Natural bentonite clay/dilute HNO3 (40%) - A mild, efficient, and reusable catalyst/reagent system for selective mono nitration and benzylic oxidations

Article abstract of DOI:10.1081/SCC-120014967

Selective mono nitration of Aromatic hydrocarbons and benzylic oxidations can be achieved in high yield using reusable catalyst/reagent system consisting of bentonite clay and dilute HNO₃ under relatively mild experimental conditions. The dual behavior of the catalyst reagent system is utilized for the regioselective synthesis of a variety of industrially important compounds.

Full text of DOI:10.1081/SCC-120014967

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NATURAL BENTONITE CLAY/DILUTE HNO3 (40%)—A MILD,
EFFICIENT, AND REUSABLE CATALYST/REAGENT SYSTEM
FOR SELECTIVE MONO NITRATION AND BENZYLIC
OXIDATIONS
Damodaran Bahulayan a , Gopinathan Narayan a , Vellalath Sreekumar b & Malathy
Lalithambika a
a Clays and Clay Minerals Division, Regional Research Laboratory (C.S.I.R), Trivandrum, 695
019, India
b Department of Chemistry, University of Calicut, India
Version of record first published: 16 Aug 2006.
To cite this article: Damodaran Bahulayan , Gopinathan Narayan , Vellalath Sreekumar & Malathy Lalithambika (2002):
NATURAL BENTONITE CLAY/DILUTE HNO3 (40%)—A MILD, EFFICIENT, AND REUSABLE CATALYST/REAGENT SYSTEM FOR SELECTIVE
MONO NITRATION AND BENZYLIC OXIDATIONS, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 32:23, 3565-3574
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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 23, pp. 3565–3574, 2002
NATURAL BENTONITE CLAY/DILUTE
HNO3 (40%)—A MILD, EFFICIENT, AND
REUSABLE CATALYST/REAGENT
SYSTEM FOR SELECTIVE MONO
NITRATION AND BENZYLIC
OXIDATIONS
*
Damodaran Bahulayan, Gopinathan Narayan,
Vellalath Sreekumar,y and Malathy Lalithambika
Clays and Clay Minerals Division, Regional Research
Laboratory (C.S.I.R), Trivandrum-695 019, India
ABSTRACT
Selective mono nitration of Aromatic hydrocarbons and
benzylic oxidations can be achieved in high yield using reusa-
ble catalyst/reagent system consisting of bentonite clay and
dilute HNO3 under relatively mild experimental conditions.
The dual behavior of the catalyst reagent system is utilized
for the regioselective synthesis of a variety of industrially
important compounds.
Key Words: Nitration; Benzylic oxidation; Bentonite clay;
Dilute HNO3
*Corresponding author. E-mail: bahulayan@yahoo.com
yCurrent address: Department of Chemistry, University of Calicut, India.
3565
DOI: 10.1081/SCC-120014967
Copyright & 2002 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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3566
BAHULAYAN ET AL.
Among the industrially important synthetic processes nitration and
oxidations are given high importance due to its wide variety of applications
in the synthesis of pharmaceuticals and fine chemicals. The first half of this
communication describes our recent achievements in selective mononitra-
tion using natural bentonite clay/dilute HNO3 system and the second half
describes the performance of the above catalyst reagent system for benzylic
oxidations.
Even though aromatic nitration is a well reviewed reaction,[1a–c] it still
remains as an important area of research among the chemists practicing
organic synthesis. In the laboratory aromatic compounds have usually
been nitrated by a mixture of conc. HNO3 and H2SO4.[1c] Most of the
industries are still using this methodology for preparing aromatic nitro
compounds. However, this process creates severe environmental pollution
caused by the gas evolution from oxidative degradation and disposal of used
acids and waste water. In the conventional process of nitration using mixed
acid, the role of H2SO4 is in the generation of the nitronium ions (NOþ2 ).
Here H2SO4 is actually acting as a catalyst for the formation of nitronium
ions.[1c] The acidity of the medium is a significant factor in this reaction.
In the industrial process, a large excess of H2SO4 is required for
compensating the retarding effect caused by the water molecule formed as
byproduct, which dilutes the acid. The disposal of the spent acids is also a
serious problem. In addition to that the reactions under high acidic
conditions inevitably brings plant corrosion and occupational hazards for
the people doing these jobs.
In recent years so many alternative methods are reported in literature
using different catalyst/reagent systems.[2a–o] More recently, Choudary
et al.[3] have reported a selective mononitration process for aromatic
molecules using HNO3 over Zeolite beta–I catalyst. This process also has
some disadvantages. The most important one being the difficulty in the
synthesis of Zeolite beta catalyst, which is an energy consuming process
involving, hydrothermal treatment of the starting materials under alkaline
conditions and it is also dependant on experimental conditions. In the
present method, we carried out the nitration reaction over a natural clay
mineral using dilute HNO3 (Sch. 1).
Scheme 1.

