Regioselective nitration of aromatics under phase-transfer catalysis conditions

Article abstract of DOI:10.1016/j.catcom.2011.07.013

Mononitration of aromatics was performed in a two-phase system using phase-transfer catalyst. The most advantageous conditions for the nitration were determined (temperature, reaction time, the type and amount of phase transfer catalyst, nitrification strength of the nitro-sulfuric acid). From the results, a significant improvement in the selectivity and conversion of the nitration of xylene was observed: the ratio of 4-nitro-m-xylene to 2-nitro-m-xylene was unprecedented increased up to 91.3%:7.7%, the ratio of 4-nitro-o-xylene to 3-nitro-o-xylene was also increased to 71.1%:27.2%; both the conversions were over 96%.

Full text of DOI:10.1016/j.catcom.2011.07.013

Catalysis Communications 14 (2011) 42–47
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Regioselective nitration of aromatics under phase-transfer catalysis conditions
a
a
b
Peng-Cheng Wang a, Ming Lu a, , Jie Zhu , Yan-Ming Song , Xian-feng Xiong
⁎
a
School of Chemical Engineering, Nanjing University of Science & Technology, Nanjing 210094, China
b
Xi'an Modern Chemistry Research Institute, Xi'an 710065, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 May 2011
Received in revised form 9 July 2011
Accepted 12 July 2011
Available online 23 July 2011
Mononitration of aromatics was performed in a two-phase system using phase-transfer catalyst. The most
advantageous conditions for the nitration were determined (temperature, reaction time, the type and amount
of phase transfer catalyst, nitrification strength of the nitro-sulfuric acid). From the results, a significant
improvement in the selectivity and conversion of the nitration of xylene was observed: the ratio of 4-nitro-m-
xylene to 2-nitro-m-xylene was unprecedented increased up to 91.3%:7.7%, the ratio of 4-nitro-o-xylene to 3-
nitro-o-xylene was also increased to 71.1%:27.2%; both the conversions were over 96%.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Phase-transfer catalysis nitration
Aromatic compounds
Quantitative calculation
Regioselectivity
1. Introduction
surfactants was reported. But the application in nitration is only
limited to phenol and its derivatives.
Nitration of aromatic compounds is a widely used reaction to
produce organic intermediates required in large amounts for the fine
chemical industry [1,2]. The earlier studies of nitration of xylene
isomers along with other benzene derivatives, reported by Olah et al.
[3,4] using conventional mixed acid and nitronium salts in organic
solvents resulted into lower p-selectivity. Uemura et al. [5] used
sodium nitrite/sodium nitrate and trifluoro acetic acid resulting into
good yield; however, only o- and m-products were formed. Coombes
and Russell [6] reported higher p-selectivity with transition metal
nitrato compounds in carbon tetrachloride, while involving expensive
and extremely hygroscopic reagents and the use of hazardous
chlorinated compounds. Nitration of xylene isomers over sulfuric
acid supported silica gel [7,8] resulted into good conversion; however,
p-selectivity was not significantly high. Choudary and co-workers [9]
reported good p-selectivity with solid acid catalyst, zeolite-β, using
chlorinated solvents. V.N. Sheemol [10] reported nitration of o-xylene
with nitric acid using various solid acids such as sulfated silica,
sulfated zirconia and zeolites. These works with different solid acids
are all facing the problem of low conversion.
The objective of our present work is to study the effect of
surfactant media on nitration of xylene utilizing the mixed acid
(sulfuric acid was used about half as much as conventional nitration
process). The selectivity and conversion was significantly improved
by comparing with the traditional results or even catalysis by zeolite
[3,4,9,10,16–18]. The ratio of 2-nitro-m-xylene to 4-nitro-m-xylene
was increased to 91.3%:7.7%; the ratio of 4-nitro-o-xylene to 3-nitro-
o-xylene was increased to 71.1%:27.2%; both the conversions were
over 96%. The application prospect of this nitration system is more
practical than other reported methods in industry. Comparing to the
improvement of commercial value, the investment is very small, for it
requires only a little optimization and renovation of existing facilities.
2. Experimental
Gas chromatography (GC) analysis was performed on an Agilent GC-
6820 equipped with a 30 m×0.32 mm×0.5 μm HP-Innowax capillary
column and a flame ionization detector. Mass spectroscopy (MS)
spectra were measured on a Trace Ultra-trace DSQ GC-MS spectrometer.
Allchemicals werepurchased from Sinopharm Chemical ReagentCo. Ltd
and used without further purification.
Surfactant molecules, used as either phase transfer catalyst or
micelle, are the simplest of the aggregates to affect selectivity, and
their applications in substitution reaction have been explored [11,12].
The selective halogenating reaction of aromatics such as chlorination
of phenol [13,14] and bromination of N,N-dimethylaniline [15] with
2.1. General procedure
In a typical reaction protocol, 0.08 mol of aromatics (about 10 ml)
and some catalyst were taken in a 50 ml two-necked round-bottomed
flask. The resulting mixture was heated to 30 °C, and then the
prepared mixed acid (0.08 mol of 65%HNO3 and 0.05 mol of 98%
H2SO4) was added dropwise for 1.5 h with stirring. After addition of
⁎
Corresponding author. Tel.: +86 25 84315514; fax: +86 25 84315030.
E-mail address: luming@mail.njust.edu.cn (M. Lu).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.07.013

