Catalysts and process for the production of benzyl toluenes

Article abstract of DOI:10.1021/op034043m

A method for the synthesis of benzyl toluenes from toluene and using molecular bromine as catalytic operator is reported here. The process can be carried out in one of two ways: the first method involves the simultaneous bromination/alkylation of toluene,

Full text of DOI:10.1021/op034043m

Organic Process Research & Development 2003, 7, 571−576
Catalysts and Process for the Production of Benzyl Toluenes
Benita Barton,* Nkululeko S. Hlohloza, Sonia M. McInnes, and Bernard Zeelie
Catalysis Research Unit, Faculty of Applied Science, Port Elizabeth Technikon, PriVate Bag X6011,
Port Elizabeth, South Africa 6000
Abstract:
mandeur et al.2 allows for the direct addition of a Friedel-
A method for the synthesis of benzyl toluenes from toluene and
using molecular bromine as catalytic operator is reported here.
The process can be carried out in one of two ways: the first
method involves the simultaneous bromination/alkylation of
toluene, followed by recycling of the liberated HBr to an
oxidation reactor to form molecular bromine; the second
method involves the formation of benzyl bromide in a separate
step with in situ bromine recycling, followed by alkylation of
toluene. HBr liberated during the alkylation step is recovered
and recycled in the bromination reactor. Solid catalysts that
maximize side-chain bromination over ring bromination, whilst
at the same time catalysing the Friedel-Crafts condensation
of toluene with benzyl bromide, are key to the success of the
process.
Crafts catalyst to the halogenating reaction mixture without
separation of the organic halide, relatively low yields of the
desired benzyl toluenes (<85%) are formed.
Salmon and co-workers4 have reported a one-pot prepara-
tion of the desired product using bromine and toluene in the
presence of a commercial bentonitic earth as catalyst with
an 85% product yield. Catalysis of organic reactions by
inorganic solids is an important dimension of preparative
organic chemistry.5 Specifically, smectite clays, doped with
transition metal ions, are known to catalyse alkylation
reactions of aromatic substrates. Clays containing zinc (II)6,7
and iron (III)8,9 have been reported to significantly enhance
catalytic activities in these types of reactions. Laszlo and
Mathy10 have extensively studied Friedel-Crafts alkylations
with halides, olefins, and alcohols using montmorillonite
K10-supported transition metal ions and have reported good
conversion rates and improved yields as compared with
standard Lewis acid catalysts.
Introduction
Benzyl toluene and isomeric mixtures thereof are com-
pounds useful as dielectric liquids for such components as
transformers, capacitors, and cables. The synthesis of benzyl
toluene and related compounds is normally carried out by
the Friedel-Crafts condensation of a benzylic halide with
an aromatic compound in the presence of a Friedel-Crafts
catalyst.1 After condensation, the catalyst is destroyed by,
for example, adding a dilute aqueous solution of hydrochloric
acid, followed by a washing of the organic phase.
The said compounds may alternatively be synthesised by
adding a catalytically effective amount of a metal halide such
as ferric chloride to a mixture of aromatic halide and aromatic
compound.2 This process has been claimed to be simpler
compared with the above process in that no downstream
destruction of the catalyst by neutralization or washing is
required.
In this study, we report a versatile, highly efficient method
for the condensation of toluene to produce benzyl toluene
(the term “benzyl toluene” implies a mixture of the o-, m-,
and/or p-isomers) in the presence of molecular bromine and
a clay-supported transition metal catalyst. The ratio of meta-:
ortho-:para-benzyl toluene obtained in this way was always
approximately 8:40:52, respectively. The process offers
several advantages, including bromination and alkylation
reactions occurring simultaneously and in one pot, selective
benzylic bromination with in situ bromine recycling, im-
proved yield and selectivity to the desired product with
insignificant amounts of side-product formation, and a
convenient method for the recovery and subsequent air-
oxidation of formed hydrobromic acid back to molecular
bromine.
In yet another method,3 a mixture of benzyl and meth-
ylbenzyl toluenes is produced by reactions involving alky-
lation and transalkylation at high temperatures, which then
requires a complex separation and purification step to obtain
the desired product.