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BENTONITE CLAY/DILUTE HNO3
3567
All the reactions were carried out in hexane medium. The experi-
mental procedure was very simple. In the first step the substrate molecule
was uniformly adsorbed on the clay surface by stirring the substrate with
the clay catalyst. To this hexane was added as solvent and the R. B.
Flask was fitted with a Dean-Stark water separator. Then 40% HNO3
solution was added to this in one lot and refluxed. The water formed as
by product was separated in the Dean-Stark apparatus and removed at
regular intervals. After the complete removal of water, the reaction was
stopped and filtered for the removal of the catalyst and the products were
separated by fractional distillation. The generality of the reaction was
proved by performing the reaction with different aromatic substrates.
All the substrates have shown a significantly high para selectivity towards
nitration. In the case of benzene the conversion and para selectivity is
ca. 100%. Toluene gave a mixture of products in which 81% of para
isomer and 10% of dinitro derivative. The nitration of anisole and chloro-
benzene yielded ca. 90% para-nitro derivative. The nitration of anisole
yielded minor quantity of another compound and our attempts for its
identification was unsuccessful.
An interesting result was obtained in the case of aniline. Laszlo et al.
have reported the nitration of aniline with ‘‘claycop’’ in presence of
Ac2O.[2k] Here the product obtained was 2-nitroacetanilide in very high
yield. This method also have some serious limitations. Mainly on the use
of large amounts of Ac2O, which is environmentally unacceptable, and the
deprotection of anilides needs hydrolysis which increases the cost of produc-
tion. While adopting our synthetic methodology we got 35% of para-nitro
aniline along with a black tarry material which could not be identified.
Results are summarized in Table 1.
Benzylic methylene compounds are converted into corresponding
carbonyl compounds by a variety of reagents. The most prominent
[4g]
being chromium based reagents,[4a–f] KMnO4,
DDQ,[4h] various poly-
mer-anchored reagents,[4i–m] and silica,[4n–o] clays,[4p–q] and zeolite[4r]
based oxidants. Some photochemical methods are also reported.[5a–b]
The major drawbacks of chromium and manganese based catalysts are
that their use in a large scale leads to the generation of large volumes of
toxic wastes. The work-up of the final reaction mixture also liberates a
large amount of acid and chromium waste. The combination of toxic
metal, acid and organic residues represents a difficult form of waste
which must be treated before disposal and this makes synthetic method-
ology environmentally unacceptable.[6] A recent communication from
Clark et al.[7] showed the use of a membrane reactor in the presence
of a phase transfer catalyst for the oxidation of benzyl alcohol to ben-
zaldehyde which could be able to minimize environmental effects.

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3568
BAHULAYAN ET AL.
Table 1. Selective Mononitration of Aromatic Molecules Using Bentonite Clay/dil.
HNO3 (40%) System
Entry
1
Substrate
Product
Time (h)
5
Yield (%)
99
2
3
4
5
5
5
81
90
91
5
6
5
3
35
91
In the present method experimental procedure was same as
described in the case of nitration. The substrates studied were ethyl ben-
zene, isobutyl benzene, benzyl alcohol and diphenylmethane. In this case
also, the oxidation reactions were carried out in a reaction vessel set up
consisting of an R. B. Flask and Dean-Stark water separator. The sub-
strate to be oxidized was uniformly mixed with the clay catalyst and was
transferred to the flask along with the required amount of dil. HNO3

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BENTONITE CLAY/DILUTE HNO3
3569
(40%) solution. It was then refluxed in hexane medium. Oxidation of
ethylbenzene yielded 89% acetophenone and diphenylmethane yielded
benzophenone (81%) in relatively low reaction time. Benzylalcohol under-
went controlled oxidation to yield benzaldehyde in 90.5% yield. The only
anomaly observed was in the case of isobutylbenzene which yielded the
corresponding ketone, but the yield was less than 50%. The results are
given in Table 2.
We wish to add an alternative synthesis for benzophenone (Entry 5 in
Table 2) using the same bentonite clay without adding HNO3. Here benzene
was mixed with excess CCl4 in the presence of catalytic amount of bentonite
clay (2g) as catalyst. This was then loaded in a Parr 50 mL pressure reactor
and the reaction was carried out at 180ꢀC under 200 psi pressure for 3 h.
The dichloro compound obtained, on hydrolysis yielded benzophenone in
greater than 90% yield. Here the yield was calculated based on the conversion
of the used benzene (Sch. 3)
Table 2. Benzylic Oxidations of Aromatic Molecules Using Bentonite Clay/dil.
HNO3 System
Entry
1
Substrate
Product
Time (h)
5
Yield (%)
89
2
5
81
3
4
5
5
41
3.5
3
90.5
92

