Nitration of simple aromatics with NO2 under air atmosphere in the presence of novel Bronsted acidic ionic liquids

Article abstract of DOI:10.1080/00397910701796758

Aromatics nitrate with NO2/air catalyzed by novel Bronsted acidic ionic liquids (ILs) without any volatile chlorinated organic solvent under mild conditions. The ILs employed were caprolactam based, [Caprolactam]X (X-=pTSO-, BSO-, BF4-, NO3-), which are o

Full text of DOI:10.1080/00397910701796758

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Nitration of Simple Aromatics with NO2 under
Air Atmosphere in the Presence of Novel
Brønsted Acidic Ionic Liquids
Xiufang Qi a , Guangbin Cheng a , Chunxu Lu a & Desheng Qian a
a School of Chemical Engineering, Nanjing University of Science and
Technology , Nanjing, China
Published online: 27 Feb 2008.
To cite this article: Xiufang Qi , Guangbin Cheng , Chunxu Lu & Desheng Qian (2008) Nitration of Simple
Aromatics with NO2 under Air Atmosphere in the Presence of Novel Brønsted Acidic Ionic Liquids, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 38:4,
537-545, DOI: 10.1080/00397910701796758
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Synthetic Communicationsw, 38: 537–545, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701796758
Nitration of Simple Aromatics with NO2
under Air Atmosphere in the Presence of
Novel Brønsted Acidic Ionic Liquids
Xiufang Qi, Guangbin Cheng, Chunxu Lu, and Desheng Qian
School of Chemical Engineering, Nanjing University of Science and
Technology, Nanjing, China
Abstract: Aromatics nitrate with NO2/air catalyzed by novel Brønsted acidic ionic
liquids (ILs) without any volatile chlorinated organic solvent under mild conditions.
The ILs employed were caprolactam based, [Caprolactam]X (X2 ¼ pTSO2, BSO2,
BF24 , NO23 ), which are of relatively lower cost and lower toxicity than traditional imi-
dazolium-based ILs. The nitration reactions were carried out at 215 to 208C first, then
at room temperature for a longer time with a little excessive NO2 (ca. 1.4 eqv.) for
moderate yield (for toluene). The IL could be reused four times.
Keywords: aromatics, nitration, NO2/air, novel Brønsted acidic ionic liquids
Nitration of aromatics is a fundamental reaction of great industrial importance
that provides key organic intermediates or energetic materials. Unfortunately,
the usual commercial process of nitration of aromatics, which uses nitric and
sulfuric acids, is not environmentally benign, resulting in disposal problems
and regeneration of used acids, and often providing poor selectivity for the
desired products. Therefore, various nitration approaches have been
explored to avoid the problems of the traditional mixed-acid method.[1]
Much success has been achieved, but some problems still exist, such as the
volatile chlorinated solvents required in some processes.
Received April 24, 2007
Address correspondence to Guangbin Cheng, School of Chemical Engineering,
Nanjing University of Science and Technology, Xiao Ling Wei 200, Nanjing
210094, China. E-mail: gcheng@mail.njust.edu.cn
537

