1572
I n d . E n g . C h e m . Res. 1995,34, 1572-1580
Phase-Transfer Catalytic Kinetics of the Synthesis of Phenyl Benzoate Yao-Sheng Lee? Mou-Yung Yeh,"and Yen-Ping Shih*?? Departments of Chemical Engineering and Chemistry, National Cheng Kung University, Tainan, Taiwan, 701, Republic of China
The reaction kinetics of the phase-transfer synthesis of phenyl benzoate from benzoyl chloride and phenol is studied using quaternary ammonium salts as well as tertiary amines as catalysts in a well-stirred batch reactor. Polar organic solvents promote high conversion from benzoyl chloride to phenyl benzoate and fast overall reaction, such as dichloromethane and chlorobenzene. Using nonpolar organic solvents such as n-hexane, benzene, and toluene, the conversion is lower and the overall reaction rate becomes slower. The order of catalytic activity for various quaternary ammonium chloride is benzyltributylammonium bromide > tetrabutylammonium hydrogensulfate > tetrabutylammonium bromide > benzyltriethylammonium chloride. When tertiary amines are used as catalysts, for the overall reaction rate the order of the catalytic activity is alkyldimethylamine > triethylamine > tributylamine > trimethylamine. As for the conversion the order is triethylamine > tributylamine > alkyldimethylamine > trimethylamine. The order of the effectiveness of organic solvents is dichloromethane > n-hexane > chlorobenzene > toluene.
Introduction The classical synthesis of phenyl benzoate (Furiness et al., 1978) is performed by the addition of benzoyl chloride to excess phenol in a strong alkaline aqueous solution. Benzoyl chloride is rather active in homogeneous and heterogeneous reaction systems. Thus, the hydrolysis of benzoyl chloride into the byproduct, benzoic acid, cannot be avoided. The conversion of phenyl benzoate with respect t o benzoyl chloride is only about 75%. A few papers (Koening and Weber, 1974; Illis, 1979; Szeji, 1980; Wilmer and Yue, 1987; Plusquellec et al., 1988; Wamer and Yates, 1989; Kuo and Jwo, 1992)were reported to minimize the extent of hydrolysis of benzoyl chloride in the organic-aqueous reaction system to obtain better conversion. The synthesis of phenyl benzoate by phase-transfercatalytic reaction using quaternary ammonium salts and tertiary amines is studied in this paper. It is known that quaternary ammonium salts are the most commonly used phase-transfer catalysts (PTCs). Tertiary amines are also used as PTCs due to their low cost (Yeh et al., 1988; Hwu, et al., 1992). In order to find a suitable condition for the two-phase reaction. The syntheses of phenyl benzoate using quaternary ammonium salts and tertiary amines as PTCs are studied respectively. Using quaternary ammonium salts, the effects of PTC concentration, sodium hydroxide concentration, solvent, inorganic ion, and volume ratio of the aqueous phase to the organic phase are investigated experimentally. The temperature effect is also studied to obtain the activation energy of the overall reaction. To facilitate the industrial application, the use of aqueous phase in repetition shows good conversion and overall reaction rate. The application of quaternary ammonium salts is more or less restricted by their high cost and their conversion into trialkylamines in strong alkaline aqueous solution by
* To whom correspondence should be addressed. Present address: National Taiwan Ocean University, Keelung, Taiwan, 202, ROC. t Department of Chemical Engineering. Department of Chemistry.
*
0888-5885/95/2634-1572$09.00/0
Hofmann degradation (Landini et al., 1986; Zerda et al., 1986). To avoid the disadvantages of quaternary ammonium salts, tertiary amines are used as phasetransfer catalysts in the synthesis of esters (Mills et al., 1962; Hennis et al., 1967,1968; Huang and Dauerman, 1969; Hwu et al., 1990) and ethers (Merker and Scott, 1961; Hwu et al., 1992). Using tertiary amines as catalysts in the synthesis of phenyl benzoate, the usual mechanism is proposed and is checked by the experimental result. However, effects of various amines, solvent, and temperature are also studied in detail.
