work, the technique of parameter estimation is provided t o determine the rate constants. As indicated in the experimental section, an independent experiment was carried out to examine the reaction of CS2 and R3N in a methanol solution. Usually, it takes about 10 min for the reaction to reach an equilibrium state following reaction R1. However, it is believed that the forward reaction dominates the whole reaction of CSZ and R3N during the first 4 min of reaction. Therefore, the intrinsic rate constant ( k l )of the forward reaction can be obtained from the kinetic data at short reaction times. A plot of W absorbance vs time is shown in Figure 1. A straight line with slope (kapp) is obtained. Using eq 3, the intrinsic rate constants ( K 1 ) are 1.49 x 2.16 x 3.06 x and 4.06 x min-l M-l at 25, 30, 35, and 40 "C, respectively. The activation energy was expressed in terms of Arrhenius equation, i.e. kapp= 4.89 x
lo1 exp(2.60 x
103/T); in MeOH solvent (26)
(B)Identification of the Reaction Mechanism. To further examine the equilibrium reaction R4, two experimental runs in different sequential orders of
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3691 I .o
TEA
T i m e (Min)
Figure 2. Effect of the sequential order of adding reactants on the conversion of o-phenylenediamine; 2.54 x mol of ophenylenediamine, 8 molar ratio of carbon disulfide/o-phenylenediamine, 2.16 x mol of triethylamine (TEA), and 50 mL of DMF, 600 rpm, at 40'"C. 1, First reaction of carbon disulfide and triethylamine for 2 h, and then reacting with o-phenylenediamine after 2 h. 2, All reactants were added to the reactor simultaneously to start the reaction.
reaction procedures were tested in this work. The first one involved conducting the reaction of carbon disulfide and triethylamine (Et3N) for 2 h in advance and then adding o-phenylenediamine. The second procedure was the introduction of carbon disulfide, triethylamine, and o-phenylenediamine dissolving in MeOWHzO (v/v = 4/11 simultaneously t o start the reaction. Results obtained from these two experimental runs are shown in Figure 2. No difference for the conversion was obtained. This result indicates that the reaction of CS2 and R3N to produce the active intermediate (R3N-CS2) is very fast. The reaction of active intermediate and o-phenylenediamine is a rate-determining step for the entire reaction. Therefore, two basic reaction steps were proposed to represent the entire reaction given by R4 and R5. (C) Factors Affecting the Kinetics of Reaction. (i)Effects of the Amount of Catalysts. The effects of the amount of catalysts on the conversion were investigated under the condition of 8 molar ratio of carbon disulfidelo-phenylenediamine. Triethylamine (TEA),tripropylamine (TPA), and tributylamine (TBA) were employed as the catalysts. A typical result obtained for the reaction catalyzed by TEA is shown in Figure 3. The reaction rate is very slow, and the conversion of o-phenylenediamine is low without adding tertiary amine to the solution. The conversion of o-phenylenediamine is obviously increased with the increase of the amount of catalyst. The roles of tertiary amine are as follows: first, the alkaline tertiary amine reacts with hydrogen sulfide, which is a byproduct of the reaction, t o enhance the reaction. Reaction R3 goes forward to the right following LeChatelier's principle. Second, the formation of an active intermediate (R3NCS2) from the reaction of CS2 and R3N also enhances the entire reaction. Therefore, the reaction is enhanced by adding a small amount of tertiary amine. As stated, three tertiary amines were used as the catalysts in the synthesis of MBI in this work. Figure 4 shows the effect of the amount of catalyst on the value of VaPqfor these three tertiary amines. The reaction is insensitive t o the catalysts used in this work. Based on the experimental data, the reaction of CS2 and o-phenylenediamine in a homogeneous solution is
o
TEA system
'Amount o f Catalyst ( M I
Figure 4. Effect of the amount of tertiary amines on the value of kapp;same reaction conditions as given in Figure 3.