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3570
BAHULAYAN ET AL.
Scheme 2.
Scheme 3.
The same reaction was reported in literature[8] using AlCl3 as
catalyst. Its main disadvantages were that one molecule of AlCl3 liberated
3 molecules of HCl and also during hydrolysis unreacted AlCl3 liberates
a large volume of HCl vapors. The present method totally avoids these
problems.
PART II
In continuation of the nitration process we tried the nitration of ethyl
benzene with bentonite/HNO3 system. The reported procedures for the
nitration of alkyl benzenes include Laszlo’s ‘‘calycop’’/Ac2O[2k] system
and Olah’s Nafion-H/Hg(NO3)2 system.[2o] These authors successfully
isolated the different nitroalkyl benzenes. But in our experiments we did not
get any nitrated product. The major product obtained was acetophenone in
89% yield. Based on these observations we tried the same reagent/catalyst
system for benzylic oxidation reactions (Sch. 2).
Recycling studies were also done with used clay. The performance of
the catalyst was unaltered upto 5 cycles of operation. The used HNO3 was
also recycled by adjusting the concentration by adding the sufficient amount
of fresh acid.
In conclusion we have developed an efficient catalyst/reagent system
based on a natural clay/HNO3 system for the regioselective nitration and

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BENTONITE CLAY/DILUTE HNO3
3571
benzylic oxidations. The studies regarding the origin of the dual behavior of
this catalyst/reagent system is under progress.
EXPERIMENTAL
General
1
All products were systematically characterized by H NMR, IR spec-
tral and GC/MS analysis. Melting points were determined in an Aldrich
Mel-temp instrument and are uncorrected. IR spectra were recorded in a
Nicolet-Magna 560 FT-IR spectrometer. 1H NMR spectra were recorded on
a Bruker Avance DPX 300 MHz spectrometer. GC/MS analysis were done
on an HP 5890 Series-II instrument. X-Ray analysis of the clay catalyst was
done with a Philips 1710 instrument using Cu K-a radiation. B. E. T. surface
area of the catalyst was measured in a Micromeritics Gemini III 2375
surface area analyzer. The reagents used were purchased from M/s Aldrich
Chemicals and used as such. The solvents used were purchased from
commercial sources and were doubly distilled and dried before use. The
yields reported are based on the isolated products.
Preparation of the Clay Catalyst
The clay samples were collected from Ashapura mines of Gujarat state
India. The preparation of the catalyst involves separation of the fines by
sedimentation technique, followed by calcination of the clay samples at
450ꢀC for removing the organic impurities usually humic acid. Character-
isation of the clay samples were done by chemical assay and X-ray analysis
[SiO2 56.19%, Al2O3 15.56% Fe2O3 7.59%, TiO2 0.79%, CaO 5.68%, MgO
4.98%, L.O.I. 14.21%, H0 (mmol) 0.043, B.E.T. surface area 98.30 m2/g].
General Procedure for Nitration
The following procedure for the preparation of nitrobenzene (I) is
representative. A dry 100 mL two-necked round bottom flask is fitted with
a Dean-Stark apparatus and was charged with benzene (7.8 g, 0.1 mol) and
bentonite clay (5 g). To this 50 mL of dry hexane was added and the setup was
fitted with a water cooled reflux condenser. Twenty milliliters of dilute HNO3
(40%) was added to this in one lot and refluxed for 5 h. At this time the
water level in the Dean-Stark apparatus remained constant and this indicated

