The effect of an electron-withdrawing group in the imidazolium cation: The case of nitro-functionalized imidazolium salts as acidic catalysts for the acetylation of glycerol

Article abstract of DOI:10.1039/c8nj02520g

The acetylation of glycerol was achieved with high conversion and selectivity towards triacetin at low temperatures and short reaction times by using acidic imidazolium salts as catalysts. Moreover, the addition of a nitro group to the imidazolium cation affords a much more competent catalyst, indicating a significant effect provided by the simple electronic change in the imidazolium cation. Theoretical calculations revealed increased polarization of the acidic hydrogen bond on the nitrated salts, which may be related to their superior catalytic behavior when compared to the non-functionalized salts. Combining the preliminary experimental and theoretical results, it is possible to suppose that the catalytic activity of acidic imidazolium salts may be better comprehended by its Br?nsted acidities, but other parameters such as hardness, electronegativity, electrophilicity and ion-pair binding energy were also evaluated in order to investigate their effects in the acetylation of glycerol promoted by these acidic imidazolium salts.

Full text of DOI:10.1039/c8nj02520g

View Article Online
View Journal
NJC
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: E. M. Morais, I. B.
Grillo, H. K. Stassen, M. Seferin and J. D. Scholten, New J. Chem., 2018, DOI: 10.1039/C8NJ02520G.
Volume 40 Number
1
January 2016 Pages 1–846
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
NJC
New Journal of Chemistry
www.rsc.org/njc
A journal for new directions in chemistry
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

Page 1 of 10
Pl Ne ae swe Jd oo u nr no at l ao df j Cu sh te mm ai sr tgr iyn s
View Article Online
DOI: 10.1039/C8NJ02520G
Journal Name
ARTICLE
The effect of an electron-withdrawing group in the imidazolium
cation: the case of nitro-functionalized imidazolium salts as acidic
catalysts for the acetylation of glycerol
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
c
a
DOI: 10.1039/x0xx00000x
Eduardo M. Morais, Igor B. Grillo, Hubert K. Stassen, Marcus Seferin and Jackson D. Scholten*
www.rsc.org/
The acetylation of glycerol was achieved with high conversion and selectivity towards triacetin at low temperatures and
short reaction times by using acidic imidazolium salts as catalysts. Moreover, the addition of a nitro group to the
imidazolium cation affords a much more competent catalyst, indicating a significant effect provided by the simple
electronic change in the imidazolium cation. Theoretical calculations revealed increased polarization of the acidic
hydrogen bond on the nitrated salts, which may be related to their superior catalytic behavior when compared to the non-
functionalized salts. Combining the preliminary experimental and theoretical results, it is possible to suppose that the
catalytic activity of acidic imidazolium salts may be better comprehended by its Brønsted acidities, but other parameters
such as hardness, electronegativity, electrophilicity and ion-pair binding energy were also evaluated in order to investigate
their effects
in the acetylation of glycerol promoted
by
these acidic
imidazolium salts.
production of triacetin in high conversions and selectivity at low
temperatures, reaction times, cost and good recyclability. Several
attempts to accomplish these goals have focused on increasing the
Introduction
The global production of biodiesel has been steadily increasing in
the past few years, coming along with the production of the process
by-product glycerol. One of the options to deal with the increasing
amounts of glycerol is adding value to glycerol by acetylation to
produce triacetin, which can be used as an additive for biodiesel,
number of Brønsted acidic sites in the ILs by adding sulfonic acid
-
groups to the side chains of the cations or/and by using HSO
4
as an
anion, since the unmodified imidazolium cation is not significantly
6
acidic. This has been the main strategy for obtaining Brønsted
7
acidic ILs since the early 2000s. However, these modifications are
1
, 2
gasoline, plastics and foods.
Traditional methods for the
expensive and synthetically slightly complicated.
industrial production of triacetin usually require long reaction times
and high temperatures; therefore, catalysts must be used to
achieve a less energy demanding process.
In this work, we propose a simple and inexpensive approach to
increase the natural acidity of the imidazolium cation by nitrating
the imidazolium ring, which produces potentially more acidic protic
imidazolium salts (the nitro-functionalized compounds are not
technically ILs since their melting points are located above 100 °C,
however, many of their properties are similar). The main goal here
is to evaluate how the electronic change produced in the
imidazolium ring by adding a nitro-functional group can influence
the acidity/activity of the catalytic systems. We expect this new
class of nitrated imidazolium salts to potentially increase the variety
of simple and inexpensive Brønsted acidic compounds. The activity
of five catalysts in the acetylation of glycerol (Scheme 1) were
compared under similar conditions and the conversion of glycerol
and selectivities towards different products were determined by
Many heterogeneous catalysts have been tested with promising
1
results for high glycerol conversion and easy separation from the
products, but high costs and inactivation are common issues.
