Regioselective mononitration of chlorobenzene using caprolactam-based Br?nsted acidic ionic liquids

Article abstract of DOI:10.1016/j.molcata.2013.11.025

Three kinds of Br?nsted acidic ionic liquids caprolactam benzenesulfonate ([CP]BSA), caprolactam hydrosulfate ([CP]HSO₄) and caprolactam p-toluenesulfonate ([CP]pTSA) were synthesized. The structures of the ILs were confirmed by ¹HNM

Full text of DOI:10.1016/j.molcata.2013.11.025

Journal of Molecular Catalysis A: Chemical 383–384 (2014) 101–105
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Regioselective mononitration of chlorobenzene using
caprolactam-based Brønsted acidic ionic liquids
Cun Zhang∗, Mei-Jing Yu, Xiao-Yu Pan, Guo Wu, Liang Jin,
Wei-Dan Gao, Meng Du, Jun-Chao Zhang
College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three kinds of Brønsted acidic ionic liquids caprolactam benzenesulfonate ([CP]BSA), caprolactam hydro-
sulfate ([CP]HSO4) and caprolactam p-toluenesulfonate ([CP]pTSA) were synthesized. The structures of
the ILs were confirmed by 1HNMR and FT-IR. Regioselective mononitration of chlorobenzene has been
investigated in HNO3–Ac2Osystem with three ionic liquids. The sequence of the nitrification activity
is [CP]HSO4 > [CP]pTSA > [CP]BSA. With the caprolactam hydrosulfate as the catalyst the influences of
reaction time, reaction temperature and the amount of the catalyst were studied in the nitration of
chlorobenzene. The results showed that the optimum reaction conditions were as follows: when the
mole ratio of chlorobenzene to ionic liquid was 10:1.5, the reaction temperature was 60 ◦C and the reac-
tion time was 2.5 h, the yield of mononitro-chlorobenzene could reach 71.22%. And the mass ratio of
para/ortho isomer was 7.74, which was much more than 2.0 of para/ortho isomer mass ratio obtained by
nitric-sulfuric mixed acids as catalyst. Besides, the ionic liquid could be used repeatedly.
Received 23 May 2013
Received in revised form
14 November 2013
Accepted 24 November 2013
Available online 2 December 2013
Keywords:
Chlorobenzene
Brønsted acid
Caprolactam
Ionic liquid
Nitration
Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction
Ionic liquids (ILs) are defined as pure compounds consisting only
of cations and anions (i.e., salts), most of which are liquid at room
Nitrochlorobenzene mainly refers to p-nitrochlorobenzene and
o-nitrochlorobenzene. As the important and basic organic raw
material, they are widely used in the synthesis of diverse inter-
mediates, including dyes, pigments, pharmaceuticals, pesticides,
rubber chemicals, engineering plastics, and so on [1,2]. How-
ever, the usual industrial preparation of nitrochlorobenzene still
uses the traditional process of nitration by nitric-sulfuric mixed
acids [3], which exists two main problems. On the one hand,
the selectivity for desired products is poor and the mass ratio of
para/ortho isomer is only about 2.0. However, the market demand
of p-nitrochlorobenzene is more than o-nitrochlorobenzene, which
results in the relative surplus of o-nitrochlorobenzene and the
waste of resources [4]. On the other hand, the reaction produces
large amount of acid wastewater which is difficult to recycle,
and not only corrodes the equipment, but also seriously pol-
lutes the environment. Therefore, more and more domestic and
foreign scholars have attached great importance to looking for
high activity catalysts to improve the selectivity of the nitra-
tion of chlorobenzene and to explore green nitration approaches
[5–12].
temperature. Ionic liquids have excellent thermodynamic stabil-
ity and good solubility. Because of the negligible vapor pressure
at room temperature and environmentally friendliness, they are
known as green solvents and novel catalysts. ILs are now widely
used in various types of chemical reactions. In addition, ionic liq-
uids can meet the need of some particular reactions by changing
the anions or cations, so they are often referred to as “designer
solvents” [13–15].