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3572
BAHULAYAN ET AL.
the completion of the reaction. After filtering the clay catalyst, the product
and solvent were separated by vacuum distillation. Yield 13.4 g, (99%).
General Procedure for Oxidation
The following procedure for the oxidation of ethylbenzene to aceto-
phenone (7) is representative. A dry 100 mL two-necked round bottom flask
was fitted with a Dean-Stark water separator and it as charged with ethyl-
benzene (5.3 g, 0.05 mol) and bentonite clay (5 g). To this 50 mL of dry
hexane was added and the setup was fitted with a water cooled reflux con-
denser. Twenty milliliters of dilute HNO3 (40%) was added to this in one lot
and refluxed for 5 h. At this time the water level in the Dean-Stark appa-
ratus remained constant and T. L. C. analysis showed a well defined spot for
a product. After filtering the clay catalyst, the solvent was removed in
vacuum and the product was purified on a silica column (20 : 1 petroleum
ether/ethyl acetate) to give acetophenone as a colorless liquid (5.3 g, 89%),
1
IR (KBr; cmꢁ1), 1700, 1600, 1580, 1450, H NMR (300MHz; CDCl3) ꢀ 2.6
(s, 3H, –CH3), 7.9 (s, 5H, –C6H5). GC/MS (OV 101) Rt 3.96 min. MS(EI)
(relative intensity): m/z 120 (Mþ)(25), 105 (100), 77 (83), 51 (30).
Benzophenone (8): Starting from 8.4 g (0.05 mol) diphenylmethane and
5 g of clay, purification on a silica column (10 : 1 petroleum ether/ethyl
acetate) afforded a white solid which crystallized from petroleum ether/ben-
zene mixture (10 : 1 v/v) as benzophenone. M.p. 49ꢀC (7.37 g, 81%) IR
(KBr; cmꢁ1), 1660, 1600, 1580, 1H NMR (300 MHz; CDCl3) ꢀ 7.5–7.7
(m, 10H, 2 ꢂ ꢁC6H5). GC/MS (OV 101) Rt 8.53 min. MS (EI) (relative
intensity): m/z 182 (Mþ) (54), 105 (100), 77 (54).
2-Methylpropiophenone (9): Starting from 6.7 g (0.5 mol) isobutyl-
benzene and 5 g of clay. Purification on a silica column (5 : 1 hexane/benzene)
afforded 2-methylpropiophenone as a viscous liquid (3.03 g, 41%). IR
1
(KBr; cmꢁ1), 1656. H NMR (300 MHz; CDCl3) ꢀ 7.3–7.8 (m, 5H, –C6H5)
1.9 (s, 1H, CH) 2.6 (m, 6H, 2 ꢂ CH3). GC/MS (OV 101) Rt 5.28 min. MS
(EI) (relative intensity): m/z 148 (Mþ) (28), 105 (100), 77 (90), 51 (89).
Benzaldehyde (10): Starting from 5.4 g benzylalcohol and 5 g clay. Yield
4.79 g (90.5%) IR (KBr; cmꢁ1), 1659, 1667, 1557, 1449. 1H NMR (300 MHz;
CDCl3) ꢀ 7.4–8.0 (m, 5H, –C6H5) 10.0 (s, 1H, –CHO). GC/MS (OV 101) Rt
3.38 min. MS (EI) (relative intensity): m/z 106 (Mþ) (90), 105 (92), 77 (100).
Benzophenone (8) frombenzene, CCl 4 and clay (Entry 5): 7.8 g (0.1 mol
of benzene, 30 mL CCl4 and 2 g of clay were loaded in a Parr 50 mL pressure
reactor and the temperature was adjusted to 180ꢀC at a stirring rate of
150 rpm for 3 h. At this condition a pressure of 200 psi was generated in
the reaction vessel. After 3 h the reactor was switched off. After cooling the

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BENTONITE CLAY/DILUTE HNO3
3573
reaction mixture was transferred into a beaker and warmed with water on a
boiling water bath for 30 min. The water was removed by filtration and the
dried residue was dissolved in benzene and filtered for removing the clay
catalyst. The filtrate after removing the charcoal was concentrated. It was
then chromatographed on a silica column (10 : 1 petroleum ether/benzene)
to yield benzophenone as a colorless solid m.p. 49ꢀC (8.37 g, 92%).
Regenarability of the Catalyst
The catalyst filtered from the reaction mixture was thoroughly washed
with acetone and dried at 110ꢀC for 1 h in a hot air oven. This was then
directly used for another fresh reaction. The process was repeated in five
cycles and no considerable change was observed in the activity of the catalyst.
ACKNOWLEDGMENTS
One of the authors, D. B., wishes to thank S.E.R.C., Department of
Science and Technology, Government of India for providing financial assis-
tance under the Fast Track scheme for young scientists. We also thank
C.S.I.R., Government of India for providing financial support under
N.M.I.T.L.I. program.
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