3696
Ind. Eng. Chem. Res. 1996,34, 3696-3702
Synthesis of Unsymmetric Acetal Compounds under Phase-Transfer Catalysis and Separation of the Products? Maw-Ling Wang* and Shahng-Wern Chang Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China
Unsymmetric formaldehyde acetal was synthesized from the reaction of two mixed alcohols (e.g., 1-butanol and l-octanol) and dibromomethane by phase-transfer catalysis (F'TC) in a n alkaline solution of a KOWorganic solvent two-phase medium. Three unique products with two alkoxide substituents were observed during the reaction. This result indicated a rather rapid rate of the second reaction in the organic phase. High yields of acetals and high conversion of dibromomethane were obtained employing tetrabutylammonium bromide (TBAB) as a phase-transfer catalyst. An investigation was also undertaken regarding the effects of the reaction conditions on the conversion of dibromomethane and the individual yield of the three final products. The reaction rate, the conversion of dibromomethane, and the yields of the three final products satisfactorily accounted for the polarities of organic solvents, concentrations of the reactants, and the amount of catalyst. Mixed products are separated by utilizing the column distillation. An optimum reaction condition was recommended to produce a maximum yield of unsymmetric acetal compound.
Introduction The development of phase-transfer catalysis (PTC) (Dehmlow and Dehmlow, 1993;Freedman, 1986; Starks and Liotta, 1978; Starks, 1985; Weber and Gokel, 1977) represents a major step forward in many organic reactions and renders useful processes for their advantages. It provides a simple and feasible method of synthesizing specialty chemicals. The primary advantages of PTC in the synthesis of organic chemicals are the high yield of products, large reaction rate, and high selectivity of the desired products. Recently, Wang and his co-workers (Wang and Wu, 1990a, 1991; Wang and Chang, 1991; Wang and Yang, 1991) successfully studied the kinetics and dynamics of phase-transfercatalyzed reaction through identifying the active catalyst (or the intermediate product) in the two phases. The kinetics and dynamics of the reaction system were more fully understood. In recent years, ethers were generally synthesized in an anhydrous condition for several hours or longer. The yields of the products in using such methods were not high. This disadvantage in the synthesis of ethers may be alleviated by phase-transfer catalysis (PTC). Synthesizing ethers in a Williamson reaction by PTC requires that the organic salt is directly synthesized in situ by the reaction of alcohols and potassium hydroxide in the aqueous phase. Diether (a kind of acetal) was first obtained through acid-catalyzed etherification of a-poly(0xymethane). The conversion of alcohol into formaldehyde acetal was carried out (Bal and Pinnick, 1979; Keller, 1986; Webb et al., 1962). However, obtaining the desired products took too long or else various formaldehyde acetals of a wide molecular weight distribution were obtained. The formaldehyde acetals were recently synthesized by Cornelis and Laszlo (1982) by reacting alcohol and dichloromethane in a 50% sodium hydroxide solution, applying Tixoget VP clay as a catalyst. However, completing a reaction for such a low reaction rate takes roughly 3 days. The technique ~~
' Prepared
for presentation on Division of Industrial and Engineering Chemistry, ACS National Meeting, Anaheim, CA, April 2-7, 1995.