Traditional homogeneous catalysts such as sulfuric acid and p-
toluenesulfonic acid monohydrate (TsOH.H O), which can be used
2
to achieve high conversion of glycerol at low cost, present a
diversity of problems, such as corrosion of equipment, effluent
2
contamination and recyclability of catalyst. In this context, ionic
liquids (ILs) have been used as alternative homogeneous catalysts
that can be easily recycled without excessive deactivation after
3
-5
many runs, since the structure of these compounds can be
tailored to have increased catalytic activity under specific reaction
conditions (task specific ionic liquids, TSILs). Therefore, it is
expected that these TSILs can be used as catalysts for the
13
C-qNMR using a similar method as already described in the
8
literature. Computational studies were also conducted to
determine the effect of the nitro group, as well as the change in
anions, on a variety of molecular descriptors, which were correlated
with the changes in catalytic activity. In order to have a broader
understanding of these effects, additional calculations were
5
performed for four ILs already described in the literature.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 2 of 10
View Article Online
DOI: 10.1039/C8NJ02520G
ARTICLE
Journal Name
small amount of reaction. This low conversion has already been
9
reported. Full conversion of glycerol with high selectivity towards
triacetin can be achieved without catalyst at high temperatures
1
0
(
120 °C) and long reaction times (120 min). In reactions with
acetic acid (AcOH) at high temperatures (110 °C) and low reaction
times (20 min), conversion of 40% and negligible selectivity towards
1
1
triacetin has been reported; therefore, the conversion and
selectivity would be even lower at lower temperatures. We were
not able to quantify these blank reactions with our method, since it
requires samples from a homogeneous reaction mixture in order to
provide an accurate representation of the concentration of the
different compounds in the mixture. In Table 1, we summarize the
conversion and selectivity for the glycerol acetylation using the
different catalysts. By comparing the activities of the catalysts, we
observed that the nitro group added to the C5 position of the
imidazolium cation increases considerably the activity of the
imidazolium salts for acetylation. Moreover, the tosylate anion also
positively affects the activity of the catalyst (Table 1). Indeed, the
best results among the imidazolium salts were achieved for the salt
Scheme 1 Acetylation of glycerol with different acidic catalysts.
[
2
NO Hmim][TsO] (entry 19, Table 1), which can be used in a 1 mol%
relation to glycerol to achieve 100% conversion to triacetin at 30 °C
within 10 minutes using AA (molar ratio of 1:4 with glycerol). In
addition, 94% of conversion of glycerol in acetins was also achieved
using AcOH (ratio of 1:8) at 120 °C for 10 minutes with 1 mol% of
Results and discussion
Acetylation of glycerol
The acetylation of glycerol with acetic anhydride (AA) using no catalyst (entry 18, Table 1). Conversions as high as 98% and
catalyst, at low temperatures (30 °C) and low reaction times (10 selectivities of 29% to triacetin can be achieved using AcOH under
min), is negligible. This can be easily observed, since under these different conditions (entry 16, Table 1).
conditions, glycerol and AA form two phases, and the volume of
glycerol remains apparently constant, indicating that occurs only a
a
Table 1 Acetylation of glycerol under different conditions
Selectivity (%)
Mass of
Catalyst (g)
0.0953
Time
(min)
10
Acetylating Glycerol:acetylating Conversion
Entry
1
Catalyst
T (°C)
1-M 2-M 1,2-D 1,3-D
T
agent
AA
agent
1:4
1:8
1:8
1:4
1:8
1:4
1:8
1:4
1:8
1:4
1:8
1:4
1:8
1:4
1:8
1:8
1:8
1:8
1:4
1:4
(%)
100
89
TsOH.H
TsOH.H
TsOH.H
TsOH.H
TsOH.H
2
O
2
O
2
O
2
O
2
O
60
60
0
51
18
0
0
6
2
0
4
5
0
0
0
0
0
0
9
0
2
3
5
8
0
0
0
11
16
0
0
29
37
0
100
3
b
2
0.0953
10
AcOH
AcOH
AA
b
3
0.0953
120
60
10
98
27
100
10
6
c
4
0.0100
10
100
97
b
c
5
0.0100
120
60
10
AcOH
AA
30
56
0
17
9
39
24
0
b
6
[Hmim]Cl
[Hmim]Cl
0.1500
0.1500
0.1500
0.1500
0.1500
0.1500
10
76
7
60
10
AcOH
AA
0
0
0
