Acetyl nitrate nitrations in [bmpy][N(Tf)2] and [bmpy][OTf], and the recycling of ionic liquids

Article abstract of DOI:10.1039/b417152g

In this work we have examined the nitration by acetyl nitrate of a range of activated and deactivated aromatic substrates in two ionic liquids and compared the results to the same reaction in dichloromethane. Both ionic liquids are stable to the reaction conditions, and in both ionic liquids the yields of reaction are higher after unit time than the same reactions in dichloromethane, although the regioselectivity is little affected by solvent choice. This result gives further support to the suggestion that in the ionic liquid, acetyl nitrate dissociates to give the nitronium ion, and that this is the effective nitrating agent here. However, it is shown that [bmpy][N(Tf)₂] is a better solvent for aromatic nitration than [bmpy][OTf]. This is due to the ease of formation of nitronium ion in the former ionic liquid, and is consistent with the fact that [bmpy][N(Tf)₂] is a weaker hydrogen bond acceptor solvent than [bmpy][OTf]. Finally, a method by which [bmpy][N(Tf)₂] may be recovered and reused for aromatic nitration has been demonstrated.

Full text of DOI:10.1039/b417152g

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A R T I C L E
Acetyl nitrate nitrations in [bmpy][N(Tf)2] and [bmpy][OTf], and the
recycling of ionic liquids†‡
Emilie Dal and N. Llewellyn Lancaster*
Department of Chemistry, King’s College London, Strand, London, WC2R 2LS, UK.
E-mail: llewellyn.lancaster@kcl.ac.uk
Received 11th November 2004, Accepted 10th December 2004
First published as an Advance Article on the web 24th January 2005
In this work we have examined the nitration by acetyl nitrate of a range of activated and deactivated aromatic
substrates in two ionic liquids and compared the results to the same reaction in dichloromethane. Both ionic liquids
are stable to the reaction conditions, and in both ionic liquids the yields of reaction are higher after unit time than the
same reactions in dichloromethane, although the regioselectivity is little affected by solvent choice. This result gives
further support to the suggestion that in the ionic liquid, acetyl nitrate dissociates to give the nitronium ion, and that
this is the effective nitrating agent here. However, it is shown that [bmpy][N(Tf)2] is a better solvent for aromatic
nitration than [bmpy][OTf]. This is due to the ease of formation of nitronium ion in the former ionic liquid, and is
consistent with the fact that [bmpy][N(Tf)2] is a weaker hydrogen bond acceptor solvent than [bmpy][OTf]. Finally, a
method by which [bmpy][N(Tf)2] may be recovered and reused for aromatic nitration has been demonstrated.
ionic liquids, or the role of the solvent. This is in contrast to
Introduction
the study of other electrophilic substitutions in ionic liquids,20
The use of ionic liquids as alternative reaction media for organic
which the results of this work can also be compared against.
chemistry is an active and important area of current research.2,3
Our first paper discussed the importance of cation choice
Ionic liquids are attractive because they have certain “green”
(showing that imidazolium-based cations were not stable to the
properties, including near zero vapour pressure. Additionally, it
reaction conditions) and demonstrated that an ionic liquid based
is possible, by choice of cation and anion, to tune the solvent
on [bmpy]+ (where bmpy = 1-butyl-1-methylpyrrolidinium) was
properties of the ionic liquid,4 for example. The potential to
suitable. The effect of the anion was ignored in that work,
design a solvent for a specific reaction (or reaction class) provides
a contrast to what is possible with molecular solvents where
choice is limited.
although we did speculate on the fate of acetyl nitrate generated
in situ.
Aromatic nitration is a reaction of synthetic and industrial
importance, despite the fact that it is a notoriously “ungreen”
Results and discussion
reaction. One particular problem is that conventional aromatic
nitration makes use of the mixed acid system, in which nitric acid
is used in conjunction with an excess of concentrated sulfuric
acid. For this reason, a range of alternative systems for aromatic
nitrations which do not require sulfuric acid have been developed
in recent years. These include the use of clays5,6 or zeolites7–9 as
solid supports; lanthanide10,11 and transition12,13 metal catalysts
or employing perfluorocarbons as solvents.14
We now report the preliminary findings of the effect of the
anion on aromatic nitrations by acetyl nitrate in ionic liquids
as illustrated in Scheme 1.