of phase-transfer catalysis (PTC) was employed by Dehmlow and Schmidt (1976) for synthesizing formaldehyde acetals from alcohols and dichloromethane in the aqueous phase. Only 50-60% of the yield was obtained for 15 h. Methods to synthesize unsymmetric formaldehyde acetals are currently unavailable. The objectives of this work are to synthesize the unsymmetric acetals by employing phase-transfer catalysis. Dibromomethane has reacted with two mixed alcohols catalyzed by quaternary ammonium salts in a high alkaline solution of KOWorganic solvent to form the unsymmetric acetal. Additionally, the products of two symmetric and one unsymmetric acetals are separated by utilizing the column distillation and identified by spectrophotometry analysis. Those operating conditions, such as the molar ratio of aqueous-phase reactants, amount of catalyst and dibromomethane, content of potassium hydroxide, organic solvents, and temperature on the conversion of dibromomethane and the distribution of the three final products, are investigated in detail. Experimental Section Materials. All reagents, including l-butanol, l-octanol, potassium hydroxide, tetrabutylammonium bromide (TBAJ3 or QBr), dibromomethane, and other reagents, were guaranteed grade (G.R.) chemicals for synthesis. Procedures. (A) Two-Phase Phase Transfer Catalytic Reaction. The reactor was a 150-mLthreenecked Pyrex flask, serving the purposes of agitating the solution, inserting the thermometer, taking samples, and feeding the feed. K n o w n quantities of l-butanol, l-octanol, and KOH were dissolved in organic solvent (50 mL) and water (10 mL). Next, the solution was put into the reactor which was submerged into a wellcontrolled temperature (50 f 0.1 "C) water bath. This two-phase solution was stirred for 1 h to reach the desired temperature. Tetrabutylammonium bromide (TBAB) and toluene (internal standard) were then introduced into the reactor. Before adding dibromomethane to start the reaction, an aliquot sample of the
0888-5885/95/2634-3696$09.00/00 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 11, 1995 3697 organic phase (0.1 mL), which was withdrawn from the reactor, was put into a test tube containing 5 mL of chlorobenzene at 4 "C and was analyzed by using method i. Finally, dibromomethane was added to start the reaction. The sample (0.5 mL), which was withdrawn from the reactor at each time interval, was put into test tubes containing 4 mL of water and 4 mL of chlorobenzene at 4 "C. The samples were analyzed by using method ii. The acetal products were also identified by mass spectroscopy, lH-NMR, and IR analysis. Method i: Detection of the Active Catalyst (QOR1 and QOR2) in the Organic Phase. Before dibromomethane was added to the solution to start the reaction for synthesizing acetals, the catalyst reacted with l-butanol and 1-octanolin the high alkaline solution of KOH to produce the organic-soluble active catalysts (&OR1 and QOR2: RI, CdHg, Rz, C8H17; Q, (C4HghN). This organic-soluble QORl and QOR2 then transferred to the organic phase for reaction with dibromomethane. The active catalysts, tetrabutylammonium alkoxides (&OR1 and QORz),were analyzed by employing the thermolysis (Lopez et al., 1989) of quaternary ammonium salts, commonly known as Hoffmann elimination. Gas chromatography was carried out for the sake of analyzing the tertiary amine, applying Shimadzu GC-SA and integrator C-R6A instruments with a 30 m x 0.525 mm i.d. capillary column containing 100% poly(dimethy1siloxane). The end of the injector was packed with steel shavings in order to furnish a large surface area for rapid heat exchange with the sample. Nitrogen was used as the carrier gas a t a flow rate of 30 mumin. The detector is a FID and the injection temperature is 360 "C for thermolysis of QORl and QOR2. Method ii: Measurement of the Acetal Products. In this work, three acetal products, i.e., (C4Hg0)2CHz, ( C ~ H ~ O ) ( C ~ H I ~ Oand )CH (C~HI~O)ZCH~, ~, were produced from the reaction solution. Samples were then withdrawn from the reaction and analyzed by gas chromatography. The analysis conditions were as follows: column, 30 m x 0.525 mm i.d. capillary column containing 100% poly(dimethylsi1oxane);injection of temperature, 220 "C; carrier gas, N2 at a flow rate of 20 m u min; detector, FID. (B) Separation of the Unsymmetric Acetal from the Symmetric Acetal. The acetal compounds in an alkaline solution are more stable than that in acidic solution. Those impurities in acetals contain alcohol, aldehyde, water, and quaternary ammonium salt. In purifying the acetal compounds, the sample is first washed with an alkaline solution of KOH. Next, the organic and aqueous phases were separated in a funnel. The above process was repeated six times. Alcohol and water in a trace amount in the organic solution were removed by treating with sodium metal. Aldehyde was also removed through precipitation by polymerization. The purity of acetal compounds is greater than 98%. The unsymmetric acetal was obtained using vacuum fractional distillation in a Vigreux column (1.5 i.d., 25 cm in length). Vacuum fractional distillation was normally required to be performed four times to obtain an extremely high purity of unsymmetric acetal.
