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Kinetics of Synthesizing Dibutanoxymethane at Very High Alkaline Concentration by Phase Transfer Catalysis.

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Kinetics of Synthesizing Dibutanoxymethane at
Very High Alkaline Concentration by Phase
Transfer Catalysis
M.=L.Wang* and S.=W. Chang
Deparfment of Chemical Engineering, National Tsing Hua University;
Hsinchu, TAIWAN, ROC 300453
The reaction of dibromomethane and 1-butam1 to synthesize dibutatwxymethane in a
very high alkaline concentration of KOH (i.e. large amount of solid KOHlorganic
solvent) by phase transfer catalysis was studied. Only one uniquefinal product with two
butanoxide substituents was observed in the organic phase, thus indicating a very fast
rate for the second reaction. The reaction rate was significantly enhanced by adding a
small amount of quaternary ammonium salt. Only a pseudo-first-order rate law can be
applied to describe the reaction at a very high concentration of KOH. This peculiar
phenomenon was explained by the limitation of the chemical reaction equilibrium in the
aqueous phase as the controlling step at a low KOH concentration, and the reaction in
the organic phase as the controlling step at a high KOH concentration. The experimental
results indicate that the mass-transfer resistance of the active catalyst
((C4H9)4N+-0CdHg; QOR)is negligible, compared with the reactions in the two phases.
When using TBAB 0s the catalyst, the only way to increase the product yield is by the w e
of a solvent with appropriate polarity and a small amount of water at a very high alkaline
KOH content.
Keywords:
Dibutanoxymethane, phase transfer catalysis, reaction mechanism and
kinetics
Introduction
The reaction of the expensive dialkyl sulfate and alkoxide salt in an aqueous solution
has been frequently used to obtain the ether product. Therefore, the low-cost alkyl
halide is the prefemd choice to act as the akylating agent in the organic synthesis.
However, the synthesis of ether compounds using akyl halide as the reactant has to
be carried out with a slow reaction rate in the organic phase. Fortunately, the
reaction rate of synthesizing ether compounds can be enhanced by adding a small
amount of quaternary ammonium salt to the two-phase reaction [l-71.
The synthesis of diakoxymethane has often been obtained from the reaction of
alcohols and formaldehyde. Alcohols and aldehydes have been used [8] to
synthesize acetals and low molecular weight oligoformals, RO(CH,O),R.
Experiments for the reaction of alcohol and poly-phosphoric acid in a DMSO solvent
have been performed [9]. However, the reaction condition resulted in the reaction of
formaldehyde dialkyl acetals with small molar quantities of formaldehyde donors to
*Authorfor correspondence.
124
Kinetics of Symhcsizing Dibuanorymcthane at Very High Alkaline Concentration
by Phase Transfer Catalysis
produce the related oligoformals [8]. Therefore, a wide molecular weight
distribution of the acetal compounds was obtained. A unique desired product was
thus not available. Acetal compounds have also been synthesized by reacting
alcohols and dichloromethane in a 50% sodium hydroxide solution using Tixoget VP
clay as the catalyst [lo]. However, the reaction rate was still very slow. Studies
were carried out using the technique of phase transfer catalysis to synthesize acetals
from alcohols and dichloromethane [l 13. Only 5040% yield was obtained after 15
hours.
The ether compounds, i.e. acetals, can be synthesized from alcohols (or phenols)
and alkyl halide by employing phase transfer catalysis [1,4,6]. However, both the
product yield and the reaction rate are still low even at the high alkaline
concentration of KOH (about 50%) used in our preliminary tests. Therefore, the
main purpose of the present study was to investigate the reaction mechanism and
kinetics for synthesizing dibutanoxymethane from I-butanol and dibromomethane in
a very high alkaline concentration of KOH (i.e. large amount of solid KOWorganic
solvent) by phase transfer catalysis (PTC) [l-51. The main advantage of using PTC
is that potassium butanoxide is synthesized in-situ during the two-phase reaction. It
is not necessary to first synthesize potassium butanoxide separately, and the
decomposition of potassium butanoxide due to humidity in the air can be prevented.
The product can also be easily separated from the reactant by evaporating the
solvent. Therefore, a high yield of unique product can be obtained by phase transfer
catalysis with a high reaction rate.
Experimental Details
Materials:
Reactants such as 1-C4H90H,CH2Br2,(C4H9),NBr (TBAB or QBr) and other
chemicals.
reagents were all guaranteed grade (G.R.)
Procedures:
(A) Two-phase phase transfer catalytic reaction
The reactor was a 150 ml three-necked Pyrex flask,serving the purposes of agitating
the solution, recording the temperature, taking samples, and introducing feed. The
reactor was submerged in a constant-temperature water bath which could be
controlled to within H.1"C. To begin a kinetic run, known quantities of KOH and
l-C,$OH were prepared and dissolved in water and organic solvent for 1 hour at
the desired temperature. The solution was then introduced into the thermostatically
controlled reactor. Measured quantities of CH2Br2(also at the desired temperature)
were then added to the reactor.
To start the reaction, tetrabutylammonium bromide (QBr or TBAF3) was added to
the reactor. At a chosen time, a sample of the organic and aqueous solution was
withdrawn from the reactor, and the organic and aqueous phases of the sample were
quickly separated. After separation, 0.5 ml of the organic-phase sample was
immediately diluted with an excess amount of water (4 ml) and organic solvent (4
ml) at 4°C to quench the reaction. The sample procedure usually took less than 20
seconds.
125
M.-L. Wang andS.-W.Chang
The product was analyzed by GC, mass spectrometry, IR and NMR. Gas
chromatography (GC) was carried out to analyze the product (C4H90)2CH2using a
Shimadzu GC-9A instrument. A 7G 0.525mm x 15m, glass column containing
100%poly(dimethylsi1oxane) was used to separate and analyze the components. The
detector was an FID and the injection temperature was 220-250°C. Mass spectra
were obtained from a JOEL JMS-100 mass specnometer with ionization potentials of
12ev. The infrared spectrophotometer (IR) was a Perkin Elmer instrument, model
no. 983. The 1H Nh4R spectra were obtained in CDC13 solvent at 400 M H z (from
Brucker Co.).
(B) Detection of the active catalyst (QOR) and its distribution between two
phases
In this study, three experimental runs were conducted to detect the active catalyst
((C4H9)4N+-OC4H9; QOR). Firstly, the experiments were carried out such that the
active catalyst was synthesized in the aqueous phase, from the reaction of 1-butanol
and tetrabutylammonium bromide in a very high alkaline concentration of KOH (or
large amount of solid KOH) with the presence of the TBAB catalyst. Thus, QOR
transferred to the organic phase without containing the dibromomethane reactant.
Secondly, the active catalyst QOR was detected directly, and measured in the organic
phase in the phase transfer catalytic reaction. These two independent experiments
provided identical concentrations of QOR, under the same reaction conditions.
Thirdly, the distribution of QOR between the two phases was obtained at equilibrium
between the two phases.
The active catalyst (QOR) was analyzed by employing the thermolysis of
quaternary ammonium salts, commonly known as "Hoffmann Elimination" [12]. A
Shimadzu GC-9A gas chromatograph was used to analyze the tertiary mine. The
detector was an FID with an injection temperature of 360°C.
Results and Discussion
It has proved difficult to identify the reaction mechanism of the two-phase phase
transfer catalytic reactions. Probably because the active catalyst (QOR) was very
difficult to synthesize, or separate from the solution. Therefore, an understanding of
the reaction mechanism and modelling of the dynamic behavior of the reaction
system by phase transfer catalysis (PTC) has bee: limited by the synthesis of QOR
[13]. In this work, the active catalyst ((C4H9)4N -OC4H9; QOR) can be analyzed
by gas chromatography (GC). Therefore, the reaction mechanism of the present
reaction system can be identified based on experimental data. QOR was detected in
the reaction, which was described previously in the Experimental Details. The
experiments were conducted such that QOR was synthesized in the aqueous phase,
and then transferred to the organic phase without containing the dibromomethene
reactant. The distribution coefficient of QOR was also obtained. It was shown that
the active catalyst (QOR) was almost observed in the organic phase (>91%). Thus,
the reaction mechanism is proposed as:
126
Kinetics of Synthesizing Dibuanorymeihane at Very High Alkalino Concentration
by Phase T r a q e r Catalysir
2ROH
+ 2KOH-2ROK
2KBr
+ 2QOR-2ROK
CH2Br2
t
+ QOR
CH2(0R)Br
+ QOR
t
+
2H20
+
+
2QBr
k1 t CH2(0R)Br + Q l r
(aqueous)
(1)
(organic)
k2 t CH2(0R)2+ QBr
As shown above, 1-butanol (ROH) first reacts with KOH to form potassium
butanoxide (ROK) in the aqueous phase. Then, the ROK salt reacts with quaternary
ammonium salt (QBr) to form quaternary ammonium butanoxide (QOR), which is
soluble in the organic solvent. In the organic phase, CH2Br2 then reacts with QOR
to form the desired product, namely dibutanoxymethane. There are two reaction
steps in the organic phase. However, the fmt product, CH2(OR)Br, was not detected
during the reaction or after the termination of the reaction. Only the final product
(CH2(OR),) was detected. This indicates that for the reaction rate constants,
k, << k2 Therefore, the first reaction (which synthesizes the fust product) will be
the rate-determining step in the organic phase reaction.
In the two-phase phase transfer catalytic reaction, the concentration of the active
catalyst (QOR) was measured as a function of time. It was found that most of the
QOR (>91%) transferred to the organic phase within one minute. The concentration
of QOR in the organic phase was then kept at a constant value. For example: using
6.8 g of 1-butanol, 4.8 g of CH2Br2,30 g of KOH, 1 g of TBAB catalyst, 10 rnl of
H20, 50 rnl of chlorobenzene, and 1020 rprn at 50°C, about 91.22% of the QOR was
observed in the organic phase throughout the reaction run. This indicates that the
mass-transfer resistance of QOR from the aqueous phase to the organic phase is
small, compared with the chemical reaction resistance in the organic phase. The
organic phase reaction obviously dominated the overall reaction rate. Thus,material
balances for the first product and the final product in the organic phase were made as
follows:
d[CH2(OR)Brlddt = k, [QORlo[CH2Br210- k~[QORlo[CH~(OR)Br]o
(2)
Since the second reaction rate is much greater than the first reaction rate in the
organic phase, the first product was not observed during the reaction. Therefore, the
production rate of the first product from Equation (2) is equal to the consumption
rate of the first product from Equation (3) in the organic phase:
127
M.-L. Wang a d s . - W .Chang
d[CH2(OR)Br]ddt = 0
Thus, Equation (2) becomes:
SubstitutingEquation ( 5 ) into Equation (3):
where kapp is given by:
In these experiments, only one unique final product was observed. Therefore, the
consumption rate of CH2Br2is equal to the production rate of CH2(ORb:
d[CH2Br2]ddt= - d[CH2(OR),]ddt
Therefore, Equation (6) can be written as:
-ln(l
- X ) = kappt
(9)
The conversion of dibromomethane is defined as:
X = 1 - [CH2Br2]d[CH2Brd:
where the superscript "0" denotes the initial concenuation of dibromomethane in the
solution.
In order to study the reaction kinetics, the effects of the reaction parameters on
the reaction rate were investigated and are summarized in the following sections.
(A) Amount of catalyst
In this study, tetrabutylammonium bromide (TBAB or QBr) was used as the catalyst
to synthesize dibutanoxymethane. The main reason for adding TBAB was to
increase contact between two immiscible reactants (1-butanol and dibromomethane)
by bringing the nucleophiIe (C4%O-) from the aqueous phase to the organic phase.
Only a 0.002% conversion was obtained after 600 minutes of reaction time without
adding the TBAB catalyst for a regular experimental run. Conversion up to 90% can
be obtained after 120 minutes by adding a small amount of the TBAB catalystThe results from synthesizing dibutanoxymethane with TBAB (QBr) as the phase
aansfer catalyst are shown in Figure 1. The apparent reaction rate constant (kapp) is
approximately a linear function of the initial concentration of the TBAB catalyst
[14]. This is probably because the aqueous solution was saturated with QOR, and
most of the QOR was forced into the organic phase. Therefore, the concentration of
128
Kinetics of Synthesizing Dibutanoxymthane at Very High Alkaline Conctniration
by Phart Tram+ Cailysir
QOR in the organic phase increased with an increase in the initial concentration of
QBr in the aqueous phase. From Equation (7), the ka value will increase with an
increase in the concentration of QOR in the organic pRase. This result is genedly
consistent with published data [15,16].
I
0.m
O
S m
O*OM
ntt
e
r
n
r
n
t
Inltlol mole r o t l o o f QBr/M,Bs
Figure 1. Effect of the initial mole ratio of QBrlCH2Brz on the apparent reaction
rate constant (kapp) [6.8g or I-butanol; 4.88 of dibromomethane; 308 of K O H ;
lOml of H 2 0 : 50ml of chlorobenrene; Ig of TBAB catalyst: 1020rpm; SO'C]
(B) Effect of agitation speed
As previously stated, the mass transfer resistance of QOR and QBr can be neglected
in comparison with the resistance of the organic phase reaction. The effect of the
agitation speed on the value of kapp is shown in Figure 2. The value of bpp
is
independent of agitation speeds greater than 200 rpm, i.e. the mass transfer
resistance of QOR and QBr is negligible for agitation speeds greater than 200 rpm.
Therefore, the agitation speed was kept at 1020 rpm in the following experiments
used to obtain kinetic data.
(C) Concentration of dibromomethane
The reaction of the organic phase in the present study can be described by an S
,2
mechanism. The conversion of 1-butanol as a function of time with different
concentrations of dibromomethane are shown in Figure 3. It can be seen that the
conversion of dibromomethane is insensitive to the concentration of
dibromomethane in the organic phase. As shown in Figure 4, the value of kapp is
unaffected by the concentration of dibromomethane in the organic phase, if greater
129
0.02
-
stlrrlng rate C r p m l
Figure 2. Effect of the agitation speed on the apparent reaction rate constant (kapp)
(other conditions as givenfor Figure I )
0.71
0
0
m
Qo
Tlme Cmln)
Figure 3. EIfect of the concentration of dibromomethane on the product yield (other
conditions as given for Figwe 1 )
130
Kinetics of Synthesizing Dibutanoxytneihane at Very High Alkaline Conceniration
by Phase Tranvfer Catalysir
0.Q
-
0.01
-
8
Y
om
I
CHrBnconcentratIon
(g/60rnl of chlorobenzwre)
Figure 4. Effect of the concentration of dibromomethane on the apparent reaction
rate constant (kapp)(other conditions as givenfor Figure I )
(D)Amount of KOH and water
Alkali is usually added in excess of the stoichiometric quantity in the two-phase
phase transfer catalytic reaction for synthesizing ether compounds. A 15% alkaline
solution will often be sufficient to obtain a quite fast chemical reaction rate.
However, a low conversion of dibromomethane was obtained using a small amount
of KOH in the present study. Only 21% conversion of dibromomethane was
obtained when using 10 g of KOH, which is equivalent to 50% KOH in the aqueous
solution. The conversion increased as the KOH increased. A high concentration of
KOH in the reaction is therefore recommended in order to obtain a high yield of
acetal compounds.
The concentration of QOR in the organic phase which was dependent upon the
amount of KOH being added to the reactor was measured in the present study.
About 69% of QOR stays in the organic phase when 7.5 g of KOH (about 37.5% of
KOH) is added to the reactor. However, most of the QOR (>90%)was observed in
the organic phase when 30 g of KOH was added to the reaction system. In general, a
high concentration of QOR in the organic phase will be favorable for the two-phase
reaction. Based on the active catalyst distribution at very high alkaline
concentrations ( ~ 1 g5 of KOH), it is reasonable to expect a high product yield (and
reaction rate) for the present reaction system.
The phase transfer catalytic reaction can usually be described by a pseudo-fitstorder rate law, allowing for convenient treatment of the experimental data. The
concentration of the aqueous reactant (1-butanol) must usually be more than three
times the concentration of the organic reactant (dibromomethane). The
concentration of KOH must also be slightly greater than that of the concentration of
I31
M.-L. Wang ads.-W. Chang
the aqueous reactant. However, as shown in Figure 5, the reaction is very slow and
does not follow the pseudo-first-order rate law in the present reaction system (at a
relatively low KOH concentration < 15 g of KOH/10 ml of H20). Regression
analysis of the experimental data indicated that a pseudo-first-orderrate law can only
be used to describe the reaction at high KOH concentrations. The regression
coefficient using a very large amount of KOH is very close to unity (see Figure 6),
indicating that a pseudo-first-order-ratelaw can describe the reaction kinetics. The
min-l when the KOH
reaction rate increases significantly from 0 . 3 ~ 1 0 -to~ 16.7~10'~
concentration is increased from 5 g to 30 g. The change in reaction rate at high and
low KOH concentrationscan be explained as follows.
Figure 5. A regression curve used to determine the conversion of dibromomethane
vs time at low KOH concentrations (other condtions as given for Figure I )
The amount of C 4 b 0 K formed is increased by increasing the amount of KOH
[17],due to the removal of a proton from the C4qOH in a KOH solution. It reaches
an equilibrium state very quickly for the reaction of 1-butanol with KOH in the
aqueous phase at a low KOH concentration. The concentration of ROK at
equilibrium decreases as the KOH concentration decreases. The decrease in the
concentration of ROK also makes the concentration of QOR decrease. From
decreases gradually as KOH is consumed in the
Equation (7), the value of k
aPP
reaction. Therefore, the reaction system is controlled by the chemical reaction
equilibrium of l-C,H,OH and KOH at a low KOH concentration in the aqueous
phase. However, the reaction system is controlled by the reaction in the organic
phase at a high KOH concentration (> 30 g of KOH/lO ml of H20).
132
Kinetics of Synthesizing Dibutanoxymethane at Very High Alkoline Concentratwn
by Phase Transfer Catalysis
Tlme C m l n l
Figure 6. Effect of the amount of KOH on the conversion of dibromomethane (other
conditions as given for Figure I )
(E)Types of solvent
Five different solvents were used in this work. The relative polarity of these five
solvents are: dichloromethane > dibromomethane > chlorobenzene> cyclohexane >
benzene. In the organic phase, dibromomethane reacts with QOR to form the frrst
and final products. Dibromomethane,which possesses a weak dipole moment, will
usually form a weak dipole-dipole bond with the organic solvent. In general, this
type of dipole-dipole bond does not significantly affect the reaction rate, although
QOR will solvate with the polar organic solvent. This solvation will ensure that the
energy of the nucleophilic agent is less than that of the transition state compound.
Therefore, a higher activation energy is obtained when using a very polar soIvent,
and it is not suitable in the present reaction system. However, the low polarity
solvent will neither solvate with QOR, nor will it pull the Q+ ion apart from the
butanoxide. Thus, the reactivity is also small for the low polarity solvent. Figure 7
shows the effect of solvent on the conversion of the reactant. Both dichloromethane
and dibromomethane give a very high product yield with a solvent of appropriate
polarity.
Q Volume ratio of water to organic solvent
In the two-phase phase transfer catalytic reaction, the reaction rate will be highly
dependent on both the volume of the aqueous phase and the volume of the organic
phase. Figure 8 shows the effect of the volume ratio of aqueous phase to organic
phase on the product conversion. For these experiments, the concentration of
133
1-21
+
qer,
o
Chiorobenzene
A
Benzene
im
0
aM
Tlme Crnln)
Figure 7 . Effect of the type of solvent on the conversion of dibromomethane (other
conditions as given for Figure 1)
aqueous reactant (or organic reactant) is constant for every run, i.e. the aqueous
reactant (or organic reactant) is added as the amount of water increases (or organic
solvent). The product conversion decreases with an increase of the volume ratio of
12.5 m l
10 m i
H20 7.5 rnl
0 . 0 ~H20
o H20
0.8.0
;;0.7-
6 0.8 E 0.5 -
U
0.2
0. I
1
0
30
60
80
Tlns.Cmln)
la0
150
Figure 8. Effect of the amount of water on the conversion of dibromomethane
(concentration of the aqueous reactant was constant for all experiments) (other
conditions as givenfor Figure 1 )
134
Kinetics of SynIhesizing Dibutanaxymethaneat Very High Alkaline Concentration
by Phaw Transfer Catalysis
aqueous phase to organic phase. Therefore, a low volume ratio of aqueous phase to
organic phase is recommended in the two-phase phase transfer catalytic reaction in
order to obtain a higher product conversion.
(C) Concentrationof aqueous reactant
Figure 9 shows the effect of the aqueous reactant concentration on the kTp value.
In these experiments, the concentration of the aqueous reactant is diluted with a large
aqueous volume, i.e. the amount of aqueous reactant was kept constant for all
experiments. An optimum value of the aqueous concentration produces a maximum
value of kaw. Using 6.8 g of 1-butanol and 4.8 g of CH2Br2in the reaction and
about 10 ml of water (corresponding to a volume ratio of 15:1 for aqueous phase to
organic phase), increases the value of kapp
Volltme r a t l o o f C W O I
Figure 9. Effect of the volume ratio of aqueous phaselorganic phase on the apparent
reaction rate constant (kapp) (concentration of aqueous reactant was diluted and
changed with the increase of the aqueous volume) (other conditions as given for
Figure I )
(H) Effect of temperature
From the Arrhenius law, the reaction rate will be enhanced by increasing the
temperature. From Figure 10, the apparent pseudo-first-orderreaction rate constant
increases when the reaction temperature is increased. The Amhenius plot gives the
activation energy as 19.69 kcal/mole.
135
M.-L. Wang and S.-W. Chang
310
32n
340
330
C 1 /T)xl$
C I /K)
Figure 10. An Arrhenius plot of kapp value vs I I T (other conditions as given for
Figure 1 )
Conclusions
The reaction mechanism and kinetics were investigated for the synthesis of
dibutanoxymethane, by reacting CH2Br2 and 1-C4%0H in a very high alkaline
concentration of KOWorganic solvent by phase transfer catalysis. Only the unique
final product with two butanoxide substituents was observed in the organic phase.
At a higher concentration of KOH, the reaction rate is a linear function of the
dibromomethane concentration in the organic phase, and follows the pseudo-fmtorder rate law. The mass-transfer resistance of QOR can be neglected, when
compared with the reactions in the two phases. The system was controlled by the
reaction equilibrium of 1-butanol and potassium hydroxide at a low KOH
concentration in the aqueous phase. The system was controlled by the reaction in the
organic phase at a very high KOH concentration. A high reaction rate can only be
obtained using a small amount of water at a very high alkaline concentration (or
large amount of solid KOH). The results were investigated in terms of the
distribution of the active catalyst (QOR) between two phases, and the chemical
reaction equilibrium of 1-butanol and KOH in the aqueous phase.
Acknowledgement
We acknowledge financial support from the National Science Council, Taiwan, ROC
(Grant NO. NSC 804402-EOO7-12).
I36
Kinetics of Synthesizing Dibutanoxymethaneat Very High Alkaline Concentralwn
by Phase Tramfer catalysis
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Received 10 March 1993; Accepted qfter revision: 12 August 1993.
137
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