538
X. Qi et al.
Because ionic liquids (ILs) have strong polarity, negligible vapor
pressure, wide solubility to organics and inorganics, and variable Lewis/
Brønsted acidity, they have been used in a series of reactions[2] as catalysts
and/or solvents (instead of volatile organic solvents). That also drew our
attention, and we desired to develop an environmentally benign nitration
process using ILs as catalysts and solvents.
Because imidazolium-based ILs have relatively higher cost and
toxicity[3] in general chemical applications, we made efforts to seek new
Brønsted acidic ILs with lower cost and lower toxicity. Lactam-based
Brønsted acidic ILs are relatively cheaper, easily available in large
amounts from industry, and more environmentally friendly than imidazo-
lium-based ILs.[4] The additional carbonyl groups in the lactam might lead
to specific functions when these lactam-based ILs are used as media or
solvents. Therefore, we chose [caprolactam]BF4 and [caprolactam]NO3,
whose Brønsted acidities are relatively stronger, and applied them to
nitration of simple aromatics. Considering that addition of large aromatic
groups to the molecular of ILs may increase the solubility of ILs to
aromatics, we synthesized two new caprolactam-based Brønsted acidic ILs,
[caprolactam]pTSO and [caprolactam]BSO, with p-toluenesulfonic acid
and benzenesulfonic acid as conjugated acid, respectively. Then the ILs
were characterized by spectrometry, and their activity as catalysts for the
nitration of toluene, chlorobenzene, and benzene were investigated. There
are several reports on aromatics nitration with HNO3 (67%)[5] or HNO3/
Ac2O[6] in the presence of ILs. However, there has been no report on the
nitration of aromatics with NO2 using ILs as solvents and/or catalysts so
far. NO2 has potential applicable ability for nitration. In this article, we
report for the first time the results of the nitration of simple aromatics
with NO2 under air atmosphere using Brønsted acidic ILs as catalysts.
EXPERIMENTAL SECTION
Preparation and Characterization of Caprolactam-Based Ionic
Liquids
The preparation of caprolactam-based ILs is illustrated in Scheme 1. The ILs
were prepared according to the method in Ref. 4.
1H NMR spectra were recorded on a Bruker Advanced Digital 300-MHz
NMR spectrometer, and mass spectra were recorded on a Finnigan TSQ
Quantun Ultra AM spectrometer. IR spectra were recorded on a Bruker
Equinox 55 IR spectrometer; melting points were measured on a XT4 micro-
melting point instrument made by Beijing Keyi Dianguang Instrument Plant.
Using pyridine as IR probe molecule, pyridine and ILs were mixed in a
given ratio (1:1 by molar) and then spread into KBr liquid films; the
Brønsted acidities of these ILs were determinated by IR spectrometer.

Nitration of Simple Aromatics
539
Scheme 1. Preparation of caprolactam-based ILs.
Nitration of Aromatic Compounds with NO2 under Air in the
Presence of Ionic Liquids: General Procedure
To a cooled (0 or 2158C), vigorously stirred mixture of the IL (quantities
indicated in the tables) and the substrate (10.0 mmol) under an air atmosphere,
0.5 mL of liquid nitrogen dioxide was added all at once. After a given time, the
reaction was quenched by addition of deionized water (ca. 5 mL), and then the
internal standard (nitrobenzene for toluene, anisole, and chlorobenzene, and
hexadecane for benzene, 0.2 mL) was added, followed by addition of
hexane (5 mL ꢀ 4). Two phases formed once the stirring was stopped. The
bottom one was an aqueous phase containing the IL, and the upper one was
an organic phase. The upper phase was removed by simple decantation
from the aqueous phase. The organic layers were combined and washed
with a saturated solution of sodium bicarbonate (5 mL), water (5 mL ꢀ 3),
and brine (5 mL ꢀ 2) in turn, dried with Na2SO4, and then analyzed by GC.
(When in preparative scale, the dried organic layers were distilled under
reduced pressure to remove solvent and the unreacted substrates.) The
recovered IL was dried at 658C under reduced pressure for 5 h to remove
water accumulated during the workup and employed for further use.
RESULTS AND DISCUSSION
Preparation and Characterization of Caprolactam-Based Ionic
Liquids
All of the caprolactam-based ILs were prepared with high yields of 85–92%.
They are moisture stable. Among them, [caprolactam]pTSO is a white solid
with a melting point of 698C, [caprolactam]BSO is a yellow liquid of high
viscosity, [caprolactam]BF4 is a colorless liquid with good fluidity, and
[caprolactam]NO3, is a white solid with a melting point of 308C. All of
them are easily soluble in water and have good moisture absorption
(Scheme 2).

