Regioselective nitration of aromatic compounds in an aqueous sodium dodecylsulfate and nitric acid medium

Article abstract of DOI:10.1039/c3ra40185e

An aqueous solution of sodium dodecylsulfate in the presence of dilute nitric acid at room temperature works as a mild medium for the nitration of aromatic compounds with high regioselectivity.

Full text of DOI:10.1039/c3ra40185e

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Response to the Reviewers’ comments
Referee: 5
Comment 1:
The authors describe a method offering a mild entry into the class of nitroaromatics
building on a rather similar literature precedence published a few years ago (ref #16), albeit the
current work is conducted in an entirely waterꢀbased medium instead of in an organic solvent
(acetonitrile). An adequate question to ask concerns the regioselectivity of this protocol and the
authors claim they cannot identify any other isomer than the main product. This obviously
depends on how intense and how deep one is looking and not finding the minor isomer in the
final product is by no means a guarantee that it might not be formed during the reaction. Thus, a
very important issue to address is whether the crude process solution, i.e. the reaction mixture
after completed reaction but before start of work up, contains anything else but the documented
product? As the yield is hovering <90%, it could be suspected that some starting materal as well
as small amounts of other regioisomers might be present. This has to be clarified!
Response:
According to our typical procedure (page 11), we did the HPLC of the crude products
before start of work up. From the HPLC traces we could detect only one isomer. The reported
yield is the isolated yield which has been indicated in Table 1 as a footnote. Since no trace of
starting material or other isomeric product was found in the HPLC trace we concluded
regioselectivity of this protocol. Since we report isolated yield, there is a likelihood of some loss
of product during work up (extraction and recrystallization). We hope that this clarifies the point
raised by the reviewer.
Comment 2:
Another key aspect of this transformation is to establish whether it is conducted in an
entirely homogenous phase or rather displays heterogeneity in some form (pseudoꢀ
heterogenous). The latter seems quite likely given the composition of the system and, therefore,
an investigation on the effects of stirring speed should render some valuable insights into these
aspects. Currently, the experimental procedure just makes the simply statement that "the mixture
was stirred", which poses questions like what type of stirrer, what stirring speed (rpm), presence
of any device (e.g. typ of baffle) that could break the laminar flow pattern and ensure better
mixing behaviour (vertical movement and not only horizontal flow). Hence, this lack of
understanding calls for a series of experiments where the reaction outcome is studied as a
function of the mixing regime in the flask!
Response:
Micellar solutions are not macroscopically heterogeneous. However, micellar solutions are
considered to be microscopically heterogeneous due to the presence of micelles which are
typically of about 2 to 3 nm radius.
We used a magnetic stirrer for stirring (this is now mentioned in the typical procedure, p.
11). We repeated the reactions by changing the speed of stirring, but found no change in the
reaction rate. Stirring only helped in the initial solubilization of the substrate in the aqueous
micellar solution.

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Comment 3:
On p. 4, it is unclear why the same reference (#14) is mentioned twice in the same sentence.
Response:
This has been corrected now.
Comment 4:
Throughout the paper, change table 1 to Table 1
Response:
As suggested ‘table 1’ is now written as ‘Table 1’.
Comment 5:
On p. 10, final sentence before acknowledgements ꢀ what does the statement "...the agreement
was better." mean? In what way were spectral data and melting points better than the ones
documented in the literature?
Response:
We have now corrected the sentence by deleting “the agreement was better”.
Comment 6:
As the actual identity of the nitration species is highly unclear, it can be seen as misleading if, as
is done in Figure 1 (p. 22), this species nitronium ion) is highlighted as the reactive component
formed under the current condition and acting as the reaction partner with the aromatic substrate.
The authors should either continue their investigations into the true nature of the nitrating species
or present their assumption of the reaction mechanism in a less ambiguous way.
Response:
To ascertain the formation of nitronium ion in the reaction mixture, we recorded the Raman
spectra. Accordingly, on pages 8 and 9 of the revised manuscript we added the following text:
Formation of nitronium ion through hydrolysis of the SDS monomers
may be shown as
+
‾
H
‾
C H SO + H O → C H OH + HSO
12
25
4
2
12 25
4
‾
+
=
HSO ⇌ H + SO
4
4
+
+
HNO + H ⇌ NO + H O
3
2
2
To confirm the generation of nitronium ion, we recorded the Raman
spectra of aqueous 0.05 M SDS and 0.05 M SDS + 1.0 M HNO solutions.
3
The Raman spectrum of the latter after subtracting the contribution from the
aqueous 0.05 M SDS solution is shown in Fig. 1A. The spectrum showed an
ꢀ1
intense peak at 1047 cm , which can be attributed to nitrate and bisulfate
3
2ꢀ36
(
produced from the hydrolysis of SDS) ions as both these ions are reported
ꢀ1
to show Raman line in the range of 1040 – 1050 cm . In view of the