catalyzed by tertiary amine. No micelle reaction was obsewed. While triethylamine, tripropylamine, and tributylamine provide good catalytic reactivity. For a s N 2 reaction, the stereo-hindered effect and the alkalinity (i.e., pKa value) are the two important factors in affecting the reaction rate. However, there is not much difference of the pKa values for these three tertiary amines. Nevertheless, based on the molecular structure of tertiary amine, the nonbonded electron pairs on the nitrogen atom will attack the carbon atom of carbon disulfide. It is thus obvious that the stereo-hindered effect is more significant when a small alkyl group is used. Triethylamine is the best one among the three tertairy amines. This is attributed to its higher reactivity for smaller alkyl groups with less stereo-hindered effect on the tertiary amine. In general, a larger reactivity and relative unstable characteristics for small alkyl group in R3N were observed. The stereohindered effect is minimized using triethylamine as the catalyst. (ii)Effect of Temperature. A typical result for the effect of temperature on the conversion for TEA as catalyst is shown in Figure 5. The reaction follows pseudo-first-order rate law. The conversion increased with the increase of temperature. A n Arrhenius plot of -lnWaPp) vs 1/T t o obtain the activation energy is shown in Figure 6. For these three tertiary amines, the
3692 Ind. Eng. Chem. Res., Vol. 34,No. 11, 1995
conversion was obtained after 2 h of reaction at 40 "C. In this work, six organic solvents, including protic and aprotic solvents (i.e., DMF, DMSO, MeCN, MeOH, EtOH, and THF) were employed as the organic solvents in the homogeneous-phasereaction to ehhance the rate. Figure 7 shows the effects of the protic and aprotic solvents on the conversion of o-phenylenediamine. The order of the reactivity for these six organic solvents is as follows: DMF > DMSO > MeCN > MeOH EtOH > THF. The corresponding dielectric constants of the organic solvents are as follows: DMF (37.71), DMSO
0.8
(46.45),MeCN(35.94),MeOH(32.66),EtOH(24.55),and
0
80 Time (Min)
30
m
120
Figure 5. Effect of temperature on the conversion of o-phenylenediamine in acetonitrile solution, 3.18 x mol of o-phenylenediamine, 8 molar ratio of carbon disulfide/o-phenylenediamine, 0.01 M of triethylamine (TEA), and 50 mL of acetonitrile, 600 rpm.
I
o
I
TER system
w.w
3. I
3.2 I/T x la"
THF (7.581, respectively. The protic solvent, such as MeOH and EtOH, which contains an oxygen atom on each molecule of organic solvent, possesses acidic property. The unpaired electrons on the oxygen atom solvate the cation, and the hydrogen atom associates with the anion. Thus, a relatively lower conversion is obtained in MeOH or EtOH solvent. This result indicates that the acidic hydrogen bond does not have a strong catalytic capability. Those aprotic solvents, which do not possess hydrogen bond, are highly polar. Therefore, the aprotic solvent possesses high alkalinity and nucleophilicity to obtain a high conversion. In general, a large conversion is obtained by using a protic solvent or aprotic solvent of high polarity. However, the structure of DMF, which is a amide, is similar to that the tertiary amine. It possesses catalytic property as that of the dimethylaminopyridine (DMAP). Therefore, the effect of DMF on the conversion is larger than that of DMSO. An Arrhenius plot of -ln(VaPp)vs 1/T to obtain the activation energy is shown in Figure 8. The obtained for using these six organic solvents rate coefficients/dapp are
DMF
3.3
(I/K)
Figure 6. Arrhenius plot of ln(kapp)vs 1/T for triethylamine (TEA), tripropylamine (TPA), and tributylamine (TBA) systems; same reaction conditions as given in Figure 5.
dependence of k'app value on temperature is expressed as
VaPp= 1.06 x 1015exp(-1.20 x 1 0 4 / n DMSO
ktapp= 7.82
x
lo1'
(31)
ktapp= 1.39 x 1013exp(-1.09 x 104/T)
(32)
MeCN
exp(-1.06 x lo4/??)
