Nitration of aromatic compounds with nitric acid catalyzed by ionic liquids

Article abstract of DOI:10.1246/cl.2004.808

Nitration of simple aromatic compounds with 62% nitric acid is successfully carried out under solvent-free condition in a biphasic mode in the presence of the Bronsted acidic ionic liquids; the only by-product is water and ionic liquids are capable of being reused without any separation.

Full text of DOI:10.1246/cl.2004.808

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08
Chemistry Letters Vol.33, No.7 (2004)
Nitration of Aromatic Compounds with Nitric Acid Catalyzed by Ionic Liquids
Kun Qiao and Chiaki Yokoyama
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
2-1-1 Katahira, Aoba-ku, Sendai, 980-8577
(Received April 6, 2004; CL-040371)
R
R
Nitration of simple aromatic compounds with 62% nitric
acid is successfully carried out under solvent-free condition in
a biphasic mode in the presence of the Brꢀnsted acidic ionic liq-
uids; the only by-product is water and ionic liquids are capable
of being reused without any separation.
Brønsted acidic IL
+
HNO3
+ H O
2
NO2
Ionic liquids:
Me
(CH )n
2
The nitration reaction of aromatic compounds with nitric
acid is an important process in chemical industry with long his-
tory.1 The nitroaromatic compounds so produced are widely
used as raw materials for manufacturing a large range of chemi-
cal products such as dyes, pharmaceuticals, perfumes, and plas-
tics. Traditionally, this reaction is carried out in the presence of
mixtures of concentrated or fuming nitric acid with sulfuric acid
or stoichiometric quantities of strong Lewis acids such as boron
trifluoride, which inevitably leads to yielding a mass of acid
waste and causing serious environment problem. Developing
this important reaction in a catalytic and green way has been
strongly desired and a host of nitrating systems has been report-
N
N
SO3H
-
CF SO
3
3
a–c
1, n=3; 2, n=4
The Brꢀnsted acidic ionic liquids are prepared according to
Ref. 6. In a typical experiment, ionic liquid (5–15% mol of aro-
matic compound), aromatic compounds (20 mmol) and 62% ni-
tric acid (20–60 mmol) are charged successively into a 50 mL-
round flask with magnetic stirrer. Then the reaction is allowed
ꢀ
to proceed at 80 C for 12–22 h. After reaction, the organic layer
is separated by an extraction funnel and components in which are
analyzed by a gas chromatograph equipped with a FID detector
(Shimadzu GC-14A, ULBON HR-52 capillary column 25 m ꢁ
2
a–c
ed.
0
.32 mm).
From either an industrial or environmental standpoint, how-
Results of nitration of some simple aromatic compounds
ever, the ideal nitration agent is aqueous nitric acid, which is in-
expensive and generates only water as waste product. Unfortu-
nately, few catalytic systems that involved using aqueous
nitric acid as nitration agent are reported up to now. Waller et
al. have demonstrated a few years ago that rear earth metal tri-
flates are effective catalysts for nitration of aromatic compounds
with 62% nitric acid in the presence of Brꢀnsted ionic liquids
are listed in Table 1. No dinitrated products are detected in
any case. As can be seen from the table, ionic liquid 1 and 2 have
shown similar catalytic activity for the nitration of benzene with
2% nitric acid (Entries 1and 2). In both cases, medium conver-
sions of benzene are achieved. Both increasing the mole ratio of
nitric acid to benzene (Entries 1, 3, and 4) and prolonging the re-
6
3
with 69% nitric acid. But in this case, a time- and energy-con-
suming process of separating and drying catalysts is indispensa-
ble in order to ensure the reusability of the catalysts. Further-
more, dichlormethane, an environmental and safety harmful
volatile organic solvent, is also used as reaction medium.
Meanwhile, room temperature ionic liquids (RTILs) have
gained more and more recognition as green and promising sol-
Table 1. Nitration of aromatics with catalytic quantities of ion-
a
ic liquids
Ionic
Conv.
/%
Product distribution
Entry
R
liquid
ortho
meta
para
4
a–e
vents for synthetic chemistry.
A few examples of application
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
1 (5%)
2 (5%)
1 (5%)
1 (5%)
1 (5%)
70.1
76.5
n/ae
n/a
n/a
n/a
n/a
n/a
n/a
4.9
of ionic liquids as reaction medium for nitration reaction are re-
ported.5 Nevertheless, example of direct using ionic liquid as
catalyst for nitration of aromatic compounds is absent of the lit-
eratures.
a,b
b
60.2
83.7
c
d
86.7
In this paper, we report what we believe to be the first exam-
ple of nitration of aromatic compounds catalyzed by ionic liq-
uids. Brꢀnsted acidic ionic liquids, i.e. 3-methyl-1-(4-sulfobu-
tyl)imidazolium trifluoromethanesulfonate and 3-methyl-1-(3-
sulfopropyl)imidazolium trifluoromethanesulfonate, are demon-
strated to be the effective catalysts for nitration of simple aro-