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2ꢀ34
ꢀ1
reported
1
Raman line of nitronium ion at 1400 cm , the Raman line at
418 cm in Fig. 1B is indicative of the presence of nitronium ion in the
solution.
ꢀ
1
3
2
000
500
B
A
1
1
5000
0000
5
000
0
2000
5
00
1000
1500
1200
1400
1600
-
1
Wave number /cm
Fig. 1. Raman spectra of (A) aqueous 0.05 M SDS + 1.0 M HNO solution
3
after subtracting the contribution from aqueous 0.05 M SDS solution and (B)
ꢀ1
the enlarged view of the spectra in the range from 1175 to 1750 cm .
In the experimental section on page 11 we have given the details of Raman spectral
measurement.
The earlier Fig. 1 which was a schematic and presumptive representation of the presence of
nitronium ion in the reaction mixture has now been replaced by the new Fig. 1 which shows the
Raman spectra as shown above.
We thank the Referee and hope that our response and clarifications would satisfy the Referee.

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Regioselective nitration of aromatic compounds in an aqueous sodium
dodecylsulfate and nitric acid medium†
Jahar Dey, Mithu Saha, Amarta Kumar Pal* and Kochi Ismail*
Department of Chemistry, NorthꢀEastern Hill University, Shillong – 793022,
India
Address for Manuscript Correspondence: Prof. K. Ismail or Dr. A. K. Pal,
Department of Chemistry, NorthꢀEastern Hill University, NEHU Campus,
Shillong ꢀ 793022, India. Eꢀmail: kismail@nehu.ac.in; kinehu@hotmail.com or
akpal@nehu.ac.in; amartya_pal22@yahoo.com
†
Electronic supplementary information available: Scanned NMR and HPLC
spectra

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Abstract
An aqueous solution of sodium dodecylsulfate in the presence of dilute nitric acid
at room temperature works as a mild medium for the nitration of aromatic
compounds with high regioselectivity.

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Introduction
Nitration of aromatic compounds is a fundamental reaction of great industrial
importance and the nitro aromatic compounds are key organic intermediates in
various important functional group transformations. A mixture of concentrated or
fuming nitric acid with sulfuric acid has been used as the most common nitrating
reagent for the nitration of benzene, alkyl benzene and less reactive aromatic
1
compounds, but highly reactive aromatic compounds such as phenols and
anilines get oxidized in the nitric acid and sulfuric acid mixture rather than
undergoing nitration. A wide variety of nitrating reagents have been developed
2
3
4
5
including the use of Fe (III) or Cu (II), montmorillonite clay, zeolites, bismuth
6
7
8
9
(
III) salts, group IV and VI metal salts, lanthanide (III) salts, indium salts and
so on. Selective nitration of aromatic compounds has been reported with solid
1
0
acid catalysts as well. Nitration of aromatic compounds such as phenol and
1
1
salicylic acid has also been carried out photochemically. Most recently, even
1
2ꢀ13
ionic liquids (ILs) have been employed as nitrating systems.
However,
majority of the reported nitration methods suffer from disadvantages such as
highly acidic media, excess nitration (diꢀ or triꢀnitro products instead of monoꢀ
nitration), tedious workꢀup, and problems associated with use of organic solvents.
Another notable disadvantage of the conventional methods of nitration is the
1
2
formation of more than one isomer, which is a problem even in case of ILs.
These disadvantages associated with the known nitration methods have led
researchers to develop from time to time newer methods by using better nitrating
mixtures.