Experimental Section Materials. All chemicals were of ultrapure synthetic grade. No further purification was done before usage. The quaternary ammonium salts and tertiary amines such as tetrabutylammonium bromide, benzyltriethylammonium chloride, tributylamine, triethylamine, and trimethylamine were obtained from E. Merck and Co. Alkyldimethylamine (40% CIZHZ~, 50% C I ~ Hand Z ~ 10% C16H33 in the alkyl group) was a product of Daian Company, Taipei, Taiwan. Phenyl chloride, benzoyl chloride, and sodium phenolate were also obtained from E. Merck and Co. Benzene, toluene, dichloromethane, and n-hexane were the products of Union Co., Hsinchu, Taiwan. Sodium hydroxide, sodium chloride, sodium bromide, potassium hydrogensulfate, sodium sulfate, and phenol were obtained from the Ishizu Pharmaceutical Co., Japan. Naphthalene used as an internal standard for the gas chromatograph was the product of Janssen Co., Belgium. Procedures. The batch reactor was a 250-mL threenecked round-bottomed flask of 6.4 cm in diameter with three creases each 1 cm in depth and 6 cm in length. The mixer was a three-blade marine-type axial propeller driven by an IKA-WERK RW 20 DZM motor. The length from the center of the stirring rod to the tip of the propeller was 2.3 cm. The flask was placed in an EYELA CAlOl low-temperature recycled water bath to maintain the temperature which was controlled within f O . l "C. Before the start of a kinetic run, benzoyl chloride, naphthalene, and organic solvent were introduced into 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1573
Scheme 1. Mechanism for the Synthesis of Phenyl Benzoate Using Quaternary Ammonium Salt as
PTC" 0
0
TI NaX
respect t o the molar concentration of benzoyl chloride in the organic phase. This molar concentration is denoted by c with initial concentration CO. Then
+Q H
O
O
-O a
-
+
N
1 1a
-a
O
NaOH + NaO
+ QX
a
In c/c, = Kt interface
aqueousphase
+ HzO
(1)
where t is the time in hours and k is an apparent reaction rate constant in (hours)-l. Letting x be the conversion of phenyl benzoate with respect to benzoyl chloride, then
a Q, quaternary ammonium cation; X, chloride, bromide, or hydrogensulfate anion.
(co - c)/c, = x
(2)
= Kt
(3)
and the flask and stirred slowly and maintained a t a reaction temperature for 112 h. Phenol, sodium hydroxide, and catalyst were mixed in water. To start the reaction, the aqueous solution was added to the flask and simultaneously the content was stirred at a definite speed. The content was sampled by taking 0.2 mL of sample at chosen times and diluting with 0.2 mL of toluene and 0.2 mL of 0.18 N HC1 aqueous solution. Shaking the contents, the organic phase was analyzed by a gas chromatograph. A Hitachi 263-30 gas chromatograph with a Hitachi D-2500 integrator was used. The separation tube was 3 mm in inside diameter and 3 m in length, and was packed with 50% SE-30 as the liquid phase and Chromosorb W/AW-DMCS in a size of 80100 mesh as the solid support. The temperature of the separation tube was program-controlled. Initially, it was set a t 110 "C and was maintained for 2 min. Then the temperature increased a t a program rate of 20 "C/ min. When the temperature reached 240 "C, it was kept at 240 "C. Nitrogen was used as the carrier gas with a flow rate of 20 mumin. The flame ionization detector was used at a temperature of 270 "C. The temperature of the injection was also kept a t 270 "C. Under this condition, the retention times of various chemicals were benzoyl chloride = 4.50 min, naphthalene = 5.39 min, and phenyl benzoate = 8.95 min.