Mechanism of the Two-Phase Reaction Two mixed 1-alcohols and dibromomethane were added t o the concentrated alkaline solution of a KOW organic solvent two-phase medium in the presence of a small amount of TBAB catalyst. One proton of alcohols
was captured by the potassium hydroxide to form potassium alkoxide. On that occasion, the potassium alkoxide and tetrabutylammonium bromide in an aqueous phase would exchange their cations with each other to form tetrabutylammonium alkoxide. The organicsoluble tetrabutylammonium alkoxide then transferred instantaneously to the organic phase. Soon afterward, S Nsubstitution ~ would take place in the organic phase. The overall reaction was expressed as
+
-
+
+
3C4HgOH 3C8H170H 6KOH 3CHzBrz QBr (C4HgO)zCHz + (C,HgO)(C8Hi,O)CHz + (C8Hl7O),CHZ 6KBr i- 6H20 (Rl)
+
The rate-controlling step of the reaction is the primary bromine substitution by an alkoxide (RO-) group (Summers, 1955). The active catalysts, tetrabutylammonium alkoxides ((C4H9)4NOR,QOR R, C4H9 and C8H17), were observed almost entirely and invariably (>91%)(Lopez et al., 1989) in the organic phase during the reaction. In this work, alcohols in excess amounts relative to their stoichiometric quantity reacted with dibromomethane in the concentrated potassium hydroxide solution and chlorobenzene two-phase system catalyzed by tetrabutylammonium bromide (TBAB or QBr). A measured constant concentration of tetrabutylammonium alkoxides during the reaction was obtained. Accompanied by this phenomenon, a mechanism of two-mixed 1-alcohols reacting with dibromomethane in a two-phase solution catalyzed by TBAB catalyst is proposed, i.e., C4HgOH + KOH CeH170H + KOH C4HgOK+ QBr CeH170K+ QBr
L
11
(C4HgO)CHzBr + QBr
k1
(CeH170)CH2Br+ QBr (C4H90)2CH2+ QBr (C4H90)(C8Hl70)CH2 + QBr (G4H90)(C8H170)CH2 + QBr (C8H170)2CH2+ QBr
C4HgOK + H20 CeH170K + H20 C4HgOQ+ KBr C8Hl70Q
+ KBr
II
1
C4H90Q + CH2Br2
(aaueousl (organic)
C8H170Q+ CH2Br2
A
-
(R2)
C4HgOQ+ (C4H90)CH2Br
ki 2
CeH170Q+ (C4H90)CH2Br
k2 i
C4HgOQ+ (CBHl70)CH2Br
k22
CeH170Q + (CaH170)CH2Br
in which Q = (C4Hg)4N+. Alcohols first reacted with KOH so as to form potassium alkoxide (R1OK and R20K) in the aqueous phase. RlOK and RzOK next further reacted with QBr in the aqueous phase to produce quaternary ammonium alkoxides (&OR1 and Q O h ) which are more soluble in the organic solvent. Dibromomethane then reacted with QORl and QORz in forming the desired products, i.e., dialkoxymethanes (R10)2CHz, (RlO)(RzO)CHz, and (R2012CHz. In this work, the intermediate products (C4HgO)CHzBr and (C8H170)CHzBr were not observed during or after the reaction. This result indicates that the reactions of intermediates with C4H90Q and C8H170Q are faster than those of CH2Br2 with C4H90Q and C8H170Q. The primary bromine substitution by an alkoxide (RO-) group in the organic phase is obviously a rate-determining step.
