Friedel-crafts benzylation of arenes over mixed oxides

Article abstract of DOI:10.1002/jccs.200400021

The replacement of liquid acid catalyst by solid acids for the Friedel-Crafts reaction of aromatic alkylation is a challenging task. Mixed oxide possessing spinel structures were found to catalyze the Friedel-Craft benzylation of arenes. It was found that the reaction was very fast at 80 °C, hence the reaction was studied with respect to more substrate. The significant point was even at this temperature monobenzylation, with the para isomer predominating, was achieved. Mixed oxides were prepared by co-precipitation method and were characterized by XRD technique.

Full text of DOI:10.1002/jccs.200400021

Journal of the Chinese Chemical Society, 2004, 51, 135-138
135
Friedel-crafts Benzylation of Arenes over Mixed Oxides
Sujit R. Jadhav and Manohar R. Sawant*
Applied Chemistry Division, Institute of Chemical Technology, University of Mumbai,
Matunga, Mumbai-400 019, India
The replacement of liquid acid catalyst by solid acids for the Friedel-Crafts reaction of aromatic
alkylation is a challenging task. Mixed oxide possessing spinel structures were found to catalyze the Friedel-
Craft benzylation of arenes. It was found that the reaction was very fast at 80 °C, hence the reaction was stud-
ied with respect to more substrate. The significant point was even at this temperature monobenzylation, with
the para isomer predominating, was achieved. Mixed oxides were prepared by co-precipitation method and
were characterized by XRD technique.
Keywords: Mixed oxide; Spinel; Friedel-crafts; Benzylation; Co-precipitation.
INTRODUCTION
The Friedel-Crafts reaction of an aromatic compound
the liquid phase and gas phase Friedel-Craft reaction using
solid acids in general as catalyst.2-11
In the present work, benzylation of benzene and few
other arenes has been carried out in presence of mixed oxide
catalyst viz Cu0.5Zn0.5Fe2O4, Zn0.5Ni0.5Fe2O4, Zn0.5Co0.5Fe2O4,
Cu0.5Co0.5Al2O4, Cu0.5Zn0.5Al2O4. Also a comparison was
done with binary mixtures and simple oxide.
with an alkyl halide requires a Lewis acid catalyst. During the
past few years, many research groups have exploited the
Friedel-Crafts benzylation of arenes using benzyl chloride.
Friedel-Crafts alkylation can be performed extremely effi-
ciently in the presence of spinels. The reasons why spinels are
emerging as the catalyst of choice for Friedel-Crafts chemis-
try are due to high yield, high conversions, reduced reaction
times, decerease of polyalkylation, lack of rearrangement
and reduction in the amount of catalyst necessary. The ben-
zylation of benzene and toluene1 in particular are used for the
preparation of substitutes of polychlorobenzenes, which are
used as dielectrics. Diphenylmethane is also used in the prep-
aration of diphenhydramine hydrochloride which is an anti-
histaminic compound. A number of reviews are available on
BENZYLATION OF BENZENE
Benzylation of benzene (1) with benzyl chloride (2) as
an alkylating agent gives mainly diphenylmethane (3). As the
Friedel-Crafts reaction is always accompanied by di- and
poly-alkylations, due to higher reactivity of the product than
the reactant, the benzylation reaction could also give diben-
zylation products (4, 5, 6) as side products.
CH2
CH2
CH2
5
3
CH2
Cl
+
CH2
1
2
CH2
CH2
CH2
6
4
* Corresponding author. E-mail: sawantmr@vdct.org

136 J. Chin. Chem. Soc., Vol. 51, No. 1, 2004
Jadhav and Sawant
Table 1. Comparison of Catalyst in Terms of Selectivity for Di-Phenyl Methane
Catalyst
Induction time
in min
Reaction time
in min
Benzyl chloride
% conversion
DPM
% selectivity
Cu0.5Zn0.5Fe2O4
Zn0.5Ni0.5Fe2O4
Zn0.5Co0.5Fe2O4
CuFe2O4
NiFe2O4
CoFe2O4
ZnFe2O4
ZnO
Fe2O4
30
-
-
45
10
5
80
10
15
120
60
50
5
120
20
100
100
100
100
100
100
100
42
92.8
93.7
93.9
91.8
91.6
93
92.8
91
92.6
-
100
Temperature: 80 °C
Benzene: 20 mL
Benzyl chloride: 5 mL
Thus dibenzylbenzene, tribenzylbenzene, etc. may be
formed in the reaction, dibenzylbenzene being the most prom-
inent side product.
The starting materials were analytical reagent grade sul-
phates. An aqueous solution containing the desired ions in the
required molar proportion was prepared by dissolving the
salts in the stoichiometric proportion in distilled water. It was
precipitated by adding sodium hydroxide and the pH of the
solution was maintained between 9 to 9.5. The solution was
kept on a water bath maintained at 80 °C for 3 hours and then
oxidized by adding the required amount of 30% H2O2 drop-
wise with constant stirring. After the reaction was over the re-
EXPERIMENTAL
Catalyst Preparation
Catalysts were prepared by co-precipitation method.12,13
Fig. 1. XRD pattern of ZN0.5Ni0.5Fe2O4.