(27)
MeOH
TPA
Yapp= 3.68 x
lo1' exp(-9.23
x 103/n (28)
ktapp= 9.62
x 1014exp(-1.24 x 104/r)
x 10l1exp(-1.02 x 104/r)
(29) The obtained activation energies are 88.05, 76.74, and 84.45 kJ/mol for TEA, TPA, and TBA, respectively. (iii) Effects of Organic Solvents. The primary purpose of selecting organic solvents entails providing the dipole-dipole bond between reactants and organic solvent. The charactersitics of the organic solvent directly affect the reaction rate and the conversion of the reactants. Several nonprotic and nonaprotic organic solvents, including n-decane, n-hexane, chlorobenzene, benzene, and dichloromethane, dissolve CS2, RsN, and C&b(NH2)2. However, the conversion of o-phenylenediamine is low in these solvents. Only about 10%
(33)
E tOH
TBA
ktapp= 7.45
lo8 exp(-7.78
x lo3/??)
x
TEA
ktapp= 2.90
(30)
ktapp= 3.84 x
lo1'
exp(-9.29 x 103/T)
(34)
THF
ktapp= 3.25 x
exp(-2.99 x 104/T) (35) As indicated in Figure 7, the reactivities of o-phenylenediamine in various organic solvents correspond t o their polarities except for DMF. Actually, DMF is also an amide that also possesses catalytic properties. Therefore, the conversion of o-phenylenediamineis quite large in DMF solvent. In order to study the reaction affected by solvent, mixtures of organic solvents with various polarities were used. Figures 9 and 10 show the effects
I .o
OVB-
I .o
Orgmic Solvents a
ow
o
DMSO
0.8.
/
. 0.8-
x
C 0
Orgmic S o t v e n t t w o pure MeCN A 80 : 20 60 : 40 40 : 60 0
: TW
20 : 80
L
$
0.4-
C 0
u
0
Figure 7. Effects of organic solvents on the conversion of o-phenylenediamine; 3.18 x mol of o-phenylenediamine, 8 molar ratio of carbon disulfiddo-phenylenediamine, 0.01 M of triethylamine (TEA), and 50 mL of acetonitrile, 600 rpm, at 40 "C.
3.2
I/T x to"
eo
63
Figure 10. Effect of the polarity of solution on the conversion of o-phenylenediamine in a mixture of MeCN and THF solution; 3.18 x mol of o-phenylenediamine, 8 molar ratio of carbon disulfide, 50 mL of solution mixture, and 0.01 M of triethylamine (TEA), 600 rpm, at 40 "C.
I
-10.0'
3.1
x)
Time (Min)
Time (Min)
3.3
(1/K)
Time ( M i n )
Figure 8. Arrhenius plot of -ln(kapp) vs 1/T in six organic solvents systems; same reaction conditions as given in Figure 7.
Orgmic Solvent (msu : WJ
Figure 11. Effect of the molar ratio of carbon disulfide/ophenylenediamine on the conversion of o-phenylenediamine in acetonitrile solution; 4.76 x mol of o-phenylenediamine, 0.01 M of triethylamine (TEA), and 50 mL of acetonitrile, 600 rpm, at 40 "C.
o pure DMSO A X
0.5
60 : 40 40 : 60
63
Time (Min)
Figure 9. Effect of the polarity of solution on the conversion of o-phenylenediamine in a mixture of DMSO and THF solution; 3.18 x mol of o-phenylenediamine, 8 molar ratio of carbon disulfidelo-phenylenediamine,50 mL of solution mixture and 0.01 M of triethylamine (TEA), 600 rpm, at 40 "C.