Results and Discussion
Scheme 1 illustrates the two possible routes investigated.
In Route A, the bromination of toluene and the Friedel-
Crafts alkylation of toluene by benzyl bromide occur in one
In all of the above-mentioned methods, a significant
disadvantage is that the organic halide has to be synthesized
in a separate process step. Although the method by Com-
(4) Salmon, M.; Angeles, E.; Miranda, R. J. Chem. Soc., Chem. Commun. 1990,
1188.
(5) Laszlo, P. Acc. Chem. Res. 1986, 19, 121.
(6) Corne´lis, A.; Dony, C.; Laszlo, P.; Nsunda, K. M. Tetrahedron Lett. 1991,
32, 1423.
* To whom correspondence should be addressed. E-mail: bbarton@
petech.ac.za.
(7) Clark, J. A.; Kybett, A. P.; Barlow, D. J.; Landon, P. J. Chem. Soc., Chem.
Commun. 1989, 1353.
(1) Mathais, H.; Commandeur, R.; Pontoglio, A.; Nebel, S. Eur. Pat. EP 8251,
(8) Cseri, T.; Bekassy, S.; Cseke, E.; Figueras, F.; De Menorval, L.-C.; Dutartre,
R.; Be´ka´ssy, S. Appl. Catal., A 1995, 132, 141.
(9) Pai, S. G.; Bajpai, A. R.; Deshpande, A. B.; Samant, S. D. J. Mol. Catal.
A: Chem. 2000, 156, 233.
1982.
(2) Commandeur, R.; Berger, N.; Jay, P.; Kervennal, J. U.S. Patent 5,186,864,
1993.
(3) Kawakami, S.; Endo, K.; Dohi, H.; Sato, A. Eur. Pat. EP282083, 1988.
(10) Laszlo, P.; Mathy, A. HelV. Chim. Acta 1987, 70, 577.
10.1021/op034043m CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/31/2003
Vol. 7, No. 4, 2003 / Organic Process Research & Development
•
571

Table 1. Bromination/alkylation of toluene with blank clays
drying temp
reaction temp
reaction
time (h)
C6H4BrCH3
(%)a
C6H5CH2Br
(%)a
BzTb
(%)a
catalyst
(°C)
solvent
(°C)
bentonitec
-
-
-
-
110
110
60
60
60
4
4
4
4
4
4
4
4
4
4
4
4
2.7
5.5
-
9.1
36.2
80.0
76.4
80.8
81.6
64.5
72.2
0
68.0
41.2
-
-
1.4
montmorillonite K10c
bentonite
180
180
180
180
180
180
120
120
90
CCl4
CCl4
montmorillonite K10
bentonite
montmorillonite K10
bentonite
montmorillonite K10
bentonite
montmorillonite K10
bentonite
9.6
1.2
3.6
trace
2.5
3.0
4.6
1.9
7.5
-
-
-
-
-
-
-
-
60
7.3
110
110
110
110
110
110
23.0
15.0
80.6
76.1
83.6
82.9
0
0
0
montmorillonite K10
90
a Yields based on Br2. b BzT ) benzyl toluene c Used as received with no pretreatment.
Scheme 1. Two possible routes for the production of
Table 2. Alkylation of toluene using commercially obtained
benzyl toluene
benzyl bromide
montmorillonite K10
bentonite
drying
temp
C6H5CH2Br yield BzTb C6H5CH2Br yield BzTb
(°C) conversion (%)a
(%)c
conversion (%)a
(%)c
d
50
90
120
180
280
55.8
85.6
76.3
60.5
51.0
61.1
50.9
79.7
71.1
57.8
49.7
56.9
70.4
86.5
78.0
81.9
70.5
81.0
66.1
77.3
74.5
74.4
65.8
75.0
a Mol %. b BzT ) benzyl toluene. c Mol %, based on benzyl bromide. d Clays
used as received.
These results show that pretreatment and drying of the
clays improves catalytic activity as compared to clays not
treated in this fashion. However, it is apparent that high
drying temperatures cause a decrease in catalyst activity, with
an optimal drying temperature being in the range between
90 and 120 °C.