Ionic liquids have also been got more and more applications in
aromatics nitration. With 68% nitric acid as nitrating agent, Martin
[16] investigated the nitration of aromatic compounds in the pres-
ence of the acidic imidazole-cation ionic liquids. In the hydrophobic
ionic liquids, the nitration of benzene was carried out smoothly
and got nearly quantitative yield of the mononitro-product. While
the nitration of chlorobenzene was much slower than that of ben-
zene, but the yield was higher and the ratio of para/ortho isomer
was about 3. N.L Lancaster [17] studied several nitration systems
on aromatic nitration using methylimidazolium-based ionic liq-
uids as the catalysts, and they demonstrated that HNO3/Ac2O was
the best nitration system. K. Smith [18] used ionic liquid as a
recyclable catalyst to investigate aromatic nitration reactions with
HNO3/Ac2O system and also achieved good results. But they all
use expensive, higher toxicity alkyl-substituted imidazolium ionic
liquids, such as 1-ethyl-3-methyl imidazolium ([emim]X), 1-butyl-
3-methylimidazolium ([bmim]X)(X: BF4−, PF6−, CF3SO3−), and so
on.
∗
Corresponding author. Tel.: +86 514 87975590 9105;
fax: +86 514 87975590 8410.
E-mail address: zc@yzu.edu.cn (C. Zhang).
1381-1169/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.11.025

102
C. Zhang et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 101–105
Fig. 1. Synthesis of caprolactam-based ionic liquids.
Fig. 2. The liquid chromatogram of the composition of the nitration products.
Caprolactam is a kind of amine derivatives, which can be formed
the quaternary ammonium cation by acid–base neutralizing reac-
tion. Thus, a new class of ionic liquids—caprolactam-based [CP]+
ionic liquids [19] are synthesized. Caprolactam-based Brønsted
acidic ILs are easy to be prepared and used, relatively cheaper, less
toxic, and easily available. Therefore, compared to more expensive
and highly toxic imidazole-based ionic liquids, they have a bet-
ter environmental friendliness and economic feasibility. Lu Chunxu
[20] used caprolactam-based ionic liquids as solvents and catalysts
for the nitration of toluene. The influences of reaction temperature,
reaction time and catalyst dosage on the yield and selectivity are
investigated in the reaction. It was found that toluene was nitrated
with higher para-selectivity in the presence of caprolactam-based
ionic liquids. The yield of mononitro-compound could reach 37.1%.
Qi Xiufang [21] researched caprolactam-based ionic liquids for the
mononitration of toluene. The yield can be above 50–60% under
suitable reaction conditions.
To date, the majority of research work in aromatic nitration
using caprolactam-based ionic liquids is about the nitration of
toluene, while it is still rarely reported about the nitration of
chlorobenzene. Three caprolactam-based ionic liquids were syn-
thesized, and respectively used in the regioselective mononitration
of chlorobenzene with HNO3/Ac2O nitration system under the
same conditions. The caprolactam-based ionic liquid which showed
the best catalytic performances was chosen to optimize the reac-
tion conditions on the nitration of chlorobenzene. The optimum
conditions were determined, and the reusability of ionic liquid was
investigated.
p-toluenesulfonic acid) (0.1 mol) was dripped slowly into the flask
within 30 min in the ice bath. The reaction lasted for another 12 h at
room temperature. Benzene was removed under reduced pressure,
and the product was further dried at 70 ◦C under vacuum for 1 h.
The stoichiometric caprolactam-based ionic liquids were almost
obtained.
2.3. Nitration of chlorobenzene
In a three-necked flask equipped with a reflux condenser, some
prepared ionic liquids were added and heated to the set reac-
tion temperature under the water bath. Then, the chlorobenzene
(50 mmol, 5.7 mL), the nitric acid (67%, 50 mmol, 3.5 mL), and
over dosage acetic anhydride (75 mmol, 7.4 mL) were successively
added under vigorously magnetic stirring. The reaction was con-
ducted at a constant temperature for a certain time. The reaction
mixture was cooled to room temperature and formed two lay-
ers after standing for some time. The bottom one was an aqueous
phase containing the IL, and the upper one was an organic phase.