540
X. Qi et al.
Data
1
The H NMR data and MS spectra of the four ILs were shown as following.
1
The data were similar with that reported in Ref. 4. In the H NMR (CDCl3)
spectra, there are two active hydrogen in the range of d 6.80–16.24 ppm, indi-
cating that they all formed N-protonated caprolactam cation.
Scheme 2. Nitration of simple aromatics with NO2/air catalyzed by ILs.
1
[Caprolactam]pTSO: H NMR (CDCl3): d 1.73–1.83 (m, 6H), 2.38 (s, 3H),
2.68 (t, J ¼ 4.3, 2H), 3.46 (s, 2H), 7.22 (d, J ¼ 7.5, 2H), 7.76 (d, J ¼ 7.7,
1
2H), 9.92 (s, 1H), 10.91 (s, 1H). H NMR (D2O): 1.61–1.77 (m, 6H), 2.41
(s, 3H), 2.49 (t, J ¼ 3.6, 2H), 3.26 (t, J ¼ 4.6, 2H), 7.38 (d, J ¼ 7.9, 2H),
7.71 (d, J ¼ 7.9, 2H). MS (ESI): m/z 114.12 (cation); calculated for cation
(C6H12NO) 114.09.
[Caprolactam]BSO: 1H NMR (CDCl3): d 1.74–1.85 (m, 6H), 2.65 (d, J ¼ 9.1,
2H), 3.43 (s, 2H), 6.80 (s, 1H), 7.45 (s, 3H), 7.90 (d, J ¼ 5.9, 2H), 10.03
1
(s, 1H). H NMR (D2O): d 1.47–1.63 (m, 6H), 2.33 (t, J ¼ 5.3, 2H), 3.11
(t, J ¼ 4.5, 2H), 7.44 (t, J ¼ 7.7, 3H), 7.68 (d, J ¼ 7.6, 2H). MS (ESI): m/z
114.13 (cation); calculated for cation (C6H12NO) 114.09.
[Caprolactam]BF4: 1H NMR (CDCl3): d 1.74–1.88 (m, 6H), 2.71–2.76
(q, J ¼ 5.6, 2H), 3.52–3.57 (t, J ¼ 7.1, 2H), 3.46 (s, 2H), 8.49 (s, 1H),
1
9.79 (s, 1H). H NMR (D2O): 1.37–1.58 (m, 6H), 2.29–2.33 (t, J ¼ 5.7,
2H), 3.05–3.09 (t, J ¼ 5.0, 2H). MS (ESI): m/z 114.09 (cation); calculated
for cation (C6H12NO) 114.09.
[Caprolactam]NO3: 1H NMR (CDCl3): d 1.64–1.84 (m, 6H), 2.62–2.65
1
(t, J ¼ 5.6, 2H), 3.40–3.43 (t, J ¼ 4.5, 2H), 8.54 (s, 1H), 16.24 (s, 1H); H
NMR (D2O): d 1.35–1.54 (m, 6H), 2.25–2.29 (t, J ¼ 5.7, 2H), 3.01–3.04
(t, J ¼ 5.0, 2H). MS (ESI): m/z 114.10 (cation); calculated for cation
(C6H12NO) 114.09.
Using pyridine as IR probe molecule, the IR spectra of the caprolactam-
based ILs were obtained. According to Refs. 7 and 8, the presence of a band
near 1450 cm21 is indicative of pyridine coordinated to Lewis acid sites,
where a band near 1540 cm21 is an indication of the formation of
pyridinium ions resulting from the presence of Brønsted acidic sites.
From each probe IR spectra of these ILs, a band is observed near

Nitration of Simple Aromatics
541
1540 cm21, confirming that Brønsted acidic sites are all present in the ILs
synthesized. The wavenumber of the band corresponding to coordination
at Brønsted acidic sites increases from 1541.59 cm21 for [caprolactam]BSO
to 1543.69 cm21 for [caprolactam]pTSO, 1544.61 cm21 for [caprolactam]
NO3 and 1549.39 cm21 for [caprolactam]BF4, indicating that the Brønsted
acidity increases in the order [caprolactam]BSO , [caprolactam]pTSO ,
[caprolactam]NO3 , [caprolactam]BF4.
NO2 Nitration of Simple Aromatics under Air in the Presence of
Brønsted Acidic ILs
Some results of NO2 nitration of simple aromatics under an air atmosphere in
the presence of Brønsted acidic ILs are listed in Table 1.
The data in Table 1 indicated that the prepared caprolactam-based ILs all
had catalytic acitivity for NO2/air nitration of aromatics, and among them,
Table 1. Results of NO2–air nitration of simple aromatics catalyzed by caprolactam-
based ILs
Product distribution (%)
Ionic
Entry
liquid
R
Yield (%)c
Ortho
Meta
Para
Ortho/para
1a
2a
IL1
IL2
IL3
IL4
IL1
IL1
IL2
IL3
IL4
IL1
IL2
IL3
IL4
None
None
None
CH3
CH3
CH3
CH3
OCH3
Cl
61.4
44.7
46.5
31.8
91.3
48.7
37.4
38.2
25.3
55.4
41.4
43.8
29.2
9.2
57.1
49.9
56.8
56.2
40.4
19.9
19.8
24.3
22.5
3.9
6.3
4.7
7.9
2.3
3.7
0.3
1.9
9.0
39.0
43.8
38.4
35.8
57.3
76.4
79.8
73.7
68.4
1.46
1.14
1.48
1.57
0.70
0.26
0.25
0.33
0.33
3a
4a
5a
6b
7b
Cl
Cl
8b
9b
Cl
H
10b
11b
12b
13b
14b
15b
16a
H
H
H
H
Cl
4.9
13.4
39.8
57.9
0
5.7
60.1
36.4
0.66
1.59
CH3
aSubstrate 10 mmol, IL 10 mol%, NO2 0.5 mL (ca. 14 mmol), 2158C/30 min, then
08C/6 h, and rt/24 h (when the air balloon was removed).
bSubstrate 10 mmol, IL 10 mol% of substrate, NO2 0.5 mL (ca. 14 mmol), 2158C/
30 min, then 08C/6 h, and rt/60 h (when the air balloon was removed).
cCalculated by quantitative GC.