P.-C. Wang et al. / Catalysis Communications 14 (2011) 42–47
43
Table 1
Nitration of m-xylene over various surfactant catalystsa.
Surfactant
Selectivity/%
1b
Conversion/
%
2c
Others
Blank
SDBSd
SDSe
81.2
90.1
90.3
90.4
86.3
87.1
86.1
16.6
9.6
9.0
2.2
0.3
0.7
0.4
0.8
1.0
0.3
82.1
98.1
98.8
97.8
96.8
98.6
96.8
SDSNf
CTABg
MTCACh
BTACi
9.2
12.9
11.9
13.6
a
1% catalyst, 0.08 mol m-xylene,0.08 mol of 65%HNO3,0.05 mol of 98%H2SO4; 1.5 h
addition at 30 °C and then 3 h at 50 °C.
b
4-nitro-m-xylene.
2-nitro-m-xylene.
Dodecyl benzenesulfonic acid sodium salt.
Sodium dodecyl sulfate.
Sodium dodecyl sulfonate.
Hexadecyltrimethyl ammonium bromide.
Methyltrioctyl ammonium chloride.
Benzyltrimethyl ammonium chloride.
Scheme 2. Possible mechanism of phase-transfer catalysis for anionic surfactant in
nitration.
c
d
e
f
more characterizations are processing. A future work will address
mechanism and how the surfactants interact with the reactants.
g
h
i
3.1. Influence of different advantageous conditions on the selectivity of
m-xylene in nitration
the mixed acid, the temperature was raised to 50 °C and maintained
for 3 h. The final solution was washed by water to remove the
surfactant and neutralized the acids by addition of 5 M NaHCO3.
Under similar conditions, the nitration of other aromatic compounds
was also performed.
Nitration of m-xylene is chosen as the typical reaction of xylene to
select a suitable surfactant (shown in Figs. 1, 2), and then the
advantageous conditions are optimized. Three most possible factors,
amount of surfactant, nitrifying capacity and temperature, are
experimented to study the influence of them on the selectivity and
conversion of the nitration of m-xylene.
3. Results and discussion
The selectivity of 4-nitro-m-xylene significantly becomes excellent
with the increment of adding amount of three surfactants: SDBS, SDS
and MTCAC, while the conversions of them are reducing. We
conjecture that when the concentration of surfactant excesses the
critical micelle concentration (CMC) and continues increasing, micelle
would generate and change the type of catalysis.
The nitrifying capacity φ, which is also called activity factor, is the
effective concentration of sulfuric acid in waste acid, numerically
equal to the concentration of sulfuric acid when nitric acid in the
mixed acid is totally used [23]. As can be seen from Fig. 3, in
experiment with SDSN surfactant, we found the proportion of 4-nitro-
m-xylene in total products was unprecedented improved to 91.3% and
still increased slowly with nitrifying capacity strengthening. Another
exciting phenomenon was that the using of H2SO4 was significantly
Nitration of m-xylene over surfactants with different function
groups is preliminary experimented and the results are listed in
Table 1.We propose that the para-directing effect in cationic
surfactant is similar to the nitration of phenol in phase-transfer
catalysis, which has been explained by several researchers in the past
[19–22]. HNO3 was extracted into the organic phase, presumably via
anion exchange or hydrogen bonding with the phase-transfer catalyst.
In this work, it is proposed that the phase-transfer catalyst has two
functions: (i) to act as the source of the strong acid (HBr or HCl) which
is required for the in situ generation of the active nitronium cation
(NO2+), the amount of H2SO4 that is obviously reduced in mixed acid is
partly for this reason; (ii) to extract the essential acids, viz., nitric acid,
hydrobromic acid and hydrogen chloride, into the organic phase
where the nitration of the substrate seems to take place (Scheme 1).
The anionic surfactants, nitration has not been completely
interpreted by former researchers. They habitually ignore the
application of anionic surfactants, for the important roles played by
both halide anion (source of strong acid) and cationic surfactant
(extraction of HNO3). The fact is that anionic surfactant performs even
better than the cationic one in this nitration system. Higher selectivity
and conversion can be observed with most anionic surfactants we
tested. The active nitronium cation can be generated from mixed acid
that is much stronger than halide anion, and it is extracted into the
organic phase by an attractive interaction with the anionic head-
group of the surfactant. This effect promotes a shift of chemical
equilibrium to reduce the requirement of too much H2SO4 (Scheme 2).
We have detected the existence of phase-transfer with surfactant and
SDBS
SDS
12
10
8
SDSN
CTAB
MTCAC
BTAC
6
0.0
0.2
0.4
0.6
Adding amount of surfactant / mmol
Fig. 1. Product distribution of m-xylene in nitration with different surfactants. Reaction
condition: 0.08 mol m-xylene, 0.08 mol of 65%HNO3, 0.05mols of 98%H2SO4, 3 h at
50 °C.
Scheme 1. Possible mechanism of phase-transfer catalysis for cationic surfactant in
nitration.