It is also of interest to compare the results described above
to results obtained for the benzylation of toluene with
commercially obtained benzyl bromide. These results are
summarised in Table 2.
As with the simultaneous bromination/alkylation reaction,
pretreatment of the clays improve catalytic activity. This is
probably the result of the combined effect of increased
surface area (by removing larger silicate particles) and
improved intercalation of (nonpolar) toluene upon removal
of excess surface and interlayer water upon drying. The
observed decrease in catalytic activity at higher drying
temperatures may possibly be the result of a decrease in the
interlayer spacing upon heating11 and/or a decrease in
Brønsted acidity with increased water loss at higher drying
temperatures.12
pot. HBr liberated is collected in glacial acetic acid and
oxidised by air in the presence of sodium nitrate. Bromine
formed in the oxidation reactor is displaced with excess air
and recycled back to the bromination/alkylation reactor. In
Route B, bromination of toluene occurs in a separate reaction
vessel with in situ HBr recycling. HBr released during the
alkylation step is recovered and recycled to the bromination
reactor.
Simultaneous Bromination/Alkylation of Toluene (Route
A). Bromination/Alkylation of Toluene Using Pretreated
Heat-ActiVated Clays. Two smectite clays, bentonite and
montmorillonite K10, were used as the basis of catalysts
investigated in this study. To ensure uniform performance
during the study, commercial samples of the two clays were
initially pretreated by suspending the clay in an excess of
deionised water for a brief period to allow any silicate
materials to settle, followed by decantation and centrifugation
to recover the clay. The clays so obtained were dried at
various temperatures. These were then used in the bromi-
nation/alkylation reaction as blanks for comparative purposes.
Results of the yields, based on Br2, of ring-brominated side-
products, intermediate benzyl bromide and final product
benzyl toluene (determined by gas chromatography) are
summarized in Table 1.
Bromination/Alkylation of Toluene Using Ion-Exchanged
Metal-Modified Clays. The pretreated, activated clays were
modified by the introduction of metal ions by means of ion
exchange into the clay structure according to a known
(11) Rowland, R. A.; Weiss, E. J.; Bradley, W. F. Natl. Acad. Sci. Publ. 1956,
456, 85.
(12) Fripiat, L.; Cloos, P.; Jornoy, A. Bull. Groupe Franc. 1962, 13, 65.
572
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Table 3. Bromination/alkylation of toluene using ion-exchanged clays
C6H4BrCH3
(%)a
C6H5CH2Br
(%)a
BzTb
(%)a
total
(%)a
selectivityc
(%)a
catalyst
K-montmorillonite K10
K-bentonite
Zr-montmorillonite K10
Zr-bentonite
Ba-montmorillonite K10
Ba-bentonite
Ce-montmorillonite K10
Ce-bentonite
Ti-montmorillonite K10
Ti-bentonite
Cu-montmorillonite K10
Cu-bentonite
Co-montmorillonite K10
Co-bentonite
Zn-montmorillonite K10
Zn-bentonite
<1
80.1
50.7
75.9
50.2
63.2
0
32.2
0
2.25
0
0
0
0
0
14.3
40.1
12.3
14.3
11.0
73.5
27.7
40.4
78.2
75.3
79.7
78.6
70.4
64.2
86.4
73.6
76.0
45.7
94.4
90.8
88.2
74.1
78.4
77.1
75.3
78.8
81.1
78.4
81.1
82.6
82.1
84.3
86.4
83.1
90.7
81.4
>99
>99
100
<1
0
9.6
4.2
87.0
94.6
95.3
79.7
48.7
99.2
96.0
98.3
95.1
85.7
76.2
>99
88.6
83.8
56.1
3.61
15.4
38.4
0.64
3.14
1.37
4.01
11.7
20.1
<1
0
0
0
0
9.46
14.7
35.7
Fe-montmorillonite K10
Fe-bentonite
a Percentage yields and selectivities based on Br2. b BzT ) benzyl toluene. c Selectivity ) [(%C6H5CH2Br + % BzT)/% total products] × 100.