Nitration has also long been of academic interest, having
been used as a tool to increase our understanding of organic
reaction mechanisms, particularly electrophilic substitution.15
The consequence of this is that the mechanisms of aromatic
nitrations are well known, and it is possible to make a compar-
ison between systems. The twin aims of this work are to study
aromatic nitrations as models of electrophilic substitution in
ionic liquids (and to use this information to describe electrophilic
substitutions in ionic liquids in general) whilst seeking an
alternative, greener methodology for aromatic nitrations.
Aromatic nitrations in ionic liquids have already received
some interest in recent years. The first published work was
by Laali,16 using a variety of conventional nitrating agents.
Complementary studies have since been made by us,1 Earle17 and
Qiao.18 During this period, Srinivasan and Rajagopal showed
that iron nitrate can be used to nitrate phenols in ionic liquids.19
Despite these numerous studies, there have been only limited
attempts to determine the mechanism of the reactions in the
Scheme 1 Acetyl nitrate nitrations in ionic liquids.
The nitrations were studied in two different ionic liquids,
[bmpy][OTf] and [bmpy][N(Tf)2], Fig. 1.§ These ionic liquids
were chosen because they can be prepared to a high standard
of purity, are stable to the reaction conditions and have known
(and different) solvent properties. For example, the Kamlet–
Taft properties a, b and p* have been determined for these
solvents.21,22 It should be noted that the reactions are subtly
different in this work than in the first paper. In the current work
we used a larger volume of ionic liquid and less reagent, such
that the ionic liquid is certainly acting as a solvent (and not
as an additive) for the nitrating agent. This has had the effect
of reducing yields after unit time (especially for the deactivated
substrates), reflecting the reduced concentration of the nitrating
agent.
† Aromatic nitrations in ionic liquids. Part 2.1
§ In this work our intention has been to study anion effects. It has already
been established that [bmpy]+ is a suitable cation for these reactions.1
There is a limited choice of anions that, when combined with [bmpy]+,
give room temperature ionic liquids which can be prepared such that
they are free of organic and inorganic impurities.
‡ Electronic supplementary information (ESI) available: kinetic data
for the nitration of toluene in dichloromethane and the nitration of
chlorobenzene in [bmpy][N(Tf)2] at 25 ◦C. See http://www.rsc.org/
suppdata/ob/b4/b417152g/
6 8 2
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 6 8 2 – 6 8 6
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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The nitronium ion thus generated would be a more potent
nitrating agent than acetyl nitrate, and therefore explains the
nitration of the deactivated aromatics in the ionic liquids.
The difference in yields in the two ionic liquids can also
be explained. The effect of the cation and anion on solvent
properties is now beginning to be understood. In this discussion
we will focus on the hydrogen bonding properties of the ionic
liquids, as determined by use of Kamlet–Taft parameters.21 It
has been shown that the hydrogen bond donor property of an
ionic liquid is largely affected by the cation (with some anion
effect), whilst the hydrogen bond acceptor property is affected
by the anion. Taking the two ionic liquids used in this work,
because they are both based on the [bmpy]+ cation they have
similar a values. The ability of such cations to act as H-bond
donors (through the hydrogen atoms on carbon atoms adjacent
to a quatenary nitrogen) has already been demonstrated.22
However, these [bmpy]+ ionic liquids have different b values;
0.252 and 0.461 for the [N(Tf)2]− and [OTf]− ionic liquids
respectively.23 These b values reveal that the triflate ionic liquid
is the stronger H-bond acceptor solvent. This also means that
the triflate anion will be more strongly coordinated to the ionic
liquid cation (as revealed by the lower a value of this ionic
liquid compared to the [N(Tf)2]− version). This gives rise to
two complementary effects. The first is that [bmpy]+ is a more
effective H-bond donor when coupled with the [N(Tf)2]− anion,
and so will be better able to coordinate (and cleave) acetate from
acetyl nitrate. The second is that because [N(Tf)2]− is the weaker
base of the two anions used, it will coordinate the nitronium
ion less strongly than [OTf]− will, thus leaving the nitronium ion
more avilable to react in [bmpy][N(Tf)2] than in [bmpy][OTf].