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The use of surfactants for nitration reactions of aromatic compounds has
gathered a lot of attention over the years. Surfactant molecules bearing both
hydrophilicity and hydrophobicity characters aggregate to form microstructures
known as micelles. The surfactant concentration at which microstructures begin to
form in a solvent is referred to as the critical micelle concentration (cmc). Chhatre
1
4
et al. carried out selective nitration of phenol to orthoꢀnitro phenol in nitric
acid/sodium dioctylsulfosuccinate (AOT)/benzene microemulsion medium. The
concentration of nitric acid used in this microemulsion varied from 5 to 10 M (M
ꢀ3
=
mol dm ). Selective nitration of phenols and anisole has also been reported in
1
5
nitric acid (8M)/cationic surfactant/isooctane microemulsions. The cationic
surfactants used are dodecyltrimethylammonium bromide and
15
16
didodecyldimethylammonium bromide. Sana et al. reported nitration of a
series of aromatic compounds in acetonitrile medium using HNO (1 mM) in the
3
presence of anionic, cationic or nonionic surfactants. Surfactants have also been
used as phase transfer catalysts during nitration of aromatics in twoꢀphase system
1
7
by Wang et al. resulting in high regioselectivity. These methods of nitration
involving surfactants also suffer from the disadvantage of using organic solvents.
Sodium dodecyl sulfate (SDS), an anionic surfactant, is known to undergo
1
8
19
hydrolysis. In a recent study on the aggregation behavior of SDS in aqueous
nitric acid, we detected from conductivity data hydrolysis of the surfactant when
the concentration of HNO was about 0.02 M or more. Visualizing that hydrolysis
3
19
of SDS monomers might facilitate formation of nitronium ions, it occurred to us
that an aqueous solution of SDS + HNO (with concentration > 0.02 M) may act
3

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as a nitrating mixture. Accordingly, we report in this paper a simple methodology
for the nitration of aromatic compounds that offers a mild route to the synthesis of
monoꢀsubstituted products in aqueous medium at room temperature. The nitrated
products have been successfully isolated using a novel method.
Results and discussion
Nitration of a number of aromatic compounds listed in Table 1 was carried
out in aqueous medium. In the nitration reactions, the concentrations of HNO and
3
SDS were kept as 1.0 M and 0.05 M, respectively. The concentration of SDS was
chosen such that it is well above the cmc (0.008 M) of SDS. Micelles formed
above cmc are known to solubilize organic compounds and this has enabled us to
carry out all the nitration reactions in water (Scheme 1).
SDS (0.05 M) + HNO (1.0 M)
3
O N
2
Aqueous medium, r.t.
R
R
1
aꢀi
2aꢀi
R = OH, NH COOH, CH , Cl, COCH
3
2
,
3
Scheme 1
The nitrating mixture was first tested for nitrating phenol and we obtained
only paraꢀisomer in excellent yield (Table 1), thus exhibiting high selectivity.
Formation of only the paraꢀisomer was confirmed by HPLC. Presence of phenol
2
0
at the micellar interface without penetrating into the core has been reported in
the case of CTAB (cetyltrimethylammonium bromide), a cationic surfactant, on

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1
the basis of HꢀNMR. Similarly, in the present nitrating mixture probably the
presence of phenol at the micellar interface and a favorable orientation of phenol
are responsible for the paraꢀselectivity.
Another interesting highlight of the present study is the direct nitration of
aniline without protecting the amino group to yield 4ꢀnitroaniline. Similar
nitration of aniline without any protecting agent has been recently reported by
1
3
16
Zolfigol et al. using an ionic liquid and by Sana et al. using surfactants and
1
3,16
dilute HNO (1 mM), but in both the reports
reactions have been carried out in
3
organic solvents.
The present protocol in aqueous medium can be successfully used to carry
out nitration of aromatic compounds even in the presence of strong electron
withdrawing groups such as the nitro, carboxylic acid, keto, etc. The nitration of
pꢀnitrophenol afforded 2,4ꢀdinitrophenol in good yield. Here, again the alignment
of pꢀnitrophenol at the SDS micelle can be compared to that of pꢀcresol at the TXꢀ
2
1
1
00 micelles, where the –OH group is pointed towards the bulk phase. Metaꢀ
directing aromatic compounds such as benzoic acid, acetophenone furnished the
expected products with good yields (Table 1). All the compounds were confirmed
from the analytical and spectroscopic data.

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a
Table 1. Nitration of aromatic compounds
Sl.
No.
Substrate
Product
Time Yield Melting Point (°C)
(min) (%)
b
Found Reported
c
1
20
89 111ꢀ113
114
HO
HO
NO2
NO2
c
c
2
3
20
30
86 147ꢀ149
85 104ꢀ106
148
113
H N
H N
2
2
O N
2
HO
NO2
HO
NO2
NO2
H C
H C
3
4
3
20
87 170ꢀ172
ꢀ
HO
HO
CH3
CH3
c
5
6
7
8
9
NO2
30
30
20
30
30
85 140ꢀ142
82 173ꢀ175
141
ꢀ
HOOC
HOOC
O N
2
HO
COOH
HO
COOH
c
c
c
86
85
83
49ꢀ51
74ꢀ76
85ꢀ87
54
81
83
H3C
NO2
H C
3
NO2
H COC
3
H COC
3
Cl
Cl
NO
2
a
Reagents and condition: Aqueous SDS (0.05 M), Nitric acid (1.0 M);
b
c
Isolated yields; Ref. 22
During the isolation of the nitrated products by the usual techniques, certain
level of SDS as impurity was always detected. Therefore, we employed a novel
technique to isolate the nitrated products. After the reaction, the temperature of
the reaction mixture was maintained below the Krafft temperature of SDS (15 °C