Friedel-crafts Benzylation of Arenes over Mixed Oxides
Table 2. Effect of Substitution of Fe+3 by Al+3
J. Chin. Chem. Soc., Vol. 51, No. 1, 2004 137
Kristalloflex diffractometer with a microprocessor control-
ler using Cu Ka radiation with nickel filter. All diffracto-
grams were recorded at room temperature. The X-ray pattern
of all the compositions indicated the formation of a single-
phase spinel12,14 and match well with standard JCPDS pat-
terns.
Catalyst
Reaction time Benzyl chloride
DPM
in min
% conversion % selectivity
Cu0.5Zn0.5Al2O4
Cu0.5Co0.5Al2O4
ZnAl2O4
NiAl2O4
CoAl2O4
180
180
120
120
120
71
-
78
-
92.8
-
91.6
-
-
-
Reaction Procedure
Temperature: 80 °C
The reaction was carried out in a flat bottom reactor
containing baffles to which 20 mL of arene, 5 mL of benzyl
chloride and 0.2 gm of catalyst was added. The flask was then
sealed and was kept in an oil bath maintained at a desired tem-
perature. The reaction mixture was stirred with an overhead
stirrer. After every 2 min the sample from the reaction mix-
ture was removed to measure the induction period. The reac-
tion mixture was filtered and was analyzed by gas chroma-
tography (Column OV-17 & FID Detector).
Amount of Benzene: 20 mL
Amount of Benzyl chloride: 5 mL
sultant precipitate was filtered and washed with distilled wa-
ter to remove excess of alkali and sulphate ions. The precipi-
tate was dried at 110 °C for 3 hours. The dried precipitate was
calcined at 900 °C for 10 hours. The formation of single
spinel phase was determined by XRD. Spinels prepared by
this method have high surface area.
Thus benzyl chloride was reacted with benzene, tolu-
ene, ethyl benzene, cumene, anisole and chloro benzene.
During the substrate study it was been found that para prod-
uct was the predominating product. This may be due to the
electron density at para position.
Catalyst Characterization
The catalysts were characterized by X-ray diffraction
technique. XRD patterns were recorded on Siemens D-500
Fig. 2. Time required for benzyl chloride conversion over Zn0.5Ni0.5Fe2O4.

138 J. Chin. Chem. Soc., Vol. 51, No. 1, 2004
Jadhav and Sawant
Table 3. Alkylation of Substrate on ZnFe2O4
Product
Meta
Substrate
Reaction time Benzyl chloride
in min
% conversion
Ortho
Para
Toluene
Anisole
Chlorobenzene
5
10
40
100
100
100
8.2
46.4
36.5
-
-
-
91.8
53.6
63.5
Temperature: 80 °C
Amount of substrate: 20 mL
Amount of benzyl chloride: 5 mL
Table 4. Alkylation of Various Substrates over Zn0.5Ni0.5Fe2O4
Product
Substrate
Benzyl chloride
% conversion
Time
in min
Ortho
Meta
Para
Chlorobenzene
Toluene
Anisole
Cumene
Ethyl benzene
100
100
100
100
100
30
5
5
25
30
25.7
7.2
43.2
16.7
43
-
-
74.3
92.8
56.8
61.9
57.0
-
21.4
-
Temperature: 80 °C
Amount of substrate: 20 mL
Amount of benzyl chloride: 5 mL
RESULTS AND DISCUSSION
Catalyst Characterization
Received March 19, 2003.
The X-ray pattern of all the compositions indicated the
formation of a single-phase spinel12,14 and match well with
standard JCPDS patterns.
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Reusability of the Catalyst
Reusability of the catalyst was tested by carrying out
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CONCLUSION
The ternary mixture Zn0.5Ni0.5Fe2O4 and binary mixture
ZnFe2O4 showed good catalytic properties in alkylation of
benzene with benzyl chloride leading to increase in para
alkylated product and decrease in polyalkylated product. The
catalyst also showed good reusable properties.
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