44
P.-C. Wang et al. / Catalysis Communications 14 (2011) 42–47
the substituted aromatics are approaching the theoretical value. In the
nitration of m-, o-xylene and m-, o-dichlorobenzene, unprecedented
selectivity is shown by this nitration system. This orientation effect in
product distribution is mainly expected to include charge distribution
and steric effect. Comparing the visual steric effect of substituted
groups in C atom, charge distribution was regularly measured by the
accurate value with method of quantitative calculation. The charge
distribution of electrophilic aromatic substitution is described with
the net charge of each C atom. By the Gaoussian03 program, with
B3LYP/6-311G** group, NBO net charge of each C atom on the benzene
ring was calculated particularly and is shown in Table 3. All molecular
structures are drawn with GaussView program.
SDSN
SDBS
SDS
BTAC
CTAB
MTCAC
0.5
0.4
Viewing from the charge distribution of m-xylene and m-
dichlorobenzene, although the NBO net charge of C(2) is most
attractive in the benzene ring, the volume restriction of NO2+Q− and
H2NO3+Q− reduces drastically the chance of nitration in this position.
In other unsubstituted C atoms, the net charge of C(4,6) is much more
attractive than C(5), so the nitration products in these positions are
the main ones. In nitration of o-xylene and o-dichlorobenzene, a
significant contradiction of charge distribution and steric effect can be
observed. Similar with the results of m-xylene and dichlorobenzene,
steric effect always plays the main role in product distribution.
But in theory, steric effect is not enough to explain the extreme
nitration product distribution of o-dichlorobenzene and the unex-
pected proportion of 3-nitro-o-xylene. To study the effect of CH3−
and Cl− groups, a discussing by the charge distribution of toluene and
chlorobenzene was calculated. Although, in the nitration of chloro-
benzene, para-product is over 65%, the net charge of ortho-position (C
(2, 6)) is more attractive than the para-position. Toluene is just
contrary: para-position is nearly the same as ortho-positions in NBO
net charge, thus the ratio of para- and ortho-product is only 35.9%:
57.3% (nearly a natural distribution of 1:2). The difference value of
each C atom in the benzene ring, we conjecture, determines a more
accurate product distribution when the general trend has been
outlined. In nitration of o-xylene and o-dichlorobenzene, the
difference value of C(3, 6) and C(4, 5) of o-xylene is smaller than for
o-dichlorobenzene. So o-xylene's ability of orientating the nitro-group
towards the special position is weaker than o-dichlorobenzene,
although the C(3, 6) of o-xylene is more attractive than the C(3, 6)
of o-dichlorobenzene.
0.3
0.2
0.1
Fig. 2. Conversion of m-xylene in nitration with different surfactants.
less than blank experiment when getting the same conversion. For
instance, when the conversion was 75%, the nitrifying capacity with
SDSN surfactant was 62% (about 2.2 mL); it is nearly half of blank
experiment (nitrifying capacity 70%, about 3.6 mL). This provided a
proper solution of decreasing wasted-acid emission by a wide margin.
Commercial value was also raised because of the more wanted isomer
required by market demand.
The conversions with either SDSN surfactant or blank increase
with temperature raising (Fig. 4). When the reaction temperature is
over 60 °C, conversion is approaching the limit. However, different
from the continuous decrease in blank test, the proportion of 4-nitro-
m-xylene using SDSN surfactant increases slowly from 84% until its
peak 90.3% at 50 °C and then slips back a little. Considering both above
factors, the optimum temperature is 50 °C.
3.2. Quantitative calculation of C atoms' net charge on substituted
benzenes
To the monosubstituted aromatics, steric effect is obviously
slighter than for the disubstituted ones: The ratio of p- and o-
products is the same as traditional mixed acid system, while m-
products are improved slightly. The nitration agent prefers to be
introduced into benzene ring under the circumstance of net charge,
Encouraged by the remarkable results obtained with the above
reaction conditions, and in order to show the generality and scope of
this new protocol, we applied SDSN on other substituted aromatics.
The results obtained are summarized in Table 2. The conversions of all
100
90
80
70
60
50
100
90
80
70
60
50
100
90
80
70
60
100
90
80
70
60
Conversion of blank
Conversion of SDSN
Selectivity of blank
Selectivity of SDSN
conversion of blank
conversion of SDSN
Selectivity of blank
Selectivity of SDSN
0
20
40
60
80
55
60
65
70
75
80
Temperature / °C
Nitrifying capacity ϕ / %
Fig. 4. Influence of temperature on the regioselectivity and conversion of m-xylene in
nitration. Reaction condition: 0.08 mol m-xylene, 0.08 mol of 65%HNO3, 0.05 mol of
98%H2SO4, 0.3 mmol SDSN.
Fig. 3. The influence of the nitrifying capacity in nitration of m-xylene with different
surfactants. Reaction condition: 0.08mols m-xylene, 0.08mols of 65%HNO3, 0.3 mmol
SDSN, 3 h at 50 °C.