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2
3
for 0.05 M SDS), which resulted in the precipitation of SDS. After removing the
SDS precipitate by simple filtration, we could extract pure nitrated product from
the filtrate with a suitable solvent followed by evaporation and crystallization.
There are reports wherein nitronium ion is considered as the active species
even in dilute nitric acid, but in all such reactions the media used are not
1
6,24,25
aqueous.
Even in calcium nitrate tetrahydrate melt (a roomꢀtemperature
molten salt known as hydrate melt) nitration of benzene was reported to take
2
6
place and nitronium ion was shown to be the nitrating species. Can nitronium
ion be responsible for nitration in the present aqueous micellar medium which is
not highly acidic? In an anionic micellar solution, polarity and acidity at the
2
7ꢀ31
micelle ꢀ solution interface are entirely different from that in the bulk.
In
aqueous solution containing SDS micelles, the pH is about 2 units less than that in
2
7ꢀ31
28,31
the bulk
and the dielectric constant at the micellar interface is reported
to
be about 60 or even less. These characteristics of the micellar interface seem to
indicate that in aqueous medium containing SDS micelles and dilute nitric acid
the interfacial region around the micelles may be conducive to the formation of
nitronium ions. This might be the reason why nitration takes place in the presence
of micelles even if the concentration of nitric acid is low. Formation of nitronium
ion through hydrolysis of the SDS monomers may be shown as
+
‾
H
‾
C H SO + H O → C H OH + HSO
12
25
4
2
12 25
4
‾
+
=
HSO ⇌ H + SO
4
4
+
+
HNO + H ⇌ NO + H O
3
2
2

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To confirm the generation of nitronium ion, we recorded the Raman spectra
of aqueous 0.05 M SDS and 0.05 M SDS + 1.0 M HNO solutions. The Raman
3
spectrum of the latter after subtracting the contribution from the aqueous 0.05 M
SDS solution is shown in Fig. 1A. The spectrum showed an intense peak at 1047
ꢀ1
cm , which can be attributed to nitrate and bisulfate (produced from the
3
2ꢀ36
hydrolysis of SDS) ions as both these ions are reported
to show Raman line in
ꢀ1
32ꢀ34
the range of 1040 – 1050 cm . In view of the reported
Raman line of
ꢀ1
ꢀ1
nitronium ion at 1400 cm , the Raman line at 1418 cm in Fig. 1B is indicative of
the presence of nitronium ion in the solution.
A
3000
B
1
1
5000
0000
2
500
5
000
0
2000
5
00
1000
1500
1200
1400
1600
ꢀ
1
Wave number /cm
Fig. 1. Raman spectra of (A) aqueous 0.05 M SDS + 1.0 M HNO solution after
3
subtracting the contribution from aqueous 0.05 M SDS solution and (B) the
ꢀ1
enlarged view of the spectra in the range from 1175 to 1750 cm .

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An interesting aspect in the present nitrating medium is that the reaction
appears to be one in which the rate is independent of the aromatic substrate.
Therefore, the rate appears to be zeroꢀorder with respect to the aryl compound
implying that the formation of nitronium ion is the rateꢀdetermining step.
Conclusions
In conclusion, a simple, mild and efficient synthesis of aromatic nitroꢀ compounds
has been reported in aqueous medium. A wide range of aromatic compounds was
tested, bearing either electronꢀdonating or electronꢀwithdrawing substituents
suggesting that this method can be used to synthesize a library of aromatic nitro
compounds. The use of SDS (0.05 M) and dilute HNO (1.0 M) as nitrating
3
mixture in aqueous medium at room temperature makes this protocol
environmental friendly. Moreover, high regioselectivity of the products with very
easy separation technique renders this methodology very attractive. The method
demonstrated for the removal of surfactant from the reaction medium can in
general be applied in the cases of micellar catalyzed reactions and also in
nanoparticle (synthesized in micellar medium) catalyzed organic reactions.
Experimental section
Materials
SDS (Fluka, ≥ 99 % assay), HNO (Merck, 69%), were used as received. All
3
solutions were prepared in Millipore water. Stock solution of the acid (10.0 M)
was prepared after standardization of HNO . The organic compounds were
3
obtained either from Merck or from SD fine chemicals and were of AR grade.