of the mixture of DMSOA'HF and the mixture of MeCN/ THF, respectively, on the conversion of o-phenylenediamine. A mixture of higher polarity leads to a high conversion. THF is the least polar organic solvent, and
the conversion is low when using THF as the organic solvent. (iv) Effect of the Molar Ratio of Carbon Disulfide to o-Phenylenediamine. The effect of the molar ratio of carbon disulfide to o-phenylenediamine on the conversion is shown in Figure 11. The conversion is insensitive to the molar ratio of carbon disulfide to o-phenylenediamine when the molar ratio is greater than 5. For a molar ratio of carbon disulfide t o ophenylenediamine smaller than 5, the conversion is increased with an increasing molar ratio of these two reactants. We recommend that the reaction was carried out at a higher concentration of CS2 rather than at a higher concentration of o-phenylenediamine, i.e, the reaction was carried out a t high molar ratio of carbon disulfide to o-phenylenediamine. Therefore, the reaction in this work was carried out using an 8 molar ratio of carbon disulfidelo-phenylenediamine in order t o eliminate the influence of the molar ratio for the two reactants. Based on the experimental data, the reaction was described by a pseudo-first-order kinetics for sustaining the amount of carbon disulfide in a large excess amount. As shown in Figure 12, the conversion of o-phenylene-
3694 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
0
39
80 Time (Min)
Bo
cot
120
Figure 12. Effect of the concentration of o-phenylenediamine on the conversion of o-phenylenediamine in acetonitrile solution; 2.68 x mol of carbon disulfide, 0.01 M of triethylamine (TEA), and 50 mL of acetonitrile, 600 rpm, at 40 "C.
diamine (or the reaction rate) is decreased with the increase of the concentration of o-phenylenediamine. This phenomenon is probably due t o the fact that the reaction system contains more than one basic reaction. The reason for this occurrence is accounted for as follows:
Y.V
Ammt o f KOH and TEA
o KOH=0.2085 g , TER4.01 M
o.e.
-
Bu~N + CS2
-
s--C=S
I
c t
S=C-S-
*!
. . TER-0
M
0.4 -
0
I
I4
KOH-O g
(R6)
H
o;>C+-S-+
.
K W O g TEAEO.01 M o KOH-0.2034 g TER-0 M A
x
.E ?
Bu~N+
I
0.03
Figure 13. Effect of the concentration of o-phenylenediamine on the value of hap,,; same reaction conditions as given in Figure 12.
t
Bu~N+
0.03 0.06 0.07 Amount o f o-pheny l ened I ami ne (HI
S2-+ 2H+ + Bu3N
I!
30
80
Bo
120
Time ( H i n ) Figure 14. Effect of the concentration of poatassium hydroxide on the conversion of o-phenylenediamine in acetonitrile solution, 3.18 x mol of o-phenylenediamine, 8 molar ratio of carbon disulfide to o-phenylenediamine, and 0.01 M of triethylamine (TEA), 600 rpm, at 40 "C.
0 : S C - S H
I I4
A plot of k'app value vs the amount of o-phenylenediamine is shown in Figure 13. At a low concentration of o-phenylenediamine (corresponding to high molar ratio of carbon disulfidebphenylenediamine), the value of klapp remains constant. However, the value of klapp decreases with the increase of the amount of o-phenylenediamine when the concentration of o-phenylenediamine is larger than 0.06 M, which corresponds to a 5 molar ratio of carbon disulfide/o-phenylenediamine. (v) Effect of the Amount of Potassium Hydroxide. Adding a solution of KOH enhances the reaction of carbon disulfide and o-phenylenediamine. The reason is that the reaction produces acidic hydrogen sulfide. Furthermore, Dalgaard et al. (1974) also considered the reaction of carbon disulfide and potassium hydroxide in the synthesis of ketene mercaptals. The role of KOH likely participated in the reaction or acted as the cocatalyst during the reaction. The effect of KOH on the conversion of o-phenylenediamine is shown in Figure 14. The conversion is increased with the increase of the amount of KOH. Both KOH and tertiary amine possess the catalytic effect in enhancing the reaction of carbon disulfide and o-phenylenediamine.