Table 4. Effect of pre-ion exchange drying temperature on
catalyst selectivity
procedure.10 Metal ions that were investigated are listed in
Table 3. Ion exchange was achieved by contacting the
pretreated clay with a 1 M aqueous solution of the metal
chloride for 24 h with stirring. After this period followed
by filtering and washing with deionised water, the catalyst
was oven-dried, powdered, and used as such in the bromi-
nation/alkylation step. Results obtained are summarised in
Table 3.
yield (mol %)
C6H4BrCH3 BzTb total selectivityc
catalyst
(%)a
(%)a
(%)a
(%)a
K10-Zn-120
56.8
4.45
<1
28.4
79.1
86.4
58.0
79.6
79.7
14.7
45.8
73.6
61.5
58.1
78.6
85.2
83.6
86.4
82.2
85.3
81.1
97.5
77.7
83.1
83.8
77.3
82.6
33.3
94.6
100
K10-120-Zn-120
K10-180-Zn-120
K10-Cu-120
K10-120-Cu-120
K10-180-Cu-120
Bent-Zn-120
Bent-120-Zn-120
Bent-180-Zn-120
Bent-Cu-120
Bent-120-Cu-120
Bent-180-Cu-120
24.2
5.7
70.6
93.3
98.3
15.1
58.9
88.6
73.4
75.2
95.1
The first significant observation from these results is that
ion-exchanged clays are much more effective than those that
have not been subjected to ion exchange. In general,
bentonite ion-exchanged catalysts were less selective than
the corresponding K10 ion-exchanged catalysts, with the best
selectivity being displayed by the K-K10, K-bentonite, Zr-
K10 and Zn-K10 catalysts. It is, however, quite noticeable
from the results shown that only the later transition metals
(Cu, Co, Zn, and Fe) result in catalysts that show a significant
improvement in Friedel-Crafts activity. In comparative
experiments using commercial benzyl bromide, all the
transition metals, with the exception of titanium, gave yields
of benzyl toluenes between 95 and 100% after a reaction
period of 20 min. Generally speaking, a high activity towards
Friedel-Crafts alkylation also leads to a higher activity for
ring bromination as can be seen from the entries for cerium,
iron, cobalt and zinc.
Effect of Pre-ion Exchange Drying Temperature on
Catalyst SelectiVity. To demonstrate the importance of
temperature treatment of the clay before ion exchange and
activation after ion exchange, several catalysts were prepared
by differing the heat treatments at different stages of catalyst
preparation. In one case, pretreatment of the catalyst support
was carried out as described previously, except that the clay
was only air-dried at room temperature prior to ion exchange.
After ion exchange, the catalyst was activated at different
temperatures. In the second case, the blank clay was dried
before ion exchange at different temperatures followed by
drying at 120 °C after ion exchange. Catalysts so prepared
1.4
82.8
31.9
9.5
22.3
19.2
4.0
a Percentage yields and selectivities based on Br2. b BzT ) benzyl toluene.
c Selectivity ) [(%C6H5CH2Br + % BzT)/% total products] × 100.
were designated according to the method and conditions used
during preparation. For example, if the clay was pretreated
at 120 °C followed by ion exchange with Zn and subsequent
activation at 180 °C, this is designated K10-120-Zn-180. The
results obtained for the simultaneous bromination/alkylation
of toluene using these catalysts are summarised in Table 4.
It is clear from these results that temperature activation
of the support prior to ion exchange significantly decreases
bromination of the aromatic ring and increases the selectivity
towards benzylic bromination and subsequent benzyl toluene
formation. Both Ca-montmorillonite and Na-montmorillonite
have been shown to undergo a gradual loss in cation-
exchange capacity on heating to 300 °C.13 It is expected that
the resultant decrease in metal loading on the clay supports
affects radical (side-chain) bromination and Friedel-Crafts
activity less than the nonradical ring bromination to result
in a more selective catalyst.
(13) Hofmann, U.; Klemen, R. Z. Anorg. Chem. 1950, 262, 95.