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The solvents used for extraction were obtained from Merck. All reactions were
done at 25 °C.
Melting points were determined in open capillaries and are uncorrected. IR
spectra of the samples in KBr disks were recorded in Perkin Elmer (Model
1
Spectrum BXFTꢀIR) spectrometer. H NMR (400 MHz and 300 MHz) spectra
were recorded on Bruker Avance IIꢀ400 and 300 spectrometers in CDCl and
3
DMSOꢀd (chemical shifts in δ with TMS as internal standard), respectively.
6
Mass spectra were recorded on Waters ZQꢀ2695 and Bruker Daltonics (Esquire
3
2
000) spectrometers. Liquid chromatography analyses were performed in Waters
489 HPLC instrument with UV detector using C column (particle size: 5ꢁm,
18
column size: 150×3.9 mm). Raman spectra were recorded in the range of 200 –
ꢀ1
1
800 cm using the 532 nm line from a solid state Nd:YAG laser (Cobolt AB,
Sweden) and Synapse CCD detector based monochromator (Horiba Jobin Yvon,
France). The laser power used was 30 mW. The sample solution was taken in a
standard 1cm x 1cm cuvette Spectra were recorded at 25 °C.
Typical Procedure
A mixture of 1 mmol substituted aromatic compound 1aꢀi and 25 ml of SDS (0.05
M) solution was taken in a round bottom flask. To this mixture HNO was added
3
such that the concentration of HNO in the mixture was 1.0 M. The mixture was
3
then stirred at 25 °C for the time mentioned in the Table 1 by using a magnetic
stirrer. After completion (monitored by TLC), the reaction mixture was kept
below the Krafft temperature of SDS (15 °C) till SDS precipitated. The mixture

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was then filtered and the filtrate was extracted with diethyl ether (3 x 10 mL) and
dried over anhydrous sodium sulfate. After removing the solvent the product was
recrystallized from acetone.
The HPLC traces of the crude product were taken. The recrystallized
1
products were characterized by comparing their IR spectra, Hꢀ NMR spectra,
mass spectra and melting points (shown in Table 1) with those in the literature.
Acknowledgements
We thank the financial support received from the Department of Science and
Technology, New Delhi. JD acknowledges the RA fellowship received from the
CSIR, New Delhi. We are also thankful to Dr. S. Mahiuddin of NEIST, Jorhat,
India for helping us in obtaining the mass spectra and to Dr. Nandita Biswas of
Radiation and Photochemistry Division, BARC, Mumbai, India for recording the
Raman spectra.

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Table of contents
SDS (0.05 M) + HNO3 (1 M)
Aqueous medium, r.t.
O N
2
R
R
R = OH, NH2, COOH, CH , Cl, COCH3
3
Regioselective nitration of aromatic compounds is achieved in
aqueous solution of sodium dodecylsulfate and dilute nitric acid at
room temperature.

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Supplementary Information
Regioselective nitration of aromatic compounds in an aqueous sodium
dodecylsulfate and nitric acid medium
Jahar Dey, Mithu Saha, Amarta Kumar Pal* and Kochi Ismail*
Department of Chemistry, North-Eastern Hill University, Shillong, India.
E-mail:
kismail@nehu.ac.in;
kinehu@hotmail.com
or
akpal@nehu.ac.in;
amartya_pal22@yahoo.com

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1
1
. H-NMR Spectra:
1
Fig. S1: H-NMR spectrum of 4-nitrophenol

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Fig. S2: H-NMR spectrum of 4-nitroaniline

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Fig. S3: H-NMR spectrum of 2,4-dinitrophenol

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Fig. S4: H-NMR spectrum of 2,5-dimethyl-4-nitrophenol

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Fig. S5: H-NMR spectrum of 3-nitrobenzoic acid

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Fig. S6: H-NMR spectrum of 4-hydroxy-3-nitrobenzoic acid

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Fig. S7: H-NMR spectrum of 4-nitrotoluene

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Fig. S8: H-NMR spectrum of 3-nitroacetophenone

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Fig. S9: H-NMR spectrum of 4-chloro-nitrobenzene