The upper organic layer was removed by simple decantation and
washed to neutral with a saturated solution of sodium bicarbonate
and deionized water in turn, then dried under vacuum and analyzed
by HPLC. The IL was recovered for reusing from aqueous solution
by removing water with rotary evaporation and further drying at
70 ◦C under vacuum.
2. Experimental
2.4. HPLC method
2.1. Instruments and reagents
The composition of the nitration products were analyzed by
HPLC. ZOBRAX Eclipse Plus-C18 3.5 ␮m 4.6 × 100 mm columns
were used. A degassed and filtered mixture of methanol and water
(70:30 v/v) were used as mobile phase. The flow rate was main-
tained at 1.0 mL/min. Detection was performed at 254 nm. The run
time for each analysis was 8 min. All separations were carried out
at ambient temperature. The liquid chromatogram was as follows.
The observed retention times of the reference compounds
were found to be 2.06, 2.48, 3.29, and 4.28 min for o-
nitrochlorobenzene, p-nitrochlorobenzene, m-nitrochlorobenzene
and 2,4-dinitrochlorobenzene in Fig. 2. There is no trini-
trochlorobenzene in the liquid chromatogram because it needs
a higher temperature (90–100 ◦C) to generate. The yield in our
paper is calculated to mononitrochlorobenzene. From the liquid
chromatogram, we can obtain the ratio of mononitrochloroben-
zene to all nitration products. The yield is the ratio of weight (wt.
in formula) which is calculated by the formula below:
Reagents: chlorobenzene, caprolactam, benzene sulfonic acid,
p-toluene sulfonic acid, sulfuric acid, nitric acid (65–68%), acetic
anhydride, the above reagents are analytical grade.
Instruments: constant temperature magnetic stirrer (79–3),
rotating evaporator (R206), UV–vis spectrophotometer (TU-1901),
liquid chromatograph (Agilent1200, Agilent Technologies, Amer-
ica), FT-IR spectrometer (Tensor27, Bruker, Germany), 1H NMR
spectrometer (Avance600, Bruker, Germany).
2.2. Preparation of ionic liquids
The synthesis of ionic liquids includes one-step synthesis and
two-step synthesis. Caprolactam-based ionic liquids were synthe-
sized by one-step synthesis. Compared to the two-step synthesis,
one-step synthesis reduces the introduction of the intermediate
product, which improves the yield of ionic liquids. Furthermore,
the product is easy to be purified without by products. Fig. 1 is the
synthetic method of ionic liquids.
wt. of mononitrochlorobenzene
Yield(wt.%) =
wt. of all nitration products
Specific steps: Benzene (30 mL) was added to a 100 mL flask
containing 11.32 g of caprolactam (0.1 mol) full dissolved in water.
Then under magnetic stirring, the certain amount of high con-
centration strong acid HX (benzenesulfonic acid, sulfuric acid,
obtained wt. of product
theoretical wt. of product
×
× 100

C. Zhang et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 101–105
103
Table 1
Effect of different ILs on nitration of chlorobenzene.
Table 2
Effect of reaction time on nitration of chlorobenzene.
Ionic liquid
Isomerdistribution(%)
Para/ortho
Yield (%)
Reaction time/h
Isomerdistribution(%)
Para/ortho
Yield (%)
Ortho
Para
Ortho
Para
[CP]HSO4
[CP]pTSA
[CP]BSA
12.35
11.38
31.14
74.49
67.78
65.85
6.04
5.94
2.12
50.45
39.75
10.90
1.0
1.5
2.0
2.5
3.0
23.94
19.55
12.35
12.32
16.45
70.96
77.25
74.49
85.48
81.40
2.98
3.95
6.04
6.94
4.95
27.22
35.43
50.45
56.30
48.26
3. Results and discussion
than that of [CP]BSA, which may be due to the structure of p-
toluenesulfonate anion is more similar to chlorobenzene compared
to benzenesulfonate anion. So the intermiscibility of [CP]pTSA is
3.1. Characterization and acidity determination of ionic liquids
The spectroscopic datas of ionic liquids synthesized above by
1HNMR and FT-IR are as follows.