Vol. 7, No. 4, 2003 / Organic Process Research & Development
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573

Table 5. Effect of the rate of bromine addition
the higher metal loadings achieved during deposition as
compared to ion exchange and the resultant increase in Lewis
acid sites on the metal-coated clays. As before, high
temperature activation of metal-coated and ion-exchanged
clays improves the selectivity towards benzylic bromination,
without impacting too greatly on the Friedel-Crafts activity
of the catalyst. Using an activation temperature of 400 °C
for the ion-exchanged catalyst (K10-180-Zn-400) in this
process afforded a yield of benzyl toluene of 97% with 99%
selectivity.
Hydrobromic Acid Scrubbing and Oxidation. The gaseous
hydrobromic acid that was released during the bromination
and alkylation reactions was scrubbed from the reactor exit
gas by bubbling through glacial acetic acid. The HBr so
recovered (typically >95%) was oxidised to molecular
bromine by air by adding a catalytic amount of sodium nitrate
to the acetic acid/HBr mixture at a reaction temperature of
100 °C.14 Molecular bromine formed during the oxidation
was displaced from the oxidation reactor by the excess air
fed to reactor. Molecular bromine recovery by cold trapping
was typically in the order of 85-90% of the HBr present in
the glacial acetic acid.
Consecutive Bromination and Alkylation of Toluene
(Route B). An alternative approach to the preparation of the
desired benzyl toluenes involves the initial bromination of
toluene in a separate reactor followed by alkylation of toluene
after isolation of benzyl bromide. This route involves two
steps but has the advantage that HBr liberated during the
bromination step can be recycled in situ. In addition, HBr
liberated during the alkylation step can be recycled directly
to the bromination reactor, thereby eliminating the need to
handle molecular bromine at any stage of the process.
To evaluate the feasibility of such an approach, toluene
was brominated with molecular bromine in glacial acetic acid
using activated silica gel as catalyst in two ways. In the first,
no recycling was attempted via oxidation using the sodium
nitrate/air system, while in the second, oxidation of HBr
liberated during the bromination step was attempted in situ.
The results obtained for these two experiments are sum-
marised in Table 7.
In the absence of NaNO3, a high yield of the desired
benzyl bromide was obtained with no detectable ring-
brominated products being formed. In the presence of NaNO3
and air, HBr released during the bromination step is
effectively oxidised back to molecular bromine to give >95%
bromine atom utilisation. Apart from benzyl bromide, a small
amount of ring-brominated toluene was also obtained. These
experiments have demonstrated that the reaction may be
carried out effectively and efficiently in two separate process
steps.
yield (mole %)
addition
time
(min)
C6H4BrCH3 C6H5CH2Br BzTb total selectivity
(%)a
(%)a
(%)a (%)
(%)a,c
0d
0
10
30
9.7
18.7
27.0
38.6
5.8
0
0
81.0 96.5
76.1 94.8
55.0 82.0
45.8 84.4
89.9
80.2
67.1
54.3
0
a Percentage yields and selectivities based on Br2. b BzT ) benzyl toluene.
c Selectivity ) [(%C6H5CH2Br + % BzT)/% total products] × 100. d Undiluted,
and added directly onto catalyst prior to toluene addition.
Bromination/Alkylation of Toluene with Various Rates of
Bromine Addition. The importance of the rate and method
of bromine addition on reaction selectivity was investigated.
Several experiments were carried out in which molecular
bromine was added to the reaction mixture, comprising
substrate and catalyst, in one of four ways: (i) added in one
batch, undiluted, (ii) added in one batch, diluted with inert
solvent, (iii) continuous addition of diluted bromine over 10
min, and (iv) continuous addition of diluted bromine over
30 min. In each case, the catalyst used was K10-100-Zn-
100, and the reaction was allowed to proceed for 15 min
after complete bromine addition. All reactions were carried
out at reflux temperature. Results obtained are summarized
in Table 5.
The results suggest that a slower bromine addition rate
favours ring bromination and hence results in a significant
reduction in benzyl toluene formation. Interestingly, addition
of bromine directly onto the catalyst such that the bromine
could be adsorbed prior to contact of the catalyst with toluene
led to higher selectivity toward benzylic bromination, as
compared to addition of bromine to the catalyst/toluene
mixture. In two further comparative experiments, toluene was
first heated under reflux in the presence of the catalyst for
one minute prior to the addition of diluted bromine as a single
batch, and in the second experiment, diluted bromine was
first heated under reflux in the presence of the catalyst before
the addition of toluene to the mixture. The first reaction gave
82.0% benzyl toluene and 16.3% bromotoluene, while the
second gave 96.5% benzyl toluene and 2.33% bromotoluene.