Quaternary Ammonium Salts as PTCs Reaction Mechanism. The reaction mechanism of the use of quaternary ammonium salts is proposed in Scheme 1 following the study of Starks (1971) and Starks and Liotta (1978) as well as the research results in the synthesis of esters in our laboratory (Chang et al., 1983, 1984; Yeh et al., 1988). In the aqueous phase the quaternary ammonium salt reacts with sodium phenolate to form the intermediate quaternary ammonium phenoxide (CsH50Q) which is soluble in the organic phase and thus transfers to the organic phase. In the organic phase, quaternary ammonium phenoxide reacts with benzoyl chloride to form the product phenyl benzoate and quaternary ammonium salt which is also soluble in the aqueous phase and thus transfers back t o the aqueous phase. In the absence of quaternary ammonium salt or other phase-transfer catalyst, benzoyl chloride reacts with sodium phenolate in a slow reaction rate with a large amount of benzoyl chloride hydrolyzed in the aqueous phase to form the byproduct benzoic acid. Kinetics. The reaction of the synthesis of phenyl benzoate with a quaternary ammonium salt as phasetransfer catalyst proceeds rather quickly and is welldescribed by a pseudo-first-order reaction model with
-ln(l-x)
The above equation shows that the plot of -ln(l-x) vs t gives a straight line with K as the slope as will be illustrated later. Note that the data of conversion vs time in Figures 2-5 and 7-11, for example, follow eq 3 very well. Therefore, it is not necessary to duplicate the diagrams of -ln(l-x) vs time. Effect of Stirring Speed. Two main factors affect the reaction rate of the phase-transfer-catalyticreaction. One is the mass transfer rates of the quaternary ammonium salt and the quaternary ammonium phenoxide between the organic and aqueous phases. Another is the reaction rate of benzoyl chloride with the quaternary ammonium phenoxide in the organic phase. The ionic reaction in the aqueous phase is very rapid as compared with the above two factors. Therefore, the reaction rate in the aqueous phase is not a control step. In general, as the stirring speed reaches a certain value, the interfacial area between the two phases no longer increases, and the reaction rate becomes a constant as reported by Menger (19701, Lee and Freedman (1985), Rabinovitz et al. (1986), and our laboratory (Hwu et al., 1992). Some interesting results are obtained. When no phase-transfer catalyst is used, phenol reacts with sodium hydroxide in the aqueous phase to form sodium phenolate which transfers to the organic phase and reacts with benzoyl chloride to yield the product phenyl benzoate. However, the conversion is low. Three-hour reaction gives a conversion of 42%-72% a t stirring speed varying from 400 to 1250 rpm. Vigorous agitation enhances the chemical reaction rate as well as the conversion. However, the hydrolysis of benzoyl chloride cannot be avoided a t high stirring speed. The reaction rate increases even for stirring speed as high as 1250 rpm without catalyst and 1500 rpm with catalyst as shown in Figure 1. This phenomenon is contradictory to the previous results just mentioned. The explanation is that the reaction rate in the organic phase is rather fast with or without the addition of quaternary ammonium salt. Therefore, interfacial mass transfer is an important control step in the overall reaction rate. Since the rate of interfacial mass transfer increases with the increase of stirring speed, an increase of the interfacial area and overall mass transfer rate with the stirring step is expected. With the addition of 10 mol % tetrabutylammonium hydrogensulfate (TBAHS04) with respect to benzoyl chloride, the conversion increases to 78% at 400 rpm and 93% at about 1500 rpm. However, hydrolysis of benzoyl chloride also occurs.
1674 Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995 5.00
, P
-
100.00
I
o
-
with catalyst no catalyst
L
80.00
K
v
E
i
3.00
60.00
I3
G8
40.00
4
$
20.00
0
1000
500
1500
stirring speed(rpm)
Figure 1. Comparison of reaction rate constants for reaction with and without catalyst (0.003 mol of benzoyl chloride, 0.003 mol of phenol, 0.00375 mol of sodium hydroxide, 0.0003 mol of TBAHSO4 (optional), 100 mL of toluene, 20 mL of water, 17 "C).
1
solvent
a
I
I
I
dichloromethane chlorobenzene hexane benzene toluene
o 0 0
A
* I
O
1
1
1
,
1
1
0.50
,
1
,
1
0
1
,
,
1
1
,
1
,
,
,
,
1.00
1. 0
TIME( hr) Figure 2. Solvent effect on reaction rate and yield (0.003 mol of benzoyl chloride, 0.003 mol of phenol, 0.00375 mol of sodium hydroxide, 0.00015 mol of TBAHSO4,lOO mL of solvent, 20 mL of water, stirring speed = 1300 rpm, 17 "C).
Table 1. Some Physical Prowrties of Solvent@ dichloromethane chlorobenzene toluene benzene n-hexane
ii
0.00 0.00
2000
Solvent
100.00
viscosity at 20 "C, cP dielectric const at 20 "C 0.410 0.799 0.590 0.650 0.326
9.080 5.708 2.391 2.284 1.890
Handbook of Chemistry and Physics (1980).