Results and Discussion The well-known Williamson synthesis of ether involves two main reaction steps. First, the reaction of
3698 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
alcohol and sodium (or sodium hydride) took place to produce metal alkoxide. Second, the organic salt reacted with organic substrates to produce ether compounds. These two reactions were generally carried out in an anhydrous condition for several hours. This disadvantage was improved by phase-transfer catalysis (PTC). To synthesize ethers in a Williamson reaction by PTC requires that the organic salt is synthesized in situ directly by the reaction of alcohol with sodium hydroxide in the aqueous phase. In this work, unsymmetric acetal compound was first synthesized by phasetransfer catalysis. The product yield obtained by PTC is generally dependent on the structure of the reactants, solvents, phase-transfer catalysts, the amount of KOH being utilized in the reaction, and temperature. No other byproducts were obtained during the reaction. For studying those factors, which affect the conversion of the reactant (CH2Br2),the conversion of dibromomethane X is defined as
X = 1 - ([CH2Br,lJICH2Br,l,,o) From the total reaction shown in (Rl), the consumption rate of CHzBrz equals the sum of the total yields of the three final products, i.e., dt
dt
Thus, the total conversion of CHzBrz is equal to the sum of the fraction of the yields of the three final products, i.e.,
x = P, + P, + P3
(3)
where PI, P z , and P3 are the fractions of the yields of the three final products, (C4Hg0)2CHz, (C4Hg0)(CaH170)CHz,and (CaH170)2CH2, respectively, and are defined as (4)
in which o and 0 represent the species in the organic phase and at the initial condition, respectively. (A) Identification of the Product. In this work, the use of l-butanol and l-octanol leads to obtaining the saturated acetals of a long chain without any branches. The fragment of these three products in a mass spectrum is similar to that of ethers. However, the chief difference between acetals and ethers is that acetals would not generate a molecular peak. The reason is that the stability period of the molecular ion of the acetal compound is relatively short SI. Nevertheless, the following cation was formed with greater stability. Therefore, form A, whose molecular weight is less than that of the original acetal by 1,can be detected by the sensor of the mass spectrum, i.e.,
A
B
It is noted that (C4HgO)(CsH170)CH2is an unsymmetric acetal. Its characteristic peak from its mass spectrum contains the characteristic properties of (C4HgO)zCHz and (C8H170)2CH2. That is, larger peaks appear at mlz =57, 78, and 143. In addition, a peak, which indicates a molecular weight 215 (i.e., less than that of the original molecular acetal (C4HgO)(CaH170)CH2 by 11, can be detected by the sensor of the mass spectrum. The products obtained from the phase-transfer catalytic reaction were detected via infrared and NMR spectral analysis (Silverstein et al., 1991). The absorbed frequencies of the corresponding bond by the infrared spectrum (1150-1070, 3000, 1450, and 1375 cm-l for C-0-C (stretch, CH aliphatic, CH2 and CH3, respectively) and the chemical shifts of the obtained dialkoxymethane (6-OCH20: 4.60,, 4.60,, and 4.59, for ( C ~ H ~ O ) ~ C(CsH170)2CHz, HZ, and ( C ~ H ~ O ) ( C ~ H ~ ~ O ) C H Z , respectively; d-OCHzC: 3.48t, 3.47t, and 3.45h for (C4Hg0)2CH2,(CaHi70)2CHz,and ( C ~ H ~ O ) ( C ~ H ~ ~ O ) C H Z , respectively) are consistent with previously published documents. Dibromomethane, which is employed as the reactant, is generally not an effective organic substrate due t o its low reactivity. Therefore, the reaction of alcohols and dibromomethane t o synthesize acetal compounds can only take place in a very high alkaline concentration even in the presence of phase-transfer catalyst. Two sequential reaction steps are illustrated in (R2) to be present in the organic phase, i.e., one is the single alkoxide substituent, and the other is the two alkoxide substituents. On the basis of experimental observations, the first products, Le., (C4HgO)CHzBr and (CaH170)CH2Br,remained undetected during and after the reaction of the single alkoxide substituent. These results indicate that the second reaction rate is quite rapid, as compared to the first reaction rate in the organic phase. The first reaction in the organic phase would therefore become the rate-controlling step. A high yield could be obtained in synthesizing formaldehyde acetals via phase-transfer catalysis. The reaction, which follows (Rl), can lead toward a higher product yield by employing different kinds of reactants and solvents. Those factors affecting the conversion of dibromomethane and the cumulative yields of the three final products are discussed as follows. (B) Factors Affecting the Conversion of Dibromomethane and the Yields of the Three Products. (i) Effect of the Molar Ratio of l-ButanoV1-Octanol. The effect of the molar ratio of l-butanoyloctanol on the yields of the three products is shown in Figure 1. In Figure 1, the yield of the first symmetric acetal (C4H90)2CH2 is increased and the yield of the second symmetric acetal (CsH170)~CHzis decreased with an increase of the molar ratio of l-butanoyloctanol. A maximum fraction of the yield of the unsymmetric acetal (C4HgO)(CsH170)CHzwas obtained when the molar ratio of l-butanovl-octanol approximately equals 1.5. As shown in Figure 1, the total conversion X , i.e., the sum of the fractions of the yields of the three final products (PI + PZ+ P3), is increased
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3699 ."
.06
Oh
0.4
I
05
I O 5
1
03
04
OS
06 070809 I
2
3
4
02
Molar ratio of 1-butanol / 1-octanol Figure 1. Effect of the molar ratio of 1-butanovl-octanol on the yields of the three final products: 4.59 x mol of 1-octanol, mol of CHZBr2,3.11 x mol of TBAB 30 g of KOH, 2.76 x catalyst, 10 mL of H20,50 mL of chlorobenzene, 1020 rpm, 50 "C, 2 h of reaction.
I
I
I
I
I
04
06
08
IO
12
Amount of catalyst ( g )
Figure 3. Effect of the amount of catalyst on the yields of the three final products: 6.88 x mol of 1-butanol, 1.5 molar ratio of 1-butanovl-octanol, 30 g of KOH, 2.76 x mol of CH2Br2, 10 mL of H20, 50 mL of chlorobenzene, 1020 rpm, 50 "C. 0 04 h
z
E
v
005
4
v
'I ;
Oo4 0
32
003
..-8
0
.-e .-8
u8
002
002
E
8
Oo3
B
001
8
001
000 0 00
03
04
05 06 070.809 1
2
3
Molar ratio of l-butanol/l-octanol Figure 2. Effect of the molar ratio of 1-butanoyl-octanol on the concentration of C4H90Q (QORd and CaH170Q (QOR-2): same reaction conditions as given in Figure 1.
1
I
04
06
,
1
os
IO
12
Amount of catalyst ( g ) Figure 4. Effect of the amount of catalyst on the concentrations of C4HgOQ (&OR11and CsH170Q (QORz): same reaction conditions as given in Figure 3.
phase reaction. The effects of the amount of TBAB catalyst on the yield of the three individual products with an increase of the molar ratio of 1-butanoYlare shown in Figure 3. The total conversion of dibromooctanol. This result indicates that the reactivity of methane ( X ) , which corresponds to the total yields of 1-butanol is larger than that of 1-octanol. The fraction P2 P3), obviously of the unsymmetric acetal compound ( C ~ H ~ O ) ( C ~ H I ~ O )the - three final products (PI increases with an increase of the amount of TBAB CH2 among the three final products (PdX = 0.459) catalyst used. All the yields of the three individual exhibits a maximum value for a 1.5 molar ratio of products also increase with an increase of the amount 1-butanoul-octanol. This maximum value of PdX alof TBAB catalyst. However, the fraction of the unsymmost corresponds to the maximum value of P2. metric acetal compound ( C ~ H ~ O ) ( C ~ H I ~ O among )CH~ As stated, two intermediate products (active catalysts) the three final products (Pdm decreases with an were generated in the aqueous phase during the reacincrease of the amount of TBAB catalyst. The concention, i.e., QORl and QOR2. The effect of the molar ratio trations of &OR1 and QORz, as affected by the amount of 1-butanoY1-octanol on the concentrations of &OR1 of TBAB catalyst, are depicted in Figure 4. At a lower and QOR2 is shown in Figure 2. As expected, the content of TBAB catalyst, the concentration of &OR1is concentration of QORl is increased and the concentralarger than that of QOR2. However, not much difference tion of QOR2 is decreased with an increase of the molar in concentration between &OR1 and QO& was obtained. ratio of 1-butanoY1-octanol. However, this change does The content of 1-butanol is larger than that of 1-octanol. not follow a linear relationship with the molar ratio of This result indicated that C~HSOK is more reactive than 1-butanoul-octanol. At a higher molar ratio of l-buthat of CsH170K in reacting with QBr to form the active tanoY1-octanol, the concentration of QORz decreased catalyst (or intermediate product). and the concentration of &OR1 increased more rapidly. (iii)Effect of the Amount of Potassium HydroxThis result also confirms that the reactivity of 1-butanol ide. The reactivity of dibromomethane is generally low. is greater than that of 1-octanol. Therefore, the role of adding KOH becomes a relatively (ii) Effect of the Amount of Catalyst. In this important factor which affects the conversion of the work, tetrabutylammonium bromide (TBAB or QBr) reactants for obtaining the products from the reaction was employed as the phase-transfer catalyst in the two-
+
+
3700 Ind. Eng. Chem. Res., Vol. 34,No. 11, 1995 04
.-z+ x 0
o.2
I
t 0
o/ /
/ /
/P
I
I
1
1
10
20
30
40
Amount of KOH ( g )
Figure 5. Effect of the amount of KOH on the yields of the three mol of mol of 1-butanol, 4.59 x final products: 9.17 x mol of CH2Br2, 3.11 x mol of TBAB 1-octanol, 2.76 x catalyst, 10 mL of HzO, 50 mL of chlorobenzene, 1020 rpm, 50 "C, 2 h of reaction.
of two mixed alcohols and dibromomethane. Figure 5 depicted the experimental results for the effects of the amount of KOH upon the conversion of dibromomethane and the yields of the three individual products. Alkaline was usually added in excess amount relative to its stoichiometric quantity during the two-phase phasetransfer-catalyzed reaction. A 15% alkaline solution would generally be sufficiently high to obtain a quite rapid chemical reaction rate for most cases (Wang and Wu, 1990b;Wang and Yang, 1990). As shown in Figure 5, however, the total conversion of CHzBr2 (or total yields of the three final products) is relatively low using such a concentration of KOH. For example, only 8.96% of the conversion of CHzBrz was obtained in using 10 g of KOH which is roughly 50% in large excess. The conversion of CHzBrz is increased with an increase of the amount of KOH used. The synthesis of acetals via phase-transfer catalysis should therefore be carried out at a large amount of KOH so as to obtain a high product yield. A maximum value of the fraction of the unsymmetric acetal compound ( C ~ H ~ O ) ( C ~ H I ~ O among )CH~ the three final products (PdX = 0.564) was obtained in using 15 g of KOH. (iv)Effect of Organic Solvents. Dibromomethane reacts with tetrabutylammonium alkoxides (&OR1and QOR2) in the organic phase to form the three final products. Dibromomethane, which possesses weak dipole moment, would form a weak dipole-dipole bond with the organic solvent. However, this kind of dipoledipole bond does not significantly affect the reaction rate. Nevertheless, QORl and QOR2 would solvate with a polar organic solvent. This solvation would result in less energy in the nucleophilic agent than that in the transition state compound. The activation energy therefore becomes high due to the solvation of &OR1 and QOR2 with a high polar solvent which is unsuitable in the current reaction system. The low polarity solvent would neither solvate with &OR1and &OR2 nor pull the tetrabutylammonium ion (Q+) apart from the alkoxide ion (RO-1. Thus, the reactivity of the low polar solvent is also low. Table 1 summarizes the effect of the organic solvent on the conversion of CH2Br2 and the yields of the three final products. This table indicates that chlorobenzene and dibutyl ether with the appropriate polarity are the best solvents to obtain a high yield of products. However, the fraction of the unsymmetric
Table 1. Effect of Organic Solvents on the Yields of the Three Final Products (6.88 x mol of 1-Butanol, 1.5 Molar Ratio of 1-ButanoYl-Octanol,30 g of KOH, 1 g of TBAB Catalyst, 2.76 x mol of CH2Br2, 10 mL of H20, 50 mL of solvent, 1020 rpm, 50 "C) solvent p1 p2 p2Ix p3 X chlorobenzene dibutyl ether 1-decanol cyclohexane
0.2467 0.2461 0.1902 0.1646
120 min 0.3259 (0.4585) 0.3072 (0.4506) 0.2256 (0.4344) 0.1986 (0.4144)
0.1382 0.1285 0.1035 0.1160
0.7108 0.6818 0.5193 0.4792
chlorobenzene dibutyl ether 1-decanol cyclohexane
0.2325 0.2085 0.1511 0.1516
90 min 0.2702 0.2649 0.1751 0.1305
(0.4334) (0.4543) (0.4163) (0.4147)
0.1207 0.1097 0.0944 0.0742
0.6234 0.5831 0.4206 0.4209
chlorobenzene dibutyl ether 1-decanol cyclohexane
0.1771 0.1552 0.1100 0.1082
60 min 0.2127 0.2163 0.1305 0.1200
(0.4352) (0.4791) (0.4147) (0.4173)
0.0990 0.0799 0.0742 0.0621
0.4888 0.4514 0.3147 0.2903
Amount of CH2Br, ( g )
-0--4.8
Time ( rnin )
Figure 6. Effect of the initial concentration of CHzBrz on the conversion of CHZBr2: 6.88 x mol of 1-butanol, 1.5 molar mol of TBAB ratio of 1-butanovl-octanol, 30 g of KOH, 3.11 x catalyst, 10 mL of HzO, 50 mL of chlorobenzene, 1020 rpm, 50 "C.
product ( C ~ H ~ O ) ( C ~ H I ~ Oamong ) C H ~the three final products (Pdx) remains almost unaffected by the organic solvents. (v) Effect of Dibromomethane. The effects of the concentration of dibromomethane in the organic phase on the conversion of CHzBr.2 and the yield of the unsymmetric product (C~H~O)(CSHI~O)CH~ are shown in Figures 6 and 7. In Figures 6 and 7, the reaction rate increases with an increase of the concentration of CH2Br2 in the organic phase. Also, the conversion of CH2Br2 and the yields of the three final products at 2 h of reaction also increase with an increase of the concentration of CH2Br2 in the organic phase. However, the fraction of the unsymmetric acetal (C4HgO)(CsH170)CH2 among the three final products (Pdx) almost remains at a constant value (0.4441).A higher concentration of CH2Br2 enhances the yield of the unsymmetric product. (vi) Effect of Temperature. The effects of temperature on the conversion of CH2Br2 and the yields of the three final products are shown in Figure 8 and Table 2. In general, the yields of the fractions of the three final products both increased with time. However, the yield of the unsymmetric acetal (C4HgO)(C8H170)CH2 increases more rapidly than that of the other two symmetric acetals in increasing the temperature. Typical
Ind. Eng. Chem. Res., Vol. 34,No.11, 1995 3701 h
L
Table 2. Effect of Temperature on the Yields of the Three Final Products (4.59 x lowzmol of 1-Octanol, 30 g of KOH, 2.76 x mol of CHBra, 3.11 x mol of TBAB Catalyst, 10 mL of HzO, 50 mL of Chlorobenzene, 1020 rpm, 2 h of Reaction)
04
v
d
-0-4.8
X
-A-3.6 -V-3.0 4 2.4
2
-0- 4.2
02
temp(OC) 30 35 40 45 50
(r
0
.-8
c)
E *
s
u
30 35 40 45 50
00 0
20
40
60
80
100
I20
Time ( min ) Figure 7. Effect of the initial concentration of CHzBrz on the yield of ( C ~ H ~ O ) ( C B H ~ ~ Osame ) C H ~reaction : conditions as given in Figure 6.
Temperature ( C )
30 35 40 45 50 30 35 40 45 50
II /" A
-0-10
PI pz pzfx P3 0.5 Molar Ratio of 1-ButanoUl-Octanol 0.0147 (0.3611) 0.1210 0.0767 0.0301 0.1172 (0.3658) 0.1731 0.0483 0.1758 (0.3681) 0.2535 0.0604 0.2189 (0.3838) 0.2910 0.0690 0.2822 (0.4102) 0.3367 1.0 Molar Ratio of 1-ButanoUl-Octanol 0.0741 0.1027 (0.4083) 0.0747 0.1101 0.1488 (0.4116) 0.1026 0.1373 0.2194 (0.4299) 0.1537 0.1631 0.2690 (0.4377) 0.1825 0.1768 0.3123 (0.4500) 0.2049 1.5 Molar Ratio of 1-ButanoV1-Octanol 0.0703 0.0840 (0.4142) 0.0485 0.1318 0.1157 (0.4212) 0.0744 0.1616 0.1903 (0.4299) 0.0907 0.2183 0.2588 (0.4309) 0.1235 0.2000 0.3259 (0.4439) 0.1382 2.5 Molar Ratio of 1-ButanoV1-Octanol 0.1450 0.0754 (0.3314) 0.0072 0.1990 0.1158 (0.3386) 0.0273 0.2521 0.1581 (0.3483) 0.0437 0.3095 0.2067 (0.3574) 0.0621 0.3956 0.2766 (0.3680) 0.0795
X 0.2124 0.3204 0.4776 0.5703 0.6879 0.2515 0.3615 0.5104 0.6146 0.6940 0.2028 0.3129 0.4426 0.6006 0.7341 0.2275 0.3421 0.4540 0.5783 0.7517
Volume of aqueous phase ( mL ) 1.5
-
0
20
0
A
15 10
l
,
h
Y
10
v
s Time ( min )
Figure 8. Effect of temperature on the yield of the final product mol of 1-butanol, 4.59 x ( C ~ H ~ O ) ( C ~ H ~ ~ O4.59 ) C HxZ : mol of 1-octanol, 30 g of KOH, 2.76 x mol of CHZBrz, 3.11 x mol of TBAB catalyst, 10 mL of HzO, 50 mL of chlorobenzene, 1020 rpm, 50 "C.
results for the dependence of the fraction of the unsymmetric acetal product ( C ~ H ~ O ) ( C ~ H I ~ O on) C time H ~a t various temperatures are shown in Figure 8. In Table 2, the fraction of the unsymmetric acetal (C4H90)(CsH170)CHz (P2Ix) among the three final products increases with temperature. Therefore, the production of unsymmetric acetal is favorable at a relatively high temperature. (vii) Kinetics of the Reaction. From the experimental data, the intermediate products (C4HgO)CHBr and (CsH170)CHBrwere not found during the reaction. These results indicate that the rates of the two first reactions in the organic phase would therefore become the rate-determining step. A pseudo-steady-state hypothesis is employed to describe the reaction rate, i.e.,
-
d[CH2Br210 = kapp[CH2Br21, dt
(7)
where Kapp is given as kapp
= k1[C4HgOQIo + k2[C&1,OQIo
(8)
For use of a large excess of C4H90H and CsH170H, the concentrations of the active catalysts C4H90Q and
0.5
0.0
20
l
40
.
l
.
l
60
.
so
l
.
100
l
I 20
Time (in)
Figure 9. Effect of the volume of the aqueous phase on the conversion of dibromomethane: 6.88 x mol of 1-butanol, 1.5 molar ratio of 1-butanoU1-octanol, 30 g of KOH, 2.76 x mol of CHZBrz, 3.11 x mol of TBAB catalyst, 50 mL of chlorobenzene, 1020 rpm, 50 "C.
CsH170Q remain constant. Therefore, (7) is integrated as -ln(l - X ) = kappt
(9)
in which X is the conversion of dibromomethane and is defined by (1). Figures 9 and 10 show the experimental data following the pseudo-first-order rate law. The effect of the volume of aqueous phase on the conversion is depicted in Figure 9. Not much difference in conversion was obtained when using the volume of the aqueous phase from 5 to 20 mL. Similarly, as shown in Figure 10, the conversion of CH2Br2 is not affected by the amount of CH2Br2.
Conclusion Various formaldehyde acetals in symmetric and
3702 Ind. Eng. Chem. Res., Vol. 34,No. 11,1995 20
Amount of CH,Br, ( 8 ) I5
0
4.8
0
4.2 3.6 3.0 2.4
0
v h
4
v
0
IO
3 05
00
Time (min)
Figure 10. Effect of the amount of CH2Br2 on the conversion of CHzBr2: 6.88 x mol of 1-butanol, 1.5 molar ratio of 1-butanol/ 1-octanol, 30 g of KOH, 1 g of TBAB catalyst, 10 mL of HzO, 50 mL of chlorobenzene, 1020 rpm, 50 "C.
unsymmetric forms were synthesized in this work by successfully reacting CH2Br2 and 1-butanol and l-octanol via phase-transfer catalysis (F'TC). The primary advantage of employing such a technique is that the unsymmetric acetal compound was synthesized. The unsymmetric acetal compound is easily separated from the other two symmetric acetal compounds through column distillation. Only a high alkaline concentration of KOH can lead t o a high yield of product and conversion of CH2Br2. Three unique final products with two alkoxide substituents were produced from the solution. No other products with only one alkoxide substituent were observed. The conversion of CH2Br2 and the yields of the three final products were affected by the molar ratio of 1-butanol/l-octanol, amount of catalyst, content of KOH, organic solvents, and temperature. However, the fraction of the unsymmetric acetal ( C ~ H ~ O ) ( C ~ H I ~ O among ) C H ~the three final products ( P d .was affected by the molar ratio of 1-butanol/ 1-octanol, content of KOH, organic solvents, and temperature.
Acknowledgment
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Received for review January 17, 1995 Revised manuscript received J u n e 21, 1995 Accepted July 6, 1995@
The authors thank the National Science Council, Taiwan, ROC, for financial support of this manuscript under Contract No. NSC 80-0402-E007-12.
IE950052A
Literature Cited Bal, B. S.; Pinnick, H. W. Convenient Conversion of Alcohols into Formaldehyde Acetals or Ethers. J . Org. Chem. 1979, 44(21), 3727-3728.
Abstract published in Advance A C S Abstracts, October 1, 1995. @