It is of interest to note that the batch-wise addition of
molecular bromine may be repeated several times without
the need to separate the catalyst or reaction products or both.
Bromination/Alkylation of Toluene Using Metal-Coated
Clays. The effect of modifying the clays by means of metal
deposition was investigated. Here the metal used was Zn,
and K10 was the support. The deposition was achieved by
dissolving the metal salt in water, adding the resultant
solution to the blank pretreated clay, evaporating off the
excess solvent by rotary evaporation and finally oven-drying
at 120 or 400 °C. After grinding finely, the modified clay
was used for the simultaneous bromination/alkylation of
toluene (Table 6).
Experimental Section
Simultaneous Bromination/Alkylation of Toluene (Route
A). Bromination/Alkylation of Toluene Using Pretreated
Heat-ActiVated Clays. Montmorillonite K10 or bentonite (20
g) was suspended in deionized water (250 mL), stirred
The ion-exchanged catalyst K10-180-Zn-120 is much
more selective towards benzylic bromination and benzyl
toluene formation than the K10-180-Zn-120 catalyst prepared
by the deposition technique. This is probably a reflection of
(14) McInnes, S. M.; Zeelie, B.; Hlohloza, S. N.; Hamilton, L. M. RSA Pat.
95/2274, Feb 28, 1996.
574
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Vol. 7, No. 4, 2003 / Organic Process Research & Development

Table 6. Method of catalyst preparation
yield (mol %)
C6H4BrCH3
C6H5CH2Br
(%)b
BzTc
(%)b
total
(%)
selectivity
(%)b,d
catalysta
(%)b
IX-K10-180-Zn-120
IX-K10-180-Zn-400
MC-K10-180-Zn-120
MC-K10-180-Zn-400
4.51
0.36
23.9
8.65
0
0.4
0
93.2
96.0
70.0
83.0
97.7
96.7
93.8
91.7
95.4
99.3
74.6
80.5
0
a IX ) ion-exchanged; MC ) metal-coated. b Percentage yields and selectivities based on Br2. c BzT ) benzyl toluene. d Selectivity ) [(%C6H5CH2Br + % BzT)/%
total products] × 100.
Table 7: Bromination of toluene
round-bottomed flask. Bromine (0.32 g; 2.0 mmol) in CCl4
(10 mL) was added, at the various rates, to the stirred reaction
mixture at 110 °C. An aliquot (5 mL) of the reaction mixture
was sampled 15-20 min after bromine addition, quenched
with sodium thiosulfate solution (0.1 M), extracted with CCl4,
and finally analysed by GC.
C6H4BrCH3
(%)a
C6H5CH2Br
(%)a
experiment
no NaNO3
with NaNO3
-
98.9
191.1
1.65
Bromination/Alkylation of Toluene Using Metal-Coated
Rather than Ion-Exchanged Clays. The metal chloride (5
mmol) was dissolved in water (5.7 mL) and the pretreated
and activated clay (5 g, activated at 180 °C) added to the
solution. After thorough mixing, a thick paste was formed.
The coated clay was then oven-dried at the required
temperature (120 or 400 °C) and ground finely. Toluene (15
mL, 0.14 mol) and the required modified catalyst (0.1 g)
were heated to 110 °C, and diluted molecular bromine (0.8
g, 5 mmol in 15 mL of CCl4) was added in one batch. After
20 min, an aliquot (5 mL) was sampled, quenched with
sodium thiosulfate (0.1 M), and extracted with CCl4. The
reaction mixture was analysed by GC.
a Yield based on initial Br2 charged.
rapidly for ca. 10 s, and left standing for ca. 1.5 min. The
supernatant slurry was decanted and filtered by suction. The
residue was further washed with deionized water four times
before drying and activation at the required temperature.