The overall reaction rate constants are shown in Figure 1. In general the overall reaction rate constant increases about 5-fold with the addition of TBAHS04 for a special stirring speed. For high stirring speed, the enhancement is more significant. Effect of Organic Solvent. The overall reaction rate is determined by the activity of the reactants as well as the mass transfer rates of quaternary ammonium salt QX and quaternary ammonium phenolate CsH50Q between the organic and aqueous phases. The stirring speed, the structure of the quaternary ammonium salt, the properties of the organic solvent such as polarity and viscosity, and the distribution coefficients of QX and C & , o Q between the organic and aqueous phases are the important factors for the mass transfer rates. In this report, dichloromethane, chlorobenzene, n-hexane, toluene and benzene are used for comparison. Table 1illustrates two basic properties of these five solvents. Figure 2 shows the conversion and overall reaction rate with various solvents. Dichloromethane is the best solvent with respect to the overall reaction rate and with a conversion over 95%. Hydrolysis of benzoyl chloride could be avoided. Chlorobenzene is only second top dichloromethane with respect to the overall reaction rate. However, due to environmental problems, these two solvents are not likely suitable for industrial application. When n-hexane or benzene is used, the hydrolysis of benzoyl chloride becomes serious with a decrease of the conversion in the later period of the reaction. Toluene is then used as the organic solvent for detail study. Comparison of Various Quaternary Ammonium Salts. The comparison of the overall reaction rate and conversion of four quaternary ammonium salts as PTC
80.00 h
K v
Et; 60.00 e:
E?
20.00
I
f
o O
TBAHSOI BnTBABr
o
TBABr
A
BnTEACl
io
Figure 3. Comparison of various quaternary ammonium salts as FTC (0.003 mol of benzoyl chloride, 0.003 mol of phenol, 0.00375 mol of sodium hydroxide, 0.00015 mol of PTC, 100 mL of toluene, 20 mL of water, stirring speed = 1300 rpm, 17 "C).
is shown in Figure 3. These are tetrabutylammonium hydrogensulfate (TBAHSO,), benzyltributylammonium bromide (BnTBABr), tetrabutylammonium bromide (TBABr)and benzyltriethylammonium chloride (BnTEACl). The reaction is remarkably enhanced with the addition of 5 mol % of the quaternary ammonium salt with respect t o benzoyl chloride. The order of the catalytic activity in overall reaction rate is BnTBABr
=- TBAHSO,
> TBABr > BnTEACl
Since QX and C6&,0&formed by BnTBABr are more lipophilic than those by the other three quaternary ammonium salts, more CgHsOQ transfers to the organic phase. Therefore, BnTBABr has better catalytic activity. In the alkaline aqueous solution, hydrogensulfate anion converts to sulfate anion which is a hard base. Then the bonding force with the tetrabutylammonium cation becomes weaker. Therefore, more tetrabutylam-
Ind. Eng. Chem. Res., Vol. 34, No. 5,1995 1676 100.00
80.00 :
80.00 h
h
K v
K v
8 E 3
2
E
60.00
E 3 z
,
I i
loo.oo
60.00 1
2
$
40.00
40.00 A
20.00
0.00
20.00
50.00
100.00
j
1
/
, , ,
/,
monium cation combines with the phenolate anion to form CsHbOQ, which transfers to the organic phase. The overall reaction rate is rather high. The bromide anion is a weaker base as compared with the sulfate anion. Therefore, the bromide anion has a stronger bonding force with the tetrabutylammonium cation. When TBABr is used, less CsH50Q is formed; then less CsH50Q transfers to the organic phase. The overall reaction rate is low as expected. As for BnTEAC1, it is the most water-soluble salt among the four quaternary ammonium salts. Its catalytic activity is the worst. It is not studied in detail. The price of BnTBABr is much higher than that of TBAHS04. The latter is then chosen for detail study. Effect of Catalyst Concentration. The conversion and overall reaction rate increase simultaneously with an increase of the TBAHS04 concentration as shown in Figure 4 a t a low stirring speed of 400 rpm. The concentration of TBAHSOI is expressed in mole percent with respect to benzoyl chloride. The conversion increases to 85% with 5 mol % catalyst being used. However, as the catalyst used increases from 5% to 12.5%,the conversion only increases from 85%to 91%. Therefore, the effectiveness of the catalyst diminishes at high catalyst concentration. Figure 5 shows a similar result at a high stirring speed of 1300 rpm. In 1/2 h the conversion increases from 31% to 72% and then to 85% when the catalyst used is 0, 5, and 12.5 mol %, respectively, with respect to benzoyl chloride. Likewise, the effectiveness of the catalyst diminishes at high catalyst concentration. The overall reaction rate constants calculated from Figures 4 and 5 are shown in Figure 6 for comparison. At low stirring speed the overall reaction rate constant is linearly related t o the molar concentration of TBAHSO4. At high stirring speed, the overall reaction rate constant is much higher and is almost linearly related to the molar concentration of TBAHS04. Effect of Sodium Hydroxide Concentration. When the moles of sodium hydroxide and phenol are the same t o form sodium phenolate stoichiometrically, the overall reaction rate and conversion are respectively maximum as shown in Figure 7. As the concentration of sodium hydroxide increases further, the conversion
TBAHSO,
, , , , , , ,
,
, ,
0.0%
10.00
20.00
30.00
TIME(min)
TIME(min) Figure 4. Effect of TBAHSOd concentration on yield and overall reaction rate at low stirring speed of 400 rpm (0.003 mol of benzoyl chloride, 0.003 mol of sodium phenolate, 100 mL of toluene, 20 mL of water, 17 "C).
4 ;,iBgS, I
2.5%
0.00 0.00
150.00
, o
30.00
TIME(min) Figure 11. Effect of sodium bromide (0.003 mol of benzoyl chloride, 0.003 mol of sodium phenolate, 0.00015 mol of TBAHSO4, 100 mL of toluene, 20 mL of water, stirring speed = 1300 rpm, 17 "C).
poison effect for the reaction. Figures 10 and 11 show that sodium chloride and sodium bromide are favorable respectively for the conversion and overall reaction rate without the occurrence of the poison effect. However, the effect of potassium hydrogensulfate is negative as illustrated in Figure 12. Addition of potassium hydrogensulfate a t high concentration almost stops the reaction. It is because the serious poison effect retards the formation of CsH50Q and causes the hydrolysis of benzoyl chloride. Figure 13 summarizes the effect of various salts expressed in overall reaction rate constant. The order of the favorable effect is Na,SO, > N a C l > NaBr > KHSO, The sulfate anion has a strong hydration ability and is a hard base, being not easy for bonding with the quaternary ammonium cation. Therefore, more C6H5-
0.00 0.00
0.02
0.04
NalSOI NaCl NaBr
wsod
0.06
0 I8
a m o u n t of salt ( m o l e )
Figure 13. Ion effect on overall reaction rate constant (0.003 mol of benzoyl chloride, 0.003 mol of sodium phenolate, 0.00015 mol of TBAHSO4,lOO mL of toluene, 20 mL of water, stirring speed = 1300 rpm, 17 "C).
OQ forms in the aqueous phase and transfers to the organic phase. The sodium sulfate has the most favorable effect. In comparison with the chloride anion, the bromide anion is larger and has less hydration ability. The bonding force of the bromide anion with the quaternary ammonium cation is stronger than that of the chloride anion. Hence, sodium chloride has more favorable effect than sodium bromide. Repeating Use of Aqueous Phase. In the experiment, initially, the mole ratio of sodium phenolate to benzoyl chloride is chosen as 3 t o 1. After the reaction is over, the aqueous phase is recovered and the mole ratio of sodium phenolate to benzoyl chloride is maintained at 3 to 1by the addition of reactants. No more catalyst is added later. Figure 14 shows that the conversion and overall reaction rate decrease when the aqueous solution is used repeatedly. This is because part of the catalyst TBAHS04 is discarded with the organic phase.
1578 Ind. Eng. Chem. Res., Vol. 34, No. 5,1995 100.00
100.00
1
I
80.00 :
80.00
h
h
Fx u
3t;
Fx v
p:
F z
E L2
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E
40.00 f
20.00 :
80*oo
40.00
20.00
0.00 0.00
4.00
2.00
0.00 0.00
10.00
8.00
6.00
TIME(min)
10.00
20.00
Figure 14. Use of aqueous solution in repetition (0.003 mol of benzoyl chloride, 0.009 mol of sodium Phenolate, 0.00015 mol of TBAHS04, 100 mL of toluene, 20 mL of water, stirring speed = 1300 rpm, 17 "C).
Figure 16. Effect of tertiary amine (0.003 mol of benzoyl chloride, 0.003 mol of sodium phenolate, 0.0003 mol of R3N, 100 mL of dichloromethane, 20 mL of water, stirring speed = 1300 rpm, 17 "C).
Scheme 2. Mechanism for the Synthesis of Phenyl Benzoate Using a Tertiary Amine as PTC
2.00
e ! ! C l
1.00 -
s
30.00
TIME(min)
0
t R3N d Q C I 0
0.00 -
d
-1.00
TBAHS0,0.00015 mol no catalyst - 0 EtSN 0.0003 mol
- 0
-2.001,
3.30
I
I
I
I
I
,
I
!
I !
I
I
I
1
1
I
I
3.40
,
I ,
1
,
I
I
3.50
I
I
I
I
I
3.60
I/T ( ~ ~ 1 0 ~ ) Figure 15. Arrhenius plot (TBAHSO4 as catalyst and no catalyst: 0.003 mol of benzoyl chloride, 0.003 mol of phenol, 0.00375 mol of sodium hydroxide, 100 mL of toluene, 20 mL of water, stirring speed = 1300 rpm. Et3N as catalyst 0.003 mol of benzoyl chloride, 0.003 mol of sodium phenolate, 100 mL of CHZC12, 20 mL of water, stirring speed = 1300 rpm).
Temperature Effect. The effect of temperature on the reaction rate is well described by the Arrhenius equation. In spite of including the interfacial mass transfer the phase-transfer-catalytic reaction is also well described by the Arrhenius equation as reported by Hwu et al. (1988, 1990).
k = A exp(-EJRT)
(4)
where A is the frequency constant, R is the gas constant, Tis the absolute temperature, and E , is the activation energy. Figure 15 is the plot of In K versus 1/T with TBAHSO4 as PTC and without PTC, respectively. The activation energies calculated are respectively 8.12 kcaU mol with TBAHS04 as PTC and 3.55 kcaymol without PTC. The activation energies are rather low.
Tertiary Amines As PTCs Reaction Mechanism. In the synthesis of phenyl benzoate, the role of tertiary amines as phase-transfer catalysts is very unique. In the organic phase, benzoyl chloride reacts with tertiary amine (R3N) to form quaternary ammonium salt which transfers to the aqueous phase to react with sodium phenolate t o form the intermediate (CsH50&,quaternary ammonium phenolate). The intermediate transfers to the organic phase to react with benzoyl chloride to yield the product phenyl benzoate. This reaction mechanism is summarized in Scheme 2 (Hwu et al., 1990). Effect of Tertiary Amine. Comparison of the conversion and overall reaction rate of four tertiary amines is given in Figure 16 for a reaction period of 30 min. Dichloromethane was used as solvent. Four tertiary amines were used. These were trimethylamine (Me3N),tributylamine (BUN), triethylamine (EtsN),and alkyldimethylamine (RMe2N) whose alkyl group contained 40% C12H25, 50% C14H29, and 10% C16H33. As for the overall reaction rate, the order is alkyldimethylamine > triethylamine > tributylamine 7 trimethylamine As for the conversion over a reaction period of 30 min, the order becomes triethylamine > tributylamine > alkyldimethylamine
7
trimethylamine
Ind. Eng. Chem. Res., Vol. 34, No. 5,1995 1679 100.00
100.00
,
I
80.00
80.00 h
h
K v
K v
2 80.00
E3 8
z
40.00
Solvent o
20.00
0 A
dichloromethane hexane chlorobenzene toluene
20.00
20.00
40.00
60.00
A
5.0%
Q
0.0%
1 I / I J I J 10.00 ~ I I J J , l l 20.00 l / l l l J , I ,30.00 IJIIJII
80.00
TIME( min)
TIME( min) Figure 17. Effect of solvent (0.003 mol of benzoyl chloride, 0.003 mol of sodium phenolate, 0.0003 mol of Et3N, 100 mL of solvent, 20 mL of water, stirring speed = 1300 rpm, 17 "C).
The steric hindrance of triethylamine is small and the solubility in the organic phase is good; therefore, it is easy t o combine with benzoyl chloride in the organic phase t o form quaternary ammonium salt. According to Scheme 2 triethylamine gives high conversion as well as high overall reaction rate. The steric hindrance of alkyldimethylamine is also small. Due to the long chain in the alkyl group, alkyldimethylamine is lipophilic. Therefore, it is easy to be quaternized in the organic phase. High overall reaction rate a t the beginning of the reaction is expected. However, the long chain alkyl group makes the quaternary ammonium water-repellent and thus difficult to transfer t o the aqueous phase. The overall reaction rate is slowed rapidly only a few minutes after the reaction starts. Tributylamine is more lipophilic than triethylamine. Since the steric hindrance of tributylamine is also larger than that of triethylamine, quaternization of tributylamine is slower. Therefore, the overall reaction rate using tributylamine is slower than that using triethylamine. As for trimethylamine, it is most easily quaternized due t o having the smallest molecular volume. However, trimethylamine and the quaternary ammonium chloride thus formed are very water soluble. According to Scheme 2 the formation of the quaternary ammonium chloride and the intermediate CsH50Q is retarded. Less intermediate transfers to the organic phase. The overall reaction rate decreases. Solvent Effect. Figure 17 illustrates the effect of dichloromethane, n-hexane, chlorobenzene,and toluene on the conversion and overall reaction rate. The order of the effectiveness of the various solvents is dichloromethane > n-hexane
20.0%
0.00 0.00
0.00 0.00
i
0
=- chlorobenzene
>
toluene It is noted that toluene is the poorest solvent. In spite of the high polarity of chlorobenzene, it is also not a good solvent. Dichloromethane is the best solvent with respect t o the conversion and the overall reaction rate. The result discloses that the interfacial mass transfer is an important factor in the overall reaction rate.