+
better and the ability to carry NO2 into the organic phase is
1. [CP]BSA: 1H NMR(D2O), ı(ppm) 1.47–1.63(m, 6H), 2.33(t,
J = 5.3 Hz, 2H), 3.11(t, J = 4.5 Hz, 2H), 7.44(t, J = 7.7 Hz, 3H), 7.68(d,
J = 7.6 Hz, 2H). FT-IR(KBr, ␯/cm−1), 3406(w), 2938(w), 2864(m),
1641(s), 1511(m), 1445(s), 1035(s), 761(s), 694(m), 497(w).
2. [CP]HSO4: 1H NMR(D2O), ı(ppm)1.34–1.54(m, 6H),
2.23–2.29(t, J = 5.7 Hz, 2H), 3.01–3.07(t, J = 5.0 Hz, 2H). FT-IR(KBr,
ꢀ/cm−1), 3101(w), 1919(m), 1603(s), 1422(s), 1056(s), 1012(s).
3. [CP]pTSA: 1H NMR(D2O), ı(ppm)1.61–1.77(m, 6H), 2.41(s,
3H), 2.49(t, J = 3.6 Hz, 2H), 3.26(t, J = 4.6 Hz, 2H), 7.38(d, J = 7.9 Hz,
2H), 7.71(d, J = 7.9 Hz, 2H). FT-IR(KBr, ꢀ/cm−1), 2948(w), 1678(m),
1505(m), 1179(s), 1026(w), 821(m), 681(s), 566(s), 484(w).
The Brønsted acidic strength of these caprolactam-based ILs
was evaluated by UV–vis spectroscopy [22]. The dimethyl yellow
(22.5 mg/L) acted as a Hammett indicator was respectively added
to the same concentration solutions (0.32 mmol/L) of the above-
mentioned ILs dissolved in methanol. UV–vis spectrophotometer
was used to measure the absorption of Hammett indicator, and
the results were shown in Fig. 3. The curve a in Fig. 3 is the
absorption curve of the dimethyl yellow solution without ionic
liquid, which showed that the absorption of the unprotonated
form of dimethyl yellow only at a wavelength of 350–460 nm.
After adding the ionic liquid, the ionized H+ from ionic liquid
is combined with dimethyl yellow to form protonated structure,
then the absorption peak appears at 460–570 nm in the spec-
trum. For the same amount of the ionic liquid, the greater the
Brønsted acidic strength and more the ionized H+. If the protonated
structure is formed more with dimethyl yellow, the absorption
at 460–570 nm will become stronger while the absorption at
350–460 nm becomes weaker accordingly. The analysis results
show that the Brønsted acidic strength of the three ionic liquids
is in the order of [CP]HSO4 > [CP]pTSA > [CP]BSA.
stronger, which make it show higher catalytic activity. Further-
more, it can be seen from Table 1, the magnitudes of the para/ortho
selectivity are affected by the nature of the ILs. This suggests
that a correlation exists between the degree of para-selectivity
and the ability of electron-withdrawing substituent to induce an
electrostatic attraction between substrate and ILs, causing greater
hindrance at ortho-positions to such substituent [24,25]. For the
same substrate and the same cation in the three ILs, the electron-
withdrawing of anions generate great effect on the para-selectivity.
For [CP]HSO4 the effects were stronger, thus enhanced the elec-
trostatic attraction between chlorobenzene and the IL, causing a
greater hindrance at ortho-position. So it showed the best para-
selectivity. As described above, the para-selectivity of [CP]pTSA is
higher than that of [CP]BSA due to the higher polarity of [CP]pTSA,
which give it good affinity to chlorobenzene. Therefore, [CP]HSO4
was chosen as the catalyst for further study.
3.3. Effect of reaction time on nitration of chlorobenzene
When the process conditions of nitration of chlorobenzene were
optimized, effect of reaction time on the yield and para-selectivity
was first investigated. Taking caprolactam hydrosulfate ionic liquid
as catalyst, other reaction conditions were the same as the previous.
The results are showed in Table 2.
As known in Table 2, with the reaction time increases, the
yield and regioselectivity increases rapidly within 2.5 h. When reac-
tion time reaches to 2.5 h, the yield and the ratio of para/ortho
achieve the maximum, which indicates that nitration reaction
has almost been completed in 2.5 h. At the beginning, ortho-
nitrochlorobenzene is formed quickly. But with the reaction
proceeding, it is more beneficial to form para-nitrochlorobenzene.
This may be because the increasing conversion of chloroben-
zene leads to that the ratio of ionic liquid to chlorobenzene is
gradually increasing, which provides ionization atmosphere for
chlorobenzene and is helpful for its polarization. After the reac-
tion time is extended to 3 h, however, the yield decreases slightly
and para/ortho also significantly reduces. The result indicates that
with the extension of reaction time, the dinitro-products are prob-
ably formed. So 2.5 h should be chosen as the appropriate reaction
time.
3.2. Effect of different ILs on nitration of chlorobenzene
Under the same conditions, three caprolactam-based ILs were
respectively used for nitration of chlorobenzene, so that to investi-
gate the influence of different anions on the reaction. The conditions
were: the mole ratio of ionic liquid to chlorobenzene was 1:5, at
50 ◦C for 2 h. The results are listed in Table 1.
As shown in Table 1, the catalytic activities of the three ILs
are in the order of [CP]HSO4 > [CP]pTSA > [CP]BSA. According to
the literature reported [23], for Brønsted acidic ionic liquid, the
stronger the acidity is, the more hydrogen protons it can provide.
In nitric acid and acetic anhydride nitration system, nitronium
3.4. Effect of reaction temperature on nitration of chlorobenzene
The reaction time was controlled in 2.5 h, and other reaction
conditions were the same as before, to examine the impact of the
reaction temperature on nitration of chlorobenzene. The results are
shown in Table 3.
It can be seen from Table 3, nitration of chlorobenzene is more
sensitive to temperature. The nitration is an endothermic reac-
tion, and below 40 ◦C the nitrification is not easy to carry out so
that the yield is low. With the reaction temperature increasing, the
+
NO2 dissociated from nitric acid attacked chlorobenzene, so as
to facilitate the formation of nitrochlorobenzene. The determina-
tion of acidity demonstrates that the acidity of [CP]HSO4 is the
strongest among the three ILs. [CP]HSO4 also can further provide
one proton as Brønsted acid to promote nitric acid to form active
agent nitronium ion favors to nitration reaction, so the yield is the
highest. Also the catalytic activity of [CP]pTSA is obviously higher

104
C. Zhang et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 101–105
Fig. 3. Absorption spectra of dimethyl yellow for ILs in methanol.
yield increases significantly. This is mainly because the increase
of temperature improves the solubility of chlorobenzene in acid
phase and decreases the viscosity and the surface tension between
two phases. Therefore, the diffusion effect will increase and be
conductive to mass transfer process. Elevating reaction tempera-
ture is also beneficial to the generation of para-nitrochlorobenzene.
As the nitration of chlorobenzene belongs to electrophilic substi-
tution reaction and the activation energy of para-nitrification is
minimum, while the ortho-product is controlled by kinetics and
the para-product is controlled by thermodynamics, so increasing
the temperature is helpful for the formation of product controlled
by thermodynamics [26]. When the temperature elevated and the
electron movement accelerated, with the frequency of collision
between the H+ and the reactant molecules increasing, the reac-
tion probably increases. But when the temperature rises to 70 ◦C,
nitric acid is easily decomposed to generate more NO2, dinitro-
byproducts increases, so that the yield declines and para/ortho
reduces. Therefore, 60 ◦C is chosen as the optimum reaction tem-
perature.
the electronegativity and nucleophilic ability of the para-position.
When the mole ratio of [CP]HSO4 catalyst to substrate increases to
2:10, the yield and para-selectivity of nitrification are beginning to
lower and significantly decrease with further increasing the cata-
lyst amount. This may be because the concentration of nitric acid is
diluted by a large number of ionic liquid, so that the activity and the
yield of nitration reduce. Excessive ionic liquid increases the solva-
tion effects, affects the polarization of the chlorobenzene substrate,
and weakens the nucleophilic ability of para-position, results in the
decline of para/ortho ratio. Therefore, the optimum mole ratio of
ionic liquid to chlorobenzene is 1.5:10.
3.6. Effect of ionic liquid reused on nitration of chlorobenzene
After the reaction is finished, the ionic liquid was recovered by
vacuum distillation to remove the water and most of acetic acid,
dried at 70 ◦C under vacuum for 2 h. Then it continued to be used
for chlorobenzene nitration as the catalyst under the same condi-
tions to investigate the effect of ionic liquid reused on the nitration
of chlorobenzene. The results are shown in Fig. 4. As can be seen
from Fig. 4, the yield and para/ortho ratio of nitrochlorobenzene
decrease a little, and [CP]HSO4 still exhibits good catalytic activ-
ity and regioselectivity after it is reused for five cycles. It not only
indicates the excellent recycling performance of the ionic liquid
[CP]HSO4, but also indicates that the structure of the ionic liquid
[CP]HSO4 is stable after regeneration process. The yield of nitra-
tion slightly decreases with increasing of the recycling times, which
may be ascribed to the accumulation of byproducts in the recovery
process for ionic liquids.
3.5. Effect of amount of ionic liquid on nitration of chlorobenzene
The reaction temperature was 60 ◦C, reaction time was 2.5 h,
other reaction conditions remained the same as above, to inves-
tigate the effect of the amount of ionic liquid on nitration of
chlorobenzene. The results are shown in Table 4.
The data in Table 4 shows that the yield of nitrochloroben-
zene increases with the increase of the amount of ionic liquid, and
the para-selectivity is significantly improved. When the mole ratio
of [CP]HSO4 ionic liquid to chlorobenzene substrate is 1.5:10, the
yield and para-selectivity are the best. The [CP]HSO4 catalyst shows
the good catalytic activity and para-selectivity, that is, para/ortho
isomer mass ratio is greatly increasing in comparison to 2.0 of
para/ortho ratio obtained by nitric-sulfuric mixed acids as catalyst.
The increasing yield with the amount of catalyst can be attributed
to more proton acid provided by more Brønsted acidic ionic liq-
uids, while para-selectivity of the nitration reaction was improved
with the increasing amount of catalyst, which may be ascribed
to a greater electron spin density on 4-C of benzene ring under
the ionic environment offered by the catalyst [24]. Thereby it is in
favor of the polarization of chlorobenzene substrate and improving
Table 3
Effect of reaction temperature on nitration of chlorobenzene.
Temperature (◦C)
Isomerdistribution(%)
Para/ortho
Yield (%)
Ortho
Para
30
40
50
60
70
21.05
14.54
12.32
12.19
20.2
75.48
83.03
85.48
86.91
79.80
3.59
5.71
6.94
7.13
3.95
32.30
46.28
56.30
65.36
45.28
Fig. 4. Effect of ionic liquid reused on nitration of chlorobenzene.

C. Zhang et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 101–105
105
Table 4
Effect of amount of ionic liquid on nitration of chlorobenzene.
Chlorobenzene:[CP]HSO4(mol:mol)
Isomer distribution (%)
Ortho
Para/ortho
Yield (%)
Para
10:1
10:1.5
10:2
14.90
11.27
12.19
14.54
81.50
87.20
86.91
83.03
5.46
7.74
7.13
5.71
57.84
71.22
65.36
56.30
10:2.5
Fig. 5. The pathway and mechanism for the nitration of chlorobenzene.
3.7. The pathway and mechanism for the nitration of
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