matic compounds with 62% nitric acid under solvent-free condi-
tion. Water is the only by-product. The reaction is carried out in
a biphasic mode, so the nitroaromatic product can be very easily
separated by decantation after reaction and the remaining ionic
liquid is capable of being reused without any separation.
1 (10%) 68.7
1 (15%) 64.3
Me 1 (5%)
93.2
42.5
56.3
<1
43.6
41.6
44
52.5
56.2
56
Cl
Br
1 (5%)
1 (5%)
2.2
trace
n/a
1
1
0
1
NO2 1 (5%)
a
ꢀ
Reaction temperature: 80 C, Reaction time: 12 h, Mole ra-
tio of aromatic compounds/ nitric acid: 1:2.
Benzene/ nitric acid: 1:1, Benzene/ nitric acid: 1:3.
Reaction time: 22 h.
Only one product formed without isomers.
b
d
e
c
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.7 (2004)
809
action time (Entry 5) can remarkably enhance the conversion of
benzene. It is very interesting, however, to observe a slight de-
creasing of the conversion of benzene when the amount of ionic
liquid is raised (Entries 1, 6, and 7).
tric acid could be easily kept at a suitable level when high con-
centrated or fuming nitric acid is employed to replenish the con-
sumed nitric acid.
The nature of the Brꢀnsted ionic liquids catalyzed nitration
of aromatic compounds is not clear at this stage. The classical
reaction mechanism of nitration of benzene in the presence of
Then ionic liquid 1 is employed as catalyst for nitration of
some other aromatic compounds, including toluene, chloroben-
zene, and bromobenzene with 62% nitric acid under unopti-
mized reaction conditions (Entries 8–10), in all of these cases,
medium to high conversions of aromatic compounds are also
achieved. When nitrobenzene is used as substrate, no reaction
is observed, which is in accord with the above results (Entry 11).
Once the reaction is over, our attention is paid to the issues
of product separation and recycling of the Brꢀnsted ionic liquids.
This nitrating system is carried out in a biphasic mode, so it is
very easy to perform the product separation by decantation or us-
ing an extraction funnel after reaction.
From the point of manufacturing view, a continuous reaction
process without catalyst separation is very attractive in industry
in the case of nitration of aromatic compounds with nitric acid.
Then we examine the reusability of the remaining aqueous
phase, consisted of nitric acid and ionic liquid, in a way that di-
rectly employed it as catalyst for next run, just replenishing a
certain amount of 62% nitric acid to the aqueous phase. Results
of recycling of nitration of benzene with nitric acid, under the
same reaction condition as Entry 2 of table 1, are listed in
Table 2. During the whole process, the mole ratio of benzene
to nitric acid is kept at 1:2. It should be noted out, however,
the concentration of nitric acid decreased steadily with the reac-
tion goes on owing to the accumulating of water, which is
formed as waste product and not separated from the system.
As can be seen from the table, with the decreasing of the concen-
tration of nitric acid, conversion of benzene decreased simulta-
neously, but it can still kept at 25.6% after 5 times recycle and
the concentration of nitric acid decreased even to about 36%,
which suggesting the Brꢀnsted ionic liquid might has a great
possibility to be applied in large scale as the concentration of ni-
1
mixture acid of nitric acid and sulfuric acid is well established,
þ
which involved the formation of NO2 as nitrifier at first and
þ
then followed by electrophilic addition of NO2 with benzene.
Because the Brꢀnsted ionic liquids have the sulfuric acid-like
group –SO3H in their molecular structure, we are inclined to be-
lieve that the traditional reaction mechanism also functions here.
In conclusion, it is demonstrated that the Brꢀnsted ionic liq-
uids are a kind of novel and reusable catalysts for nitration of
simple aromatic compounds with aqueous nitric acid. The reac-
tion is carried out under solvent-free condition in a biphasic
mode. In comparison with other nitrating system, it is a more
green process with promising prospect for practical application.
References
1
For books and reviews on nitration, see a) J. G. Hoggett,
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2
3
4
F. J. Waller, A. G. M. Barrett, D. C. Braddock, and D.
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a
Table 2. Recycling of ionic liquid 2 for nitration of benzene
3
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nitric acid
Concentration of
nitric acid/%
Conversion
/%
Run
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a) K. K. Laali and V. J. Gettwert, J. Org. Chem., 66, 35
1
2
3
4
5
1:2
1:2
1:2
1:2
1:2
62
51
45
40
36
76.5
65.2
52.1
37.5
25.6
5
6
(
8
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a
The reaction condition is same as Entry 2 of Table 1.
Published on the web (Advance View) June 7, 2004; DOI 10.1246/cl.2004.808