542
X. Qi et al.
Table 2. Recycling of IL1 for NO2–air nitration of toluenea
Product distribution (%)
Run
Yield (%)b
Ortho
Meta
Para
Ortho/para
1
2
3
4
5
61.4
60.7
57.3
57.8
54.8
57.1
56.3
54.2
54.3
52.6
3.9
4.6
6.8
7.7
5.7
39.0
39.1
39.0
37.9
41.7
1.46
1.44
1.39
1.43
1.26
aSubstrate 10 mmol, IL 10 mol%, NO2 0.5 mL (ca. 14 mmol), 2158C/30 min, then
08C/6 h, and rt/24 h (when the air balloon was removed).
bCalculated by quantitative GC.
[caprolactam]pTSO had the best affinity to aromatics. Comparing the result of
entry 1 with the result of entry 2, the latter showed a little lower yield but a
slight higher paraselectivity than the former. This could be explained by the
stronger polarization ability of IL2 than IL1, which because BSOs had
stronger polarity than pTSOs. Under the condition indicated in the table,
nitration of chlorobenzene had a smaller yield (and also benzene) but gave
better paraselectivity than without IL. From analyzing the results of NO2/air
nitration of toluene catalyzed by different ILs (entries 1–4 in Table 1) and
the Brønsted acidity of the ILs (which increases in the order [caprolactam]
BSO , [caprolactam]pTSO , [caprolactam]NO3 , [caprolactam]BF4), it
can be seen that the acidity and affinity to aromatics of ILs have important
roles on their catalytic activity.
Table 3. Synthesis of nitro compounds in preparative scale using the method upon
Product distribution
(%)d
Amount of
products (g) Yield (%)c Ortho Meta Para Ortho/para
Entry
R
1a
2a
3b
4b
OCH3
CH3
H
26.3
17.0
13.1
14.6
85.9
62.0
53.4
44.6
39.6
54.2
2.9
3.4
57.5
42.4
0.69
1.28
Cl
22.3
3.5
74.2
0.30
aSubstrate 0.2 mol, IL1 10 mol%, NO2 10 mL, 2158C/30 min, then 08C/6 h, and
rt/24 h (when the air balloon was removed).
bSubstrate 0.2 mol, IL1 10 mol% of substrate, NO2 10 mL, 2158C/30 min, then
08C/6 h, and rt/60 h (when the air balloon was removed).
cIsolated yield.
dCalculated by quantitative GC.

Nitration of Simple Aromatics
543
Figure 1. Possible pathway for NO2–air nitration of aromatics catalyzed by Brønsted
acidic ILs.
Recycling of IL1 for NO2/air nitration of toluene was studied, and the
results are listed in Table 2. Though the yield decreased and ortho/para
ratio lowered after each recycling because of the loss of IL in the disposal
process, the IL still had catalytic activity after four runs.
To check the utility of this process for mass production, nitro compounds
were synthesized with good yields using this method in preparative scale. The
details are shown in Table 3.
The mechanism of the present process at this stage is mysterious. The
nitration of aromatic compounds with N2O5 is very reasonable. N2O4 is
partially converted to N2O5 with O2. Meanwhile, knowing that dinitrogen
tetroxide is almost completely ionized in anhydrous nitric acid,[9] we
imagine that NO2-air nitration of simple aromatic compounds catalyzed by
Brønsted acidic ILs may also proceed via the pathway showed in Fig. 1. As
shown in the figure from formula (1) to formula (6), nitration products are
produced, and the side-product is nitric acid. When reaction temperature
rises, catalyzed by Brønsted acidic ILs, NOþ2 is generated by HNO3 and
reacts with the aromatics. Thus, the usage of nitrogen dioxide is increased.
The most ideal result is that water is the only by-product.
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