P.-C. Wang et al. / Catalysis Communications 14 (2011) 42–47
45
Table 2
Product distribution of different aromatics in nitration with SDSNa.
Substrate
Conversion/%
99.2
distribution
1.7%b
CH3
CH3
CH3
CH3
CH3
CH3
27.2%
71.1%
NO2
NO2
CH3
100
1.0%b
CH3
CH3
NO2
CH3
7.7%
91.3%
CH3
CH3
NO2
99.8
CH3
CH3
NO2
100%
CH3
Cl
CH3
NO2
96.3
NO2
Cl
Cl
Cl
1.2%
98.5%
Cl
Cl
Cl
97.7
93.3
Cl
Cl
NO2
100%
Cl
Cl
NO2
Cl 100%
Cl
Cl
CH3
99.4
97.7
1.3%b
CH3
CH3
CH3
NO2
57.3%
31.9%
5.5%
35.9%
NO2
NO2
Cl
0.5%b
Cl
Cl
Cl
NO2
2.5%
65.1%
NO2
NO2
a
All compounds are reacted following general procedure.
Others.
b
although the combination of phase-transfer catalyst and NO2+ or
H2NO3+ produces an increasing steric exclusion. And the difference
value of each C atom is so small that the regioselectivity is not very
distinct.
using the minimum nitration agent nitronium and varying the
solvent's polarity, one can obtain a different trend of product
distribution. A deeper study of selective nitration in this kind of
substituted aromatics will be meaningful.
The significant contradiction of charge distribution and steric
effect mentioned above provides a possibility to adjust and control the
product distribution. Increasing the steric effect such as the nitration
system we reported here can significantly decrease the attack of the
nitro-group on C atoms with crowded space, and improve the
regioselectivity. Moreover, using the charge distribution, such as
Acknowledgments
We thank the NSAF and NSFC (No: 11076017) of China for support
of this research.

46
P.-C. Wang et al. / Catalysis Communications 14 (2011) 42–47
Table 3
NBO charge number of all carbon atoms in benzene ringa.
Substrate
Net charge number of every carbon atom in benzene ring
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
1.53221
1.53225
−0.93500
0.00732
0.00732
−0.93498
CH3
1
6
CH3
2
3
5
4
CH3
2.12367
1.20506
−1.60774
−0.30259
1.79132
−1.58800
0.71400
−0.10952
−0.24225
1
6
2
3
5
CH3
4
CH3
1
−0.30714
1.20512
−0.23771
2
3
6
5
4
CH3
0.39846
0.39846
−0.99947
−0.07956
−0.07956
−0.99947
Cl
1
Cl
6
2
3
5
4
Cl
1.13764
0.80120
−1.78475
−0.70121
1.13764
−0.82775
0.05001
−0.82775
−0.70121
1
6
2
3
5
4
Cl
Cl
1
−0.70121
0.80120
−0.70121
2
3
6
5
4
Cl
−0.01521
−0.01453
−0.20722
−0.22883
−0.19059
−0.18112
−0.20887
−0.20381
−0.19057
−0.18112
−0.20711
−0.22883
CH3
1
2
6
5
3
4
Cl
1
2
6
5
3
4
a
All compounds are calculated under B3LYP/6-311G** group.
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