b
8
[Hmim][TsO]
[Hmim][TsO]
60
10
100
0
0
3
24
0
73
0
9
60
10
AcOH
AA
0
0
b
1
0
[NO
[NO
2
Hmim]Cl
Hmim]Cl
60
10
100
0
0
3
20
0
77
0
1
1
2
2
60
10
AcOH
AA
0
0
1
[NO
[NO
[NO
[NO
[NO
[NO
[NO
[NO
[NO
2
2
2
2
2
2
2
2
2
Hmim][TsO] 0.1500
Hmim][TsO] 0.1500
Hmim][TsO] 0.1500
Hmim][TsO] 0.1500
Hmim][TsO] 0.1500
60
10
100
61
0
0
0
100
0
b
1
3
60
10
AcOH
AA
76
0
3
13
0
1
1
1
1
4
5
6
7
30
10
100
98
0
100
26
29
4
120
120
120
120
30
10
AcOH
AcOH
AcOH
AcOH
AA
19
16
46
40
0
17
16
15
14
0
36
35
30
33
0
120
10
98
c
Hmim][TsO] 0.0158
Hmim][TsO] 0.0158
Hmim][TsO] 0.0158
Hmim][TsO] 0.0158
92
b
c
c
c
1
8
10
94
5
1
9
0
10
100
100
100
100
2
60
10
AA
0
0
0
a
13
b
c
Conversion and selectivities determined by C-qNMR; 10 to 15 minutes of vacuum at 70 °C to eliminate the acetylating agent; 1% mol catalyst/mol glycerol.
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 10
Pl Ne ae swe Jd oo u nr no at l ao df j Cu sh te mm ai sr tgr iyn s
View Article Online
DOI: 10.1039/C8NJ02520G
Journal Name
ARTICLE
Noteworthy, in cases without high conversions and selectivities number of Brønsted acidic sites is not necessarily the only option to
towards triacetin, most of the acetylating agent was separated from increase the activity of imidazolium salts.
the final reaction mixture using a Kugelrohr distillation apparatus in Computational studies
order to prevent the acetylation of glycerol and acetins inside the
NMR tube. Entries 17 and 18 show that the distillation of the the nitro-functionalized imidazolium salt might be attributed to the
acetylating agent at the end of the reaction may have a small effect
on the selectivities, but we do not consider that to be of great imidazolium ring, increasing the acidity of the cationic species. In
relevance. Another consideration is that equal mass amounts of
catalysts were used because these compounds may serve as performed comparing the molecular descriptors of the non-
Our preliminary evaluation suggests that the increased activity of
electronic effect of the nitro group on the N-H bond of the
order to evaluate this effect, theoretical calculations were
solvents as well as catalysts, so we attempted to compare them
under similar conditions. Evidently, it would be more desirable to
compare them in equal molar amounts as well, even though it is
quite clear that there is a considerable difference in activities.
Analyzing the entries 6, 8, 10 and 12 in Table 1 we noticed that by
changing the Cl anion by the tosylate anion the conversion of
functionalized and functionalized imidazolium salts. Conceptual
Density Functional Theory reactional descriptors were calculated at
the B3LYP-D3 and TPSS-D3 levels of theory using the basis sets 6-
3
1+G* and 6-311++G(3df,3dp), respectively. The same calculations
5
were performed using four ILs already described in the literature in
order to verify if the same hypothesis formulated to explain the
glycerol and the selectivities towards triacetin increased activity of these protic imidazolium salts can be used to explain the
significantly. The effect of the nitro group can be seen clearly when
behavior of more commonly used sulfonic acid functionalized ILs.
comparing entries 7, 9, 11 and 13. Moreover, it is worth to note
2
that the catalyst [NO Hmim][TsO] is present in the smallest molar
Electronic effect of the nitro group. As stated before, the electron
withdrawing effect of the nitro group is likely the main responsible
for increasing the acidity, and therefore the catalytic activity of the
nitrated salts. This can be confirmed when comparing the maps of
the molecular electrostatic potential of [Hmim][TsO] and
concentration when compared to the others, but it is still the most
active.
To be able to comprehend the capabilities of [NO
activity of this imidazolium salt was compared with a very common
and effective acid catalyst, TsOH.H O. Surprisingly, the results
observed in entries 1 and 12 (Table 1) show that TsOH.H O and
NO Hmim][TsO] provide the same conversion and selectivities
when AA was used as acetylating agent. In the case of AcOH, entries
and 13 show that [NO Hmim][TsO] is not as active as TsOH.H
2
Hmim][TsO], the
2
[
2
NO Hmim][TsO] (Fig. 1).