Figure 18. Effect of triethylamine concentration (0.003 mol of benzoyl chloride, 0.003 mol of sodium phenolate, 100 mL of CH2Clz, 20 mL of water, stirring speed = 1300 rpm, 17 "C. Table 3. Overall Reaction Rate Constants Using Triethylamine" triethylamine, mol
k, h-l
0.00000
1.44 4.16 6.57 8.17 12.48
0.00015 0.00030 0.00045 0.00060 a
Reaction condition is shown in Figure 18.
Chlorobenzene and toluene are not favored for the reaction, perhaps due to their high viscosity. Effect of Catalyst Concentration. The effect of triethylamine concentration expressed in molar percentage with respect t o benzoyl chloride on the conversion and overall reaction rate is shown in Figure 18. Both increase with the increase of triethylamine concentration. The overall reaction rate constants as calculated from Figure 18 are also shown in Table 3. Temperature Effect. The Arrhenius plot of temperature versus overall reaction rate for triethylamine used as catalyst is given in Figure 15. The activation energy calculated is 3.503 kcallmol. It is rather low.
Conclusion The synthesis of phenyl benzoate is studied experimentally in detail using quaternary ammonium salts and trialkylamines as phase-transfer catalysts in an organic-aqueous two-phase system. The mechanism of the reaction using quaternary ammonium salts follows the normal phase-transfer-catalytic scheme, based on which the kinetics can be well explained. As trialkylamine is used as catalyst, quaternization of trialkylamine with benzoyl chloride proceeds in the organic phase. Then, the formation of phenyl benzoate occurs in the organic phase following the normal phasetransfer-catalytic scheme.
Acknowledgment The financial support from the National Science Council of the Republic of China is appreciated. The project number is NSC-80-0402-E006-29.
1680 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995
Literature Cited Chang, J. R.; Yeh, M. Y.; Shih, Y. P. Studies of Phase Transfer Catalytic Preparation of Benzyl Benzoate. J . Chin. Znst. Chem. Eng. 1983,14,457-462. Chang, J. R.; Yeh, M. Y.; Shih, Y. P. Kinetics of Phase Transfer Catalytic Preparation of Benzyl Benzoate. J . Chin. Chem. SOC. 1984,-31, 185-90. Do. J. S.: Chou. T. S. Kinetics of the Oxidation of Benzvl Alcohol by HhochloAte Ion in the Presence of Phase-Transfe;Catalyst. Ind. Eng. Chem. Res. 1990,29,1095-1103. Furniss, B. S.; Hannaford, A. J.; Rogers, V.; Smith, P. W. G.; Tatchell, A. G. Vogel's Textbook of Practical Organic Chemistry, 4th ed.; Wiley: New York, 1978. Handbook of Chemistry and Physics, 61st ed.; CRC Press: Boca Raton, FL, 1980. Hennis, H. E.; Easterly, J. D., Jr.; Collins, R.; Thompson, L. R. Esters from the Reactions of Alkyl Halides and Salts of Carboxylic Acids: Reactions of Primary Alkyl chlorides and Sodium Salts of Carboxylic Acids. Ind. Eng. Chem. Prod. Res. Deu. 1967, 6, 193-195. Hennis, H. E.; Thompson, L. R.; Long, J. D. Esters from the Reactions of Alkyl Halides and Salts of Carboxylic Acids: Comprehensive Study of Amine Catalysis. Znd. End. Chem. Prod. Res. Dev. 1968, 7, 96-101. Huang, I. D.; Dauerman, L. Exploratory Process Study: BaseCatalyzed Reaction of Organic Chlorides with Sodium Acetate. Znd. Eng. Chem. Prod. Res. Dev. 1969,8, 227-232. Hwu, D. H.; Hwang, C.; Yeh, M. Y.; Jung, H. M.; Shih, Y. P. Kinetics of Phase-Transfer Synthesis Benzyl Esters Using Tertiary Amines. Ind. Eng. Chem. Res. 