(vi) Identification of the Reaction Order in Synthesizing MBI. As shown in eq 12, the reaction is reduced to zero-order or first-order rate of kinetics, depending on the concentration of o-phenylenediamine and the values of kl, 12-1, and k2. The conventional experiments to determine k-1 and k2 are not available, because R3N-CS2 is not isolated and purified. In this work, the method of initial rate is provided to determine the rate constant (81) of the forward reaction R1. In the reaction of synthesizing MBI, the intrinsic rate constant (k-1) of the backward reaction R1 and the intrinsic rate constant (k2) were determined from eq 25 by utilizing the technique of parameter estimation. For example, the reaction of synthesizied MBI was carried out 40 "C with [ C S ~ I= O 0.539 M, [CsH4(NH2)210= 0.074 M, and [(C4H&N]o = 0.010 M in MeOH. The rate constant (kl) of the forward reaction R1 is 4.056 x min-l M-l. Using the technique of parameter estimation, the values of k-1 and k2 are 1.796 x and 1.117 x 10-1min-' M-l, respectively. The corresponding value of equilibrium constant (K) is 4.427 x At a higher concentration (0.074 M) and low concentration (9.250 x M) of CsHd(NH2)2, the values of (kdk1)[CsHdNH2)21 are 2.037 x and 2.545 x respectively. Therefore, the ratios of the two terms in
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3695 sitive to the molar ratio of carbon disulfide to ophenylenediamine when its molar ratio is larger than 5.
the denominator of eq 12 are:
Acknowledgment The authors would like to thank the National Science Council of the ROC for the financial support of this manuscript under Contract NSC 83-0402-E-007-004. Literature Cited Then, eq 25 becomes 4.240X - 73.67 In(1 - X ) = t; for low [C,H,(~H,),],
33.88X - 73.67 ln(1 - X ) = t; for high [C6H,(~H2),lO The value of the term (-73.67 ln(1 - 11)) from the contribution of first-order rate kinetics is much larger than that of the term (4.24011) from the contribution of zero-order rate kinetics a t low concentration of C6H4("212. This result indicates that the reaction follows a pseudo-first-order rate law. However, the value of the term (33.8827 is less than one-tenth of the value of the term (-73.67 ln(1 - 11)) at higher concentrations of C~H~(NHZ)Z. This result indicates that the reaction both follows zero-order and first-order rate law. Nevertheless, the first-order rate kinetics still dominates the whole reaction. Therefore, a pseudo-first-order rate law is sufficient to describe the reaction.
Conclusion The reaction of carbon disulfide and o-phenylenediamine catalyzed by tertiary amine to produce MBI in a homogeneous organic solution has been successfully carried out in this work. The pseudo-first-order rate law sufficiently describes the kinetic data for molar ratio of carbon disulfide to o-phenylenediamine larger than 8. A kinetic model has also been proposed, which involves the equilibrium reaction between carbon disulfide and tertiary amine to form an active intermediate (CSZ-R~N) and the production of MBI from the reaction of active intermediate and o-phenylenediamine. The reaction rate of carbon disulfide and tertiary amine forms the active intermediate relatively faster than the reaction of the active intermediate and o-phenylenediamine. Additionally, the synthesis of active intermediate has also been expressed by a pseudo-first-order kinetics. The rate constant is linearly dependent on the amount of tertiary amine. Among the six organic solvents, DMF contributes the greatest reactivity to obtain the largest conversion. Except for DMF, the conversion is increased with the increase of polarity of organic solvent. Furthermore, the conversion is insen-
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Received for review March 13, 1995 Revised manuscript received J u n e 27, 1995 Accepted July 6, 1995@ IE950173A Abstract published in Advance ACS Abstracts, September 15, 1995. @