2
[
2
2
2
2
O
under these conditions, but the preferred selectivity for the mono-
acetylated compound for the reaction in the imidazolium salt is also
an interesting result. To demonstrate the true potential of our class
of nitro-functionalized imidazolium salts, we started decreasing the
reaction temperatures and the catalyst amounts. In fact, the
reaction using AA in the presence of a minor amount of catalyst
yields total conversion and selectivity to triacetin even at 30 °C
within 10 minutes (entry 19, Table 1). However, to achieve high
conversion with AcOH, higher reaction temperatures are required;
therefore, entries 15 and 16 show that increasing the temperature
to 120 °C can increase the conversion of glycerol drastically,
whereas increasing the reaction time does not have the same
effect. These results indicate that the reaction approaches
equilibrium in very short times, close to 10 minutes. Decreasing the
amount of the catalyst to 1 mol% in relation to glycerol produces a
significant decrease in selectivity towards triacetin, but the
conversion of glycerol continues excellent and the 1-M product was
formed preferably (entry 17, Table 1). If we compare this result with
Fig. 1 Molecular electrostatic potential maps at the B3LYP-D3/6-31+G* level
of theory for [Hmim][TsO] (left) and [NO Hmim][TsO] (right). Red color:
negative potential; blue color: positive potential. Isosurface value of
.016803 and grid from -0.09 to 0.09.
2
entry 3, it is possible to note that [NO
remarkable activity, approaching the one observed for the
TsOH.H O catalyst. This can be considered as an outstanding result,
2
Hmim][TsO] has a
0
2
taking into consideration the comparison of an imidazolium salt
with a strong organic acid. In comparison to a previous work, our
results evidenced the considerable effect provided by the nitro
group in the imidazolium cation. Indeed, the acetylation of AcOH
It is clear that the nitro group increases the charge polarization
around the acidic hydrogen, which may increase its acidity. This
polarization is evident due the presence of two electron
6
withdrawing regions in the molecule, the SO
and the NO group on the cation. This effect appears to be quite
strong, since at the B3LYP-D3/6-31+G* level of theory the acidic
hydrogen is transferred to the SO group of the tosylate anion.
Similar results have been found for the [NO Hmim]Cl. For a non-
3
group on the anion
using the salt [NO
than an analogous protic imidazolium salt [Hmim][HSO
to a double SO H-functionalized IL [(HSO -p) im][HSO
4
2
Hmim][TsO] afforded much superior conversions
] and similar
], which is
2
4
3
3
2
3
one of the most active systems based on acidic ILs reported to date.
Therefore, we propose that a simple change in the electronic
properties of the imidazolium cation by adding an electron-
withdrawing group produces comparable results to those systems
using several Brønsted acidic groups attached to the structure of
imidazolium salts. These results indicate that simply increasing the
2
nitrated salt, this effect was only observed in the case of [Hmim]Cl
at the TPSS-D3/6-311++G(3df,3dp) level of theory. For all other
non-nitrated salts, in both levels of theory, including the sulfonic
acid functionalized ILs, this effect was not observed. This indicates
that the acidic hydrogen in the nitrated salts is more weakly bonded
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 4 of 10
View Article Online
DOI: 10.1039/C8NJ02520G
ARTICLE
Journal Name
to the cation, which means that it is more available to be hydrogen becomes so weakly bonded to the imidazolium ring that it
transferred to another species, acting as a more effective acid is behaving more likely the free hydrochloric acid or tosylic acid,
catalyst when compared to non-nitrated salts.
and not as a salt. Therefore, we can hypothesize that the farther
away the hydrogen is from the cation, and the closer it is to the
Bond lengths and charges. The effect of the nitro group on the anion, the more acidic it may be, and then more catalytically active
acidity can also be observed in the bond lengths of the acidic in the context of acid catalysis. Since our sample of compounds is
hydrogens and the charge densities around the acid group. We too small (only four imidazolium salts), we thought that it would be
expected to find a correlation between increased bond lengths of interesting to apply our hypothesis in a different set of catalysts.
the acidic hydrogens and the increased activity of the catalysts. This Hence, we decided to perform all calculations for the present
increase in bond length could indicate that these bonds are imidazolium salts and other four ILs previously reported in
becoming weaker, which relates to more acidic hydrogens when literature (Fig. 3), which were used for the same task under similar
5
compared with those exhibiting shorter bond lengths. And as can conditions.
3
, 6, 12
be observed in other works in the literature,
high Brønsted acidity (characterized by their low values of Hammett
acidity (H )) are usually more active and capable of achieving higher
catalysts with
0
selectivities of triacetin.
Doubