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Kinetic Study of Synthesising 2 4 6-tribromophenyl Benzyl Ether by Phase Transfer Catalysis.

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Kinetic Study of Synthesising
2,4,6=tribromophenyl Benzyl Ether by
Phase Transfer Catalysis
Maw-Ling Wang' and Yu-Ming Hsieh
Department of Chemicai Engineering, National Tsing Hua
University, Hsinchu, Taiwan 30043, Republic of China
The reaction of 2,4,6-tribromophenol and benzyl bromide in an alkaline solution of
KOH/organic solvent by phase transfer catalysis was studied. The main product
C ~ H S C H ~ O { C ~ and
H ~ )the
B ~ intermediate
~
product BuqN-O(C&)Br3 we re
synthesised, and subsequently identijied analytically. The kinetics of the reaction
were studied in detail. The effects of the operating conditions such as agitating speed,
amount of catalyst, types of solvent and catalyst, salt effects, amount of water, mole
ratio of benzyl bromide to 2,4,6-tribromophenol, content of KOH and temperature,
on the conversion and the reaction rates were investigated. The negligible
mass-transfer resistance of the catalyst between the two-phase reaction was indirectly
identified from experimental observations. The organic-phase reaction is the
rate-controlling step for the two-phase reactions. During the reaction, a constant
concentration of the intermediate product was obtained in the organic phase.
Therefore, a pseudo-first-order rate law can be applied to describe the two-phase
reaction. The activation energy for this reaction system was obtained as 18.31
kcal mar'.
Introduction
In order to obtain fast reaction rates from two immiscible reactants, the usual
approach is to use a solvent which can dissolve both reagents. Use of such
solvents is not always convenient and on an industrial scale can be expensive.
This reaction problem of two immiscible reactants is solved by adding a small
quantity of a catalytic quaternary salt. The use of quaternary salts as
phase-transfer catalysts in the two-phase reaction to synthesise speciality
chemicals has been extensively studied.'-3 It is now considered to be one of the
most effective tools for synthesising organic chemicals from two immiscible
reactant^.^.'
The conventional method used to synthesise organic ethers is the Williamson
synthesis. Sodium alkoxide was first produced by the direct contact of sodium
metal and an alcohol in an anhydrous condition. Sodium alkoxide reacted with
~~
~
'To whom correspondence should be addressed.
Developments In Chemlcal Englneerlng and Mlneral Processing, Vol. 1,
No. 4, page 225
226
M. Wang and Y. Hsieh
alkyl halide to form the desired product. The synthesis of organic ethers by phase
The main advantage of using
transfer cataiysis (PTC) has been
PTC to synthesise organic ethers is that the reaction can be carried out avoiding
the use of a strong alkali, e.g. metallic sodium. Sodium alkoxide, which is
produced in-situ in the reaction solution, reacts with the quaternary ammonium
halide to form the organic-soluble quaternary ammonium alkoxide. The organic
ethers are then produced by the reaction of quaternary ammonium alkoxide and
alkyl halide in the organic phase. The main aim of this study was to investigate
the kinetics of the reaction of 2,4,6-tribromophenol and benzyl bromide in an
alkaline solution of KOWorganic solvent by phase transfer catalysis. In addition,
the synthesis and purification of the intermediate product ( B u ~ N - O ( C ~ H ~ ) B ~ ~ ;
ArOQ) was obtained from the aqueous phase reaction and the two-phase phase
transfer catalytic reaction. Having obtained a constant concentration of the
intermediate product (ArOQ) in the organic phase during the two-phase reaction
within a very short time, the reaction can be described by the pseudo-first-order
rate law.
Experimental Details
Materials
2,4,6-tribromophenol (ArOH), benzyl bromide (RBr), tetra-n-butylammonium
bromide (Bu@Br-; QBr or TBAB), and other reagents are all guaranteed grade
(G.R.) chemicals, supplied by Merck Co., Darmstadt, Germany.
Procedures
The reactor was a 300 rnL three-necked Pyrex flask, which enabled solution
agitation, temperature measurement, sample removal and feed addition. The
reactor was submerged in a constant temperature water bath (controlled to within
k 0.l"C). To begin a kinetic run, known quantities of potassium hydroxide and
2,4,6-tribromophenol were prepared and dissolved in water. The solution was then
introduced into the reactor, which was maintained at the desired temperature.
Measured quantities of benzyl bromide and diphenyl ether (internal standard),
which were also at the desired temperature, were dissolved in the organic solvent
and then added to the reactor. To start the reaction, tetra-n-butylammonium
bromide (TBAB or QBr) was added to the reactor. During the reaction, an 0.8 mL
aliquot sample was withdrawn from the reaction solution at a chosen time. The
organic and aqueous phases of the sample separated within a few seconds. After
separation, 0.1 mL of the organic-phase sample was immediately diluted with
4.5 mL of methanol. Usually, it took less than 20 seconds to take a sample.
To ensure that the experimental accuracy was not affected by the sampling
procedures, an alternative sampling procedure was also adopted. In this case, an
excess amount of concentrated HCI solution was poured into the reactor to
terminate the reaction, by reacting with KOH in the solution. Then, the agitator
was stopped and the sample withdrawn from the organic phase for HPLC analysis.
It was found that good reproducibility and consistent results were obtained using
both procedures.
The 2,4,6-tribromophenyl benzyl ether (C6H$H20(C6H2)Br3) obtained from
the two-phase reaction was identified. The intermediate product,
Synthesising 2,4,6-tribromophenyl Benzyl Ether
22 7
tetra-n-butylammonium 2,4,6-tribromophenoxide (BudN-O(C6H2)Bq; ArOQ),
which can be synthesised from the reaction of tetra-n-butylammonium bromide
(TBAB or QBr) with 2,4,6-tribromophenol and KOH in the aqueous phase, was
also identified from the two-phase reaction. The HPLC model was a Water model
440, variable wavelength detector with a Water 740 data model integrator. The
column used was Lichrosorb RP-18 (5 m), from the Merck Corn any. The eluent
was CH30H:CH3CN:H20 = 4: 1:1, with a flow rate 1.1 mL min-? at 254 nm (UV
detector).
Interpretation of Results
In this study, the final product and the intermediate product were synthesised and
identified by NMR, mass spectroscopy and elemental analyser. The product,
2,4,6- t ri bromop hen y 1 benz y 1 ether (C ~ H S C H ~ O ( C ~ H ~A)rB0~R)~ , was
synthesised by reacting 2,4,6-tribromophenol (ArOH) and benzyl bromide (RBr)
in an alkaline solution of KOWorganic solvent using QBr as a phase-transfer
catalyst. The organic phase was separated from the aqueous phase and washed
with potassium hydroxide solution and pure water several times in order to
remove the residue of ArOH and QBr. The organic solvent was then stripped from
the solution in a vacuum rotary evaporator, and a solid-phase product was
obtained. The product can be purified by recrystallization using cyclohexane as
t h e s o l v e n t , producing white crystals. The intermediate product
( B u ~ N - O ( C ~ H ~ )ArOQ)
B ~ ~ , was a white precipitate from the reaction of ArOH
and QBr in an alkaline solution. The white crystal form of the intermediate
product (ArOQ) was washed several times with distilled water in order to remove
QBr.
It was observed that QBr and ArOQ dissolved mainly in the aqueous phase and
in the organic phase respectively. The aqueous reactant, ArOH, reacted with
potassium hydroxide to form potassium 2,4,6-tribromophenoxide (ArOK) in the
aqueous phase. The ArOK then reacted with QBr to form the organic-soluble
intermediate product (ArOQ). The final product (ArOR) was synthesised in the
organic phase by reacting ArOQ and RBr. QBr (or TBAB) also produced from the
organic-phase reaction, then transferred to the aqueous phase to reactivate its
reactivity. Therefore, the reaction mechanism can be expressed by the following
equation:
ArOH
+
KOH
+ ArOK +
H2O
L
KBr
+
ArOQ t ArOK
+
RBr + ArOQ
QBr(aqueous)
t
I
J.
+
ArOR
+
QBr (organic)
During the reaction, a constant concentration of the intermediate product
(ArOQ) was obtained. Therefore, on the basis of the experimental data, a
generalised approach to the phase-transfer catalysed reaction system can be
described by a pseudo-first-order reaction [equation (2) below], if equation (3)
also holds:
M. Wang and Y. Hsieh
228
0.02
1-
Weight o f TBflB ( g l
Figure 1 Effect of amount of TBAB catalyst on the apparent reaction rate
constant (kapp) (4 g of 2,4,6-tribromophenol;0.9 g of KOH; 0.6 mL of benzyl
bromide; 50 mL of H,O; 50 mL of chlorobenzene; 40°C).
where the superscript "org" denotes the characteristics in the organic phase. The
constant value of kapp is called the pseudo- steady-state first-order reaction rate
constant.
Equation ( 2 ) can be integrated to obtain:
ln(1 - X) = kappt
(4)
where X is the conversion of organic reactant (RBr), defined by:
where C R B ~ ,represents
O ~ ~ ~ the initial concentration of the organic reactant (RBr).
In order to determine the optimum conditions and to study the kinetics, the effects
of the reaction parameters and variables on the reaction rate were studied and are
summarised below.
229
Synthesising 2,4,6-tribromophenyl Benzyl Ether
0.05
TBAB
0.04
geo'm
u
0.Q
0.01
im
Time (mln)
150
m
Figure 2 Concentration profile of the intermediate product (ArOQ) versus time of
reaction [same reaction conditions as given in Figure I ] .
j
I
o
Chlorobenzene
\
M. Wang and Y. Hsieh
230
0.m
I
I
Table lReaction of six types of quaternary ammonium salts on
2,4,6-tribromophenol and benzyl bromide in an alkaline solution of KOH and
chlorobenzene organic solvent.
Phase transfer
catalyst
TEAC
BTEAC BTBAB TBAHS TBAB
kaPP
0.0008
0.0013
TEAC :
BTEAC:
BTBAB:
TBAHS:
TBAB :
TBAI:
tetra-ethylammonium chloride.
benzyl tri-ethylammonium chloride.
benzyl tri-n-butylammonium bromide.
tetra-n-butylammonium hydrogen sulfate
tetra-n-butylammonium bromide.
tetra-n-butylammoniurn iodide.
0.0085
0.0093
0.01
TBAI
0.011
Discussion of Experimental Results
Agitation speed
As shown by equation ( I ) , the conversion is dependent on the reaction rate in the
organic phase and the transfer rate of ArOQ from the aqueous phase to the organic
231
Synthesising 2,4,6-tribromophenyl Benzyl Ether
0
m
1w
Tlme (mln)
180
2al
Figure 5 Effect of amount of water on conversion of organic phase reactant
[same reaction conditions as for Figure I , except amount of water].
phase, as well as the transfer rate of QBr from the organic phase to the aqueous
phase. The effect of agitation speed on the reaction was studied. The operating
conditions were: 4 g of ArOH, 0.9 g of KOH, 0.3 g of the TBAB catalyst, 0.6 mL
of benzyl bromide, 50 mL of H20, 50 mL of chlorobenzene, 40°C. On the basis
of the experimental data, the conversion is not affected by an agitation speed
greater than 300 rpm, i.e. the corresponding value of kapp is independent of an
agitation speed greater than 300 rpm. Therefore, the kinetic data was obtained
from experiments using an agitation speed of 600 rpm.
nrnvuni VJ curaiysr
The addition of tetra-n-butylammonium bromide (TBAB) to the reaction solution
enhances the reaction rate. The possibility of colliding two immiscible reactants
is greatly increased by using a small amount of catalyst. Therefore, the reaction
rate will also increase with the addition of catalyst. Figure 1 shows the effect of
the amount of TBAB added on the apparent reaction rate constant (kapp). The
value of kappwas found to be a linear function of the amount of TBAB being used
in the reaction.
As stated previously, the conversion is highly dependent on the reactions in
the organic phase and in the aqueous phase, the mass transfer and distribution of
the catalyst (QBr), and the intermediate product (ArOQ) between two phases.
However, it has usually been difficult to identify the organic-phase reaction rate
as the rate-determining step.'-' The reason being that the reaction mechanism was
not previously fully understood, i.e. the intermediate product (ArOQ) could not
232
M. Wang and Y.Hsieh
Figure 6 Effect of amount of water on apparent reaction rate constant (kapp)
[same reaction conditions as given in Figure 1, except amount of water].
be identified during the reaction and hence isolated from the reaction. Fortunately,
in this study ArOQ can be identified during the two-phase phase transfer catalytic
reaction, and removed from the reaction by using 2,4,6-tribromophenol as the
aqueous phase reactant. Therefore, it was easy to determine the rate-controlling
step in this case.
Figure 2 shows that the concentration of the intermediate product ArOQ
remains at a constant value for the entire reaction, except for a very short initial
period. This supports the validity of using equations (2) and (3) in the present
study. The mass-transfer resistance of the intermediate product can thus be
neglected. The pseudo-steady-state approach is also confirmed by Figure 1, where
the reaction is shown to be controlled by the reaction rate in the organic phase.
Solvents
In the two-phase phase transfer catalytic reaction, the solvent significantly affects
the reaction rate. The main reason is that the distribution of QBr and ArOQ
between two phases is highly dependent upon the polarity of the organic solvent.
In general, it is desirable for most of the intermediate products to stay in the
organic phase and react with the organic reactant. Therefore, a solvent with high
polarity will be preferred for the reaction. Figure 3 shows the effect of different
solvents on the conversion of the reactant. It was expected that dichloromethane
would give an increased reaction rate when used as the solvent. The order of
relative reactivity of the solvents is: dichloromethane > chlorobenzene > toluene,
which is consistent with the prediction.
Synthesising 2,4,6-tribromophenyl Benzyl Ether
233
1 .o
x
I
c
0.I
0
50
100
150
200
Time (minl
Figure 7 Effect of molar ratio of organic-phase reactant to aqueous-phase
reactant on conversion of organic-phase reactant [same reaction conditions as for
Figure I with 4 g of ArOH, except amount of organic-phase reactant].
Types of catalyst
As stated by Starks and Liotta,’ the quaternary cation will be an important factor
affecting the reactivity of the catalyst. In the present study, six types of quaternary
ammonium salts were used as the phase transfer catalyst to determine their
reactivities on the reaction of 2,4,6-tribromophenol and benzyl bromide in an
alkaline solution of KOH and chlorobenzene organic solvent. The results are
shown in Table 1 , where it is seen that a higher reaction rate was obtained when
using TBAHS, BTBAB, TBAB and TBAI as the phase transfer catalyst. The
symmetrical form of quaternary cations, such as TBAHS, TBAB and TBAI, give
a higher reactivity.
Also from the above data, the anion of the quaternary salt is shown as an
important factor affecting the reaction rate, where the order of the relative
reactivity of the anion is: I- > Br- > HSO4. The reason is that I- will substitute in
benzyl bromide to form an active benzyl iodide, whereas the hydrogen sulfate ion
(a weak acid) will lower the reactivity in the two-phase reaction.
Salts
In the present study, potassium bromide was produced as the by-product in the
aqueous phase. Previous reported studies’ examined the effect of inorganic salts
on the reaction rate, although no definite conclusions were obtained to judge their
influence.” Additional potassium bromide was added to the reaction solution to
234
M. Wang and Y. Hsieh
1
x
I
L
0
50
loo
im
2m
Tlme (mini
Figure 8 Effect of amount of KOH on conversion of organic-phase reactant [same
reaction conditions as for Figure I , except amount of KOH].
determine its effect, the results show that the reaction rate decreases with the
increase in the amount of extra potassium bromide. With 5 g of KBr added, the
reaction rate did not decrease further, probably because the solution is then
saturated with KBr.
When extra KBr is added to the reaction solution, the amount of ArOQ
decreases as shown in Figure 4. This probably occurs because the aqueous phase
reaction is retarded by the extra addition of KBr, as predicted by LeChatelier’s
principle. An independent experiment was performed to measure the distribution
of ArOQ between two phases, as influenced by the addition of KBr. The results
are presented in Figure 4 where the concentration of ArOQ in the organic phase
decreases with the increase of the KBr amount in the two-phase solution. This
result is consistent with a study of the two-phase reaction.12
Amount of water
Results showing the effect of the volume of water on the reaction rate are given
in Figure 5 . In general, the concentration of compounds in the aqueous phase is
decreased by increasing the amount of water. The addition of water probably also
decreases the concentration of the intermediate product in the organic phase.
Hence, both the mass transfer rate and the degree of hydration with the anion are
decreased, which also decreases the reaction rate.’ However, Figure 5 indicates
that the conversion in the two-phase reaction is not affected by the amount of
water added. From Figure 6, the corresponding value of kapp is seen to be
independent of the amount of water in the two-phase reaction, as expected.
Synthesising 2,4,6-tribromophenyl Benzyl Ether
0.02
235
’
0
0
PF
OQ:
u
0.01
0
’
0
0
0.m
’
0
L
I
2
I
3
4
weight o f KOH ( g )
Figure 9 Effect of amount of KOH on concentration of intermediate product
(ArOQ) in the organic phase [same reaction conditions as for Figure I , except
amount of KOH].
Mole ratio of benzyl bromide to 2,4,6-tribromophenol
The influence of the mole ratio of benzyl bromide (RBr) to 2,4,6-tribromophenol
(ArOH) on the conversion is shown in Figure 7. The conversion is not affected by
the mole ratio of RBr to ArOH for reaction times below 60 minutes. After 60
minutes, the conversion is shown to decrease the higher the value of the mole
ratio. However, the pseudo-first-order rate law cannot be applied for a mole ratio
of RBr:ArOH > 1. The reaction rate constant obtained by experiment is
independent of the mole ratio of RBr:ArOH indicating that the mass transfer
resistance of QBr and ArOQ can be neglected. Therefore, the reaction kinetics of
the organic phase will be the rate-determining step of the two-phase reaction.
Concentration of potassium hydroxide
The reaction rate in the aqueous phase, and the distribution of ArOQ between the
two phases, will probably be affected by the concentration of KOH in the aqueous
phase. Therefore, the reaction rate or the conversion should be influenced by the
concentration of KOH in the aqueous phase. In the present study, the reaction rate
is shown to increase as the KOH concentration increases in the aqueous phase.
From Figure 8, the conversion is highly dependent on the KOH concentration in
the aqueous phase, and an optimum value of the KOH concentration was obtained
to increase the conversion. The concentration of ArOQ in the organic phase as a
236
M. Wang and Y. Hsieh
Figure 10 Arrhenius plot of apparent reaction rate constant (kapp)versus
temperature [same reaction conditions as for Figure 1, except temperature].
function of the KOH concentration in the two-phase reaction is shown in Figure 9.
The concentration of ArOQ in the organic phase decreases with the increase in the
aqueous-phase KOH concentration. Therefore, the value of kapp decreases with an
increase of the KOH concentration in the aqueous phase.
The results obtained from independent experiments, which were used to
measure the distribution of ArOQ between the two phases as a function of KOH
concentration, are presented in Figure 9. A maximum conversion was obtained
corresponding to a maximum concentration of ArOQ in the aqueous phase. The
conclusion is that the synthesis of 2,4,6-tribromophenyl benzyl ether can be
favourably performed at a low concentration of KOH. Higher concentrations will
decrease the reaction rate and conversion. This probably occurs because the phase
transfer catalyst may decompose in very concentrated KOH solutions. The
optimum ratio of K0H:ArOH is between 1.32:1 and 1.80:1.
Temperature
In general, the reaction rate increases with an increase in the temperature. The
effect of temperature on the conversion is shown in Figure 10. An Arrhenius plot
is used to obtain the activation energy of the reaction, i.e. 18.31 kcal mol-'.
Synthesising 2,4,6-tribromophenyl Benzyl Ether
237
Conclusions
The synthesis of 2,4,6-tribromophenyl benzyl ether was carried out by reacting
2,4,6-tribromophenol and benzyl bromide in an alkaline solution of KOH/organic
solvent by phase transfer catalysis. The intermediate product (ArOQ) can also be
synthesised and purified by independent experiments. The identification of the
intermediate product from the reaction will provide valuable information for
dynamic modelling. The pseudo-first-order rate law can be used to describe the
reaction when the mole ratio of RBr:ArOH < 1. Several conclusions were
obtained from the reaction kinetics:
(a) The reaction rate is a linear function of the amount of catalyst being used in the
reaction.
(b) A polar solvent will increase the reaction rate, and the order of relative
reactivity of the solvents is: dichloromethane > chlorobenzene > toluene.
(c) The order of the relative reactivity of the catalysts is: TBAI > TBAB > BTBAB
> TBAHS > BTEAC > TEAC.
(d) The addition of KBr will retard the reaction rate.
(e) The reaction is controlled by the organic phase reaction. The mass-transfer
resistance of the catalyst between the two phases can be neglected.
(f) The reaction is not favourable when carried out at higher KOH concentrations.
The optimum ratio K0H:ArOH is between 1.32: 1 and 1.80: 1.
References
1 Starks, C.M. and Liotta, C. 1978. Phase Transfer Catalysis, Principles and Techniques.
Academic Press, New York.
2 Dehmlow, E.V. and Dehmlow, S.S. 1983. Phase Transfer Catalysis. Verlag Chemie,
Weinheim.
3 Weber, W.P. and Gokel, G.W. 1977. Phase Transfer Catalysis in Organic Synthesis.
Springler Verlag, New York.
4 Freedman, H.H. 1986. Industrial applications of phase transfer catalysis (FTC): past,
present and future. Pure Appl. Chem., 58(6), 857-868.
5 Starks, C.M. 1985. Phase transfer catalysis: an overview. Am. Chem. SOC.Symp. Series,
326, 1-7.
6 Freedman, H.H. and Dubois, R.A. 1975. An improved Williamson ether synthesis using
phase transfer catalysis. Tetrahedron Letters, 38, 325 1-3254.
7 McKillop, A., Fiaud, J.C. and Hug, R.P. 1974. The use of phase-transfer catalysis for the
synthesis of phenol ethers. Tetrahedron Letters, 30, 1379-1382.
8 Wang, M.L. and Wu, H.S. 1990. Kinetic study of the substitution reaction of
hexachlorocyclotriphosphazene with 2,2,2-trifluoroethanoI by phase transfer catalysis
and separation of the products. lnd. Eng. Chem. Res., 29,2137-2142.
9 Wang, M.L. and Yang, H.M. 1990. Kinetic study of synthesising 4-bromophenyl ally1
ether by phase transfer catalysis. J. Molecular Catalysis, 62, 135-146.
10 Wang, M.L. and Yu,C.C. 1992. Kinetic study of synthesizing 2,4-dibromophenylally1
ether in a phase transfer catalytic reaction. J. Chinese Inst. Chem. Eng., 23(3),
153-159.
11 Gordon, J.E. and Kutina, R.E. 1977. On the theory of phase transfer catalysis. J. Am.
Chem. SOC., 99,3903-3909.
12 Hsieh, Y.M. 1990. A Study of Synthesizing 2,4,6-Tribromophenyl Benzyl Ether by
Two-Phase Phase Transfer Catalytic Reaction. M.S. Thesis, Dept. of Chemical
Engineering, National Tsing Hua University, Taiwan.
Received: 10 September 1992; Arcepted after revision: 20 January 1993.
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