1990,29,2214-2221. Hwu, D. H.; Hwang, C.; Shih, Y. P.; Yeh, M. Y.; Chao, C. L. Kinetics of the Synthesis of Benzyl Ethers Using Tertiary Amines as Catalysts. Znd. Eng. Chem. Res. 1992,31, 177-182. Illis, V. 0. Phase Transfer Catalyzed Acylation. Tetrahedron Lett. 1979,2431-2432. Koening, K. E.; Weber, W. P. Synthesis of Benzoyl Cyanides by Phase-Transfer Catalysis. Tetrahedron Lett. 1974,2275-2278. Kuo, C. S.; Jwo, J. J. Inverse Phase-Transfer Catalysis. Kinetics and Mechanism of the Pyridine 1-Oxide-Catalyzed Substitution Reaction of Benzoyl Chloride and Benzoate Ion in a Two-Phase Water/Dichloromethane Medium. J . Org. Chem. 1992, 57, 1991-1995. Landini, D.; Maia, A,; Rampoldi, M. Stability of Quaternary Onium Salts under Phase-Transfer Conditions in the Presence of Aqueous Alkaline Solutions. J . Org. Chem. 1986, 51, 31873191. Lee, G. A,; Freedman, H. H. Phase-Transfer Oxidations with Hypochlorites: Scope and Mechanism. Zsr. J . Chem. 1985,26,229234. Mathias, K.; Vaidya, R. A. Inverse Phase Transfer Catalysis. First Report of a New Class of Interfacial Reactions. J . A m . Chem. SOC.1986,108, 1093-1094.
Menger, F. M. Interfacial Physical Organic Chemistry, ImidazoleCatalyzed Ester Hydrolysis at a Water-Heptane Boundary. J . A m . Chem. Sac. 1970,92,5965-5971. Merker, R. L.; Scott, M. J. The Reaction of Alkyl Halides with Carboxylic Acids and Phenols in the Presence of Tertiary Amines. J . Org. Chem. 1961,26, 5180-5183. Mills, R. H.; Farrar, M. W.; Weinkauff, 0. J. Synthesis of Esters from Simple Alkyl Halides and Tertiary Amine Salts of Carboxylic Acids. Chem. Ind. 1962, Dec 29, 2144. Plusquellec, D.; Roulleau, F.; Lefeuvre, M.; Brown, E., A New Synthesis of Carboxylic Carbonic Acid Anhydrides Using Phase Transfer Reactions. Tetrahedron 1988, 44, 2471-2476. Rabinovitz, M.; Cohen, Y.; Halpern, M. Hydroxide Ion Initiated Reactions under Phase-Transfer Catalysis Conditions: Mechanism and Implications. Angew. Chem., Znt. Ed. Engl. 1986, 25,960-970. Starks, C. M. Phase-Transfer Catalysis. I. Heterogeneous Reactions Involving Anion Transfer by Quaternary Ammonium and Phosphonium Salts. J . Am. Chem. Soc. 1971,93, 195-199. Starks, C. M.; Liotta, C. Phase-Transfer Catalysis, Principle and Techniques; Academic Press: New York, 1978; p 21. Szeji, W. The Phase-Transfer Catalyzed Synthesis of Esters of Carboxylic Acids. Synthesis 1980, 5, 402-403. Wamer, C. C.; Yates, J. A. Kinetics and Mechanisms for the TwoPhase Reaction between Aqueous Aniline and Benzoyl Chloride in Chloroform with and without Pyridine Catalysis. J . Org. Chem. 1989,54, 150-154. Wang, T. T.; Huang, T. C.; Yeh, M. Y. Benzyl Alcohol from PhaseTransfer Catalyzed Weakly Alkaline Hydrolysis of Benzyl Chloride. Chem. Eng. Commun. 1990,92, 139-151. Wilmer, K.; Yue, X. Inverse Phase-Transfer Catalysis: Probing Its Mechanism with Compositive Transacrylation. J . Am. Chem. SOC.1987,109,1278-1279. Yeh, M. Y.; Guo, H. H.; Chen, L. C.; Shih, Y. P. Kinetics of PhaseTransfer Catalytic Esterification of Phthalic Acid Monobutyl Ester. Znd. Eng. Chem. Res. 1988,27, 1582-1587. Zerda, J. D. L.; Neumann, R.; Sasson, Y. Hofmann Decomposition of Quaternary Ammonium Salts under Phase-Transfer Catalytic Conditions. J . Chem. SOC.,Perkin Trans. 2 1986, 823826.
Received for review September 9, 1994 Accepted January 10, 1995@ IE9400309 Abstract published in Advance ACS Abstracts, April 1, 1995. @