These processes are illustrated in Scheme 2, and are consistent
with the data reported in Table 1.
Fig. 1 Cations and anions used to prepare the ionic liquids used.
Effect of ionic liquid anions
The nitration of a range of activated and deactivated aromatic
substrates was studied in [bmpy][OTf] and [bmpy][N(Tf)2]
and a comparison was made with the same reaction in
dichloromethane. The results are shown in Table 1.
There are a number of comparisons that can be made.
The first is between the ionic liquids and dichloromethane.
Although dichloromethane is a good solvent in which to conduct
nitrations, only the activated substrates were nitrated. When
deactivated substrates were employed, no reaction was observed.
Additionally it can be noted that the yields of the nitroaromatics
after 1 hour were lower in dichloromethane than under the
same conditions in either ionic liquid. The regioselectivity of
nitration (both of toluene and of anisole) was not much affected
by the ionic liquid, and was comparable to that observed in
dichloromethane.
As shown in earlier work, although acetyl nitrate is ineffective
in the nitration of the deactivated haloaromatics under the
conditions employed in dichloromethane, the reactions proceed
in ionic liquids to give the nitroaromatic. The limit in nitration
seems to be reached somewhere between 2-nitrotoluene and
nitrobenzene. The former reacts slowly in [bmpy][N(Tf)2] over
24 hours, and gives a low yield in [bmpy][OTf] over the same
time period. Nitrobenzene barely reacts over 24 hours in either
solvent.
This overview misses the subtler points about the nitration
reactions in the ionic liquids. Whilst it is true that there is little
between the yields and regioselectivities where the substrates
are activated, the deactivated substrates reveal differences. It is
striking that the nitration of halobenzenes in [bmpy][N(Tf)2]
reaches ca. 60% yield in 1 hour, whilst in [bmpy][OTf], the yield
is less than 10%. Again, in the nitration of 2-nitrotoluene, the
yield is an order of magnitude higher in [bmpy][N(Tf)2] than
in [bmpy][OTf]. This suggests that the nitrating agent is more
“reactive” in the former ionic liquid than in the latter.
We can extend the discussion further. In our earlier work, we
proposed that, although acetyl nitrate is generally regarded as
a molecular nitrating agent, the solvent environment might give
rise to dissocation of this molecule and subsequent formation of
the nitronium ion, as illustrated in eqn. (1) below.
Scheme 2
Additionally we examined the nitration of the same substrates
in [bmpy][N(Tf)2] using nitronium tetrafluoroborate under anal-
ogous conditions. Using this reagent, the effective nitrating agent
is necessarily the nitronium ion. It was found that nitronium
tetrafluoroborate is poorly soluble in this ionic liquid, but that
as the reaction proceeds the suspension clears as the solid is
consumed. The yields of nitroaromatics using the activated sub-
strates were low, but this is not surprising given that the reactions
were being conducted at 25 ◦C. Extensive decomposition of
the aromatic (though not of the ionic liquid) was observed in
these cases. However, the nitration of the deactivated substrates
by nitronium tetrafluoroborate gave comparable yields and
regioselectivities to those observed when using acetyl nitrate.
AcONO2 ꢀ [AcO]− + [NO2]+
(1)
Table 1 Nitration of aromatic substrates by acetyl nitrate in dichloromethane, [bmpy][N(Tf)2] and [bmpy][OTf] and a comparison with nitration by
nitronium tetrafluoroborate in [bmpy][N(Tf)2]. Yields after 1 h at 25 ◦Ca
Ac2O/HNO3
CH2Cl2
[NO2][BF4]
[bmpy][N(Tf)2]
Yield (%)
[bmpy][N(Tf)2]
Yield (%)
[bmpy][OTf]
Yield (%)
Substrate
Yield (%)
Ratio o/p
Ratio o/p
Ratio o/p
Ratio o/p
Toluene
Mesitylene
Anisole
Chlorobenzene
Bromobenzene
2-Nitrotoluene
Nitrobenzene
61
45
52
0
1.5
—
3
—
—
—
—
70
89
66
1.6
—
1.8
0.29
0.33
0.56d
m only
66
80
64
8
1.7
—
2.5
57b
28b
24b
54
1.4
—
1.1
0.20
0.37
0.56d
m only
64
0.17
0.28
0
59
7
57
0c
—
30c
<1c
3c
—
38c
1c
e
<1c
m only
a All reactions in ionic liquids were heterogeneous, except where chlorobenzene and bromobenzene were used as substrates. b Decomposition observed.
c 24 h. d 2,6-Dinitrotoluene/2,4-dinitrotoluene. e 2,4-Dinitrotoluene only.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 6 8 2 – 6 8 6
6 8 3

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These results support our earlier suggestion that, in ionic liquids,
acetyl nitrate gives rise to nitronium ion, and that this is the
effective nitrating agent in this system.
liquid. In dichloromethane we studied nitration under analogous
conditions, but used toluene as substrate (chlorobenzene being
unreactive in this solvent). It was assumed that the reaction
would be first order in [substrate] in both solvents. Where the
substrate is particularly activated, a zeroth order dependence on
[substrate]0 can be observed, but the reaction of toluene with
AcONO2 normally shows first order dependence on [toluene].24
It is known that, depending on the reaction conditions, nitra-
tions by acetyl nitrate in acetic anhydride show a dependence
on [HNO3]0 which is between 2nd and 3rd order. The order
depends on [HNO3]0, with lower concentration leading to 3rd
order and higher concentration leading to 2nd order dependence
on [HNO3]0.24
In this work we found that, in both the ionic liquid and in
dichloromethane, the order of reaction with respect to [HNO3]
was between 2 and 3. This observation is in accord with nitration
by nitric acid in acetic anhydride, even if the reactions are not
necessarily by acetyl nitrate in both solvents studied in this work.
Attempts were made to establish the effect of [Ac2O]0 con-
centration on the rate law, and determine the order of reaction
with respect to this reagent, but these were inconclusive, save
to suggest that the reaction is of high order with respect to the
concentration of acetic anhydride in both solvents.
The effect of the anion has been investigated by choosing two
which show very different hydrogen bond acceptor properties. It
has been demonstrated that the ionic liquid which is the poorest
H-bond acceptor ([N(Tf)2]−) is the best suited for aromatic
nitrations because it does not coordinate the electrophile as
strongly as the triflate anion can. In a reaction such as this,
where the rate determining step is normally the formation
of the Wheland intermediate (Scheme 3), the solvent which
binds the electrophile least strongly will give the highest yield
after unit time. Given that b is largely anion dependent, even
if other common anions had been available to us, it is worth
noting that [BF4]− is a stronger H-bond acceptor, whilst [PF6]−
is comparable, if slightly weaker, and that only [SbF6]− is
a significantly weaker H-bond acceptor.21 Given that [PF6]−
readily evolves F− (as HF) and that [SbF6]− might be expected
to behave similarly, these alternatives are not attractive.
Scheme 3
Recycling the ionic liquid
Whilst we have not completely excluded the possibility that
acetyl nitrate is capable of acting as a molecular nitrating agent
in the ionic liquid, the evidence to date from our studies does
not suggest that acetyl nitrate acts as a molecular nitrating agent
in ionic liquids.
It is instructive to compare the nature of the intermediate
of this reaction with the transition state formed in electrophilic
bromination by [Br3]− in ionic liquids (Fig. 2).20 In the bromi-
nation reactions, the [Br3]− ion dissociates to form bromine
and bromide. Bromine then attacks the double bond and this
complex is then attacked by Br−. Therefore, in the bromination
reaction, as one bromide---cation bond is broken, so another is
formed. This suggests that the identity of the ionic liquid will
have very little effect on this part of the reaction (since each
cation---bromide adduct will have identical bond strength).
Before ionic liquids are adopted as solvents for the production of
nitroaromatics, a regime by which the solvent may be recovered
and re-used must be developed. A possible means of achieving
this is described here. For ease of handling, the reactions
were performed on a larger scale, starting with 4 cm3 of ionic
liquid. However, the reaction was carried out using the same
proportions of reagent, substrate and solvent as in the other
reactions. Chlorobenzene was chosen as the substrate because
it was anticipated that the reaction would not go to completion
within the reaction time, allowing us to observe whether the
yield after unit time (which gives an idea of the rate, although
is not equal to it) was affected. The ionic liquid chosen was
[bmpy][N(Tf)2], which is immiscible in water.
Recycling of the ionic liquid was achieved by dissolving the
post-reaction mixture into dichloromethane and adding water.
It was then possible to extract any unreacted nitric acid, plus
the acetic acid generated by reaction with water. After removing
the dichloromethane, the organics were removed from the ionic
liquid by steam distillation, extracted from the water with
dichloromethane and rotary evaporated to remove the solvent
and substrate. This gave the product as a mixture of the 2- and
4- isomers of chloronitrobenzene; the ratio was determined by
GC. Meanwhile, the ionic liquid was recovered by extracting
with dichloromethane and then heating in vacuo. This process is
illustrated in Fig. 3. A sample of the ionic liquid was withdrawn
Fig. 2 Transition state of electrophilic bromination by [Br3]−.
This contrasts with nitration, where the rate-determining step
is attack of the substrate by the nitronium ion. The identity
of the ionic liquid will affect the rate of this step because the
interactions (this time of the anion with either the nitronium ion
or with the Wheland intermediate) will not be of equal strength.
So far we have only been able to consider the effect of the ionic
liquid on its interactions with the nitronium ion. We can only
speculate about any interactions between the ionic liquid and
the Wheland intermediate.
1
after each run, and before it was reused it was analysed by H
NMR spectroscopy. This revealed that there was no difference
between the recovered ionic liquid and the original.
Kinetic studies of aromatic nitrations
Kinetic studies of the aromatic nitrations were attempted in
[bmpy][N(Tf)2] and in dichloromethane. An excess of substrate
relative to acetyl nitrate was used in the hope of obtaining pseudo
first order kinetics. However, the reactions did not obey a simple
model and it proved impossible to obtain a satisfactory fit of the
data. For this reason, it was decided to make a study of the initial
rates of reaction. In this work, the effect of both nitric acid con-
centration and acetic anhydride concentration was investigated.
Chlorobenzene is completely soluble in [bmpy][N(Tf)2] and
reacts slowly enough to be conveniently sampled and quantified.
Thus we chose this as a model substrate for nitration in the ionic
Fig. 3 Schematic representation of the ionic liquid recycling process.
The results of the nitrations are shown in Table 2, as are data
to show the amount of ionic liquid recovered after each reaction
(after removal of the sample for NMR analysis).
6 8 4
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 6 8 2 – 6 8 6

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and hexadecane. At a known time the substrate (1 cm3) was
added and the mixture stirred for 1 h before sampling.
Table 2 Nitration of chlorobenzene and recycling of [bmpy][N(Tf)2]
ionic liquid
Run
Yield (%)
Ratio o/p
Mass of ionic liquid recovereda/g
Nitration by nitronium tetrafluoroborate
1
2
3
4
5
76
70
71
76
74
0.26
0.24
0.25
0.24
0.24
5.23
5.15
5.07
4.83
4.57
A portion of nitronium tetrafluoroborate (0.08 g, 0.6 mmol)
was added to a tared Schlenk flask in a dry nitrogen atmosphere
glovebox. The sealed flask was removed from the glovebox and
to it was added [bmpy][N(Tf)2] (1 cm3) and hexadecane. At a
known time the substrate (1 cm3) was added and the mixture
stirred for 1 h before sampling.
a Initial mass of ionic liquid 5.45 g (ca. 4 cm3).
Recycling
It is clear that there is one major problem with the recy-
cling of the ionic liquid as described above; the reliance on
dichloromethane does not make this process appear green at
first glance. It should be stated that the role of dichloromethane
in this work was to enable the determination of both product
and recovery of the ionic liquid. In an industrial setting, it would
be quite possible to perform a steam distillation of the entire re-
action mixture. The organics will form a separate phase from the
aqueous acids distilled and could probably be removed by simple
decantation. And because this ionic liquid ([bmpy][N(Tf)2]) is
immiscible in water, it should be possible to decant any water
from the ionic liquid and re-use it immediately. Thus recycling
the ionic liquid does not require the use of dichloromethane.
Table 2 does show a steady loss of ionic liquid after each run.
There was no degradation of the ionic liquid during the reaction,
as evidenced by the NMR spectra of the ionic liquid after each
run and the fact that there was no discolouration. Therefore it
is reasonable to propose that these losses are purely mechanical,
arising from the washing procedure employed.
The reactions were conducted by a method analogous to that
described for the acetyl nitrate nitrations, but using four times
the quantities for ease of handling.
After 1 h, the reaction mixture was added to water (100 cm3)
and rinsed in with dichloromethane (4 × 5 cm3). The ionic
liquid and organics were removed, and the aqueous phase was
extracted with dichloromethane (4 × 20 cm3). The combined
dichloromethane phase was combined and the solvent removed
in vacuo. The organics were removed from the ionic liquid by
steam distillation, isolated by extraction with dichloromethane
(4 × 10 cm3) from the aqueous phase and evaporated to constant
mass, yielding a mixture of chloro-2-nitrobenzenes and chloro-
4-nitrobenzenes. The ratio was determined by GC-MS.
Likewise, the ionic liquid was extracted from the aqueous
phase using dichloromethane (4 × 10 cm3) and heated under
1
high vacuum to constant mass. The H NMR spectrum of the
recovered ionic liquid was identical to that of the original, with
no evidence of impurities.
The recovery of the ionic liquid and the yields of each reaction
are documented in Table 2.
Conclusions
This work has clearly shown that there is potential for ionic
liquids to be used to achieve nitrations of even quite deactivated
compounds at ambient temperatures using only a stoichiometric
amount of nitric acid and without the use of excess (or indeed
any) sulfuric acid.
Sampling
Except in the recycling study (where the product was isolated) the
reactions were monitored by taking samples at a known time,
which were then quenched into saturated sodium bicarbonate
solution and extracted with dichloromethane. Analysis was
by GC-MS using hexadecane as an internal standard. Reten-
tion times were determined by using authentic samples of the
nitroaromatics. Note that where the reaction mixture was not
homogeneous, then the entire reaction mixture was quenched
and extracted as described above.
Acetyl nitrate is a more potent nitrating agent in ionic
liquids than in molecular solvents. This is because acetyl nitrate
dissociates to form nitronium acetate in the ionic liquid and it is
therefore the nitronium ion which is the effective nitrating agent
in this system. The importance of knowing the mechanism of a
given reaction in order to inform our decision of which solvent
to use has also been demonstrated. In examples of electrophilic
substitutions where the rate-determining step is the addition
of the electrophile, the solvent of choice should be a weaker
hydrogen bond acceptor. The study of the kinetics of nitrations
in ionic liquids reveals that the dependence of rate upon nitric
acid concentration is similar to that observed when the same
reactions are conducted in acetic anhydride. Further study of the
kinetics will be made and will form another part of this study.
Finally, it has been shown that the ionic liquids can be recycled
and reused for aromatic nitrations. The losses of ionic liquid were
mainly mechanical (including withdrawal of a sample for nmr
from each run). These can be overcome by a combination of
working on a larger scale and of not removing the ionic liquid
from the reaction vessel between each run.
Acknowledgements
Thanks go to Mr Andy Cakebread and Mr Roger Tye (King’s
College London) for performing GC-MS analyses.
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Experimental
The ionic liquids were prepared and purified as described
elsewhere.25 Dichloromethane was distilled prior to use. All
of the nitrations were conducted at 25 ◦C under a nitrogen
atmosphere using standard Schlenk techniques. All substrates
and reagents purchased were used as received.
Nitration by acetyl nitrate
To the ionic liquid (1 cm3) was added a known mass of nitric acid
(70% w/w, ca. 0.03 cm3, 0.6 mmol), acetic anhydride (0.25 cm3)
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 6 8 2 – 6 8 6
6 8 5

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6 8 6
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 6 8 2 – 6 8 6