Toluene (15 mL), optionally diluted with CCl4 (15 mL) and
containing the activated clay (0.30 g), was treated with
diluted molecular bromine (1 g in 15 mL of CCl4) over a
period of 45 min. The reaction was heated at the required
temperature for 4 h, the catalyst filtered off, and the reaction
mixture analysed by GC.
Alkylation of Toluene Using Commercially Obtained
Benzyl Bromide. The pretreated clay (0.25 g), toluene (15.0
mL, 141 mmol) and benzyl bromide (0.64 g, 3.74 mmol)
were weighed into a 100-mL round-bottomed flask, and the
mixture was heated under reflux for 8 min. After this period,
an aliquot (5 mL) was removed, quenched with a cold
saturated NaCl solution (5 mL), and extracted with CCl4
(3 × 5 mL). The organic layer was dried (Na2SO4), the
drying agent filtered off, and the filtrate analyzed using GC.
Bromination/Alkylation of Toluene Using Ion-Exchanged
Metal-Modified Clays. The clays were modified by metal
ion exchange according to a known procedure.10 To the metal
ion solution (1 M, 50 mL), prepared by dissolving the metal
chloride (0.1 mol) in deionized water (100 mL), was added
the clay (4.0 g), pretreated as described above, while stirring
rapidly with a Teflon-coated magnetic stirrer bar. Stirring
was continued for 24 h after which the solid was filtered
off, and washed with deionized water (10 × 15 mL) until
no chloride could be detected in the wash water (AgNO3
test). The catalyst was dried (120 °C, 8 h), finely ground,
and stored in a desiccator until required. Toluene (15 mL)
was treated with molecular bromine (1.0 g in 15 mL of CCl4)
over a period of 50 min in the presence of the catalyst (0.3
g). The reaction was heated under reflux and terminated after
4 h, and the reaction mixture was analysed by GC.
Hydrobromic Acid Scrubbing and Oxidation. HBr scrub-
bing and oxidation were carried out according to a known
procedure.14
Consecutive Bromination and Alkylation of Toluene
(Route B). Bromination of Toluene Using Molecular Bro-
mine in the Absence of NaNO3. Toluene (28.19 g, 0.31 mol)
and silica gel 60 (0.30 g) were weighed into a 100-mL round-
bottomed flask. The mixture was heated under reflux while
a solution of bromine (0.80 g, 5.0 mmol) in acetic acid (15
mL) was added dropwise. After the addition was complete,
the reaction mixture was heated and stirred until complete
discolouration had occurred, indicating a completed reaction.
The mixture was cooled, the silica removed by filtration,
and the filtrate analysed using GC.
Bromination of Toluene Using Molecular Bromine in the
Presence of NaNO3. The reaction was repeated as described
above with the exception that NaNO3 (0.52 g, 6.1 mmol)
was added to the mixture in the round-bottomed flask prior
to reflux and bromine addition.
Alkylation of Toluene with Benzyl Bromide. This was
carried out according to the procedure described previously
herein.
Conclusions
Bromination/Alkylation of Toluene with Various Rates of
Bromine Addition. The catalyst K10-100-Zn-100 (0.225 g)
was suspended in toluene (60 mL, 0.56 mol) in a 100-mL
A simple, one-pot synthetic route to benzyl toluene, from
toluene as feedstock, has been investigated using molecular
bromine over modified inorganic clays. The two main
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reactions comprising the synthetic procedure, benzylic bro-
mination of toluene and Friedel-Crafts benzylation of
toluene, are both catalysed by these modified clays. The
activity of montmorillonite K10 and bentonite for the
Friedel-Crafts alkylation reaction was dramatically increased
by modifying the clays with metal ions, either through the
process of ion exchange or by surface coating. These
catalysts were mild alkylation catalysts, affording few or no
unwanted side reactions. The selectivity and yield achieved
in the optimum process was 99% and 97%, respectively. The
preparation of benzyl toluene was also successfully carried
out in two process steps, with near-quantitative bromine atom
utilisation being noted in the recycle stage of the process.
Acknowledgment
We thank the National Research Foundation, the joint
venture Merichem-Sasol LP, as well as the Port Elizabeth
Technikon for their continued support, financial and other-
wise.
Received for review March 26, 2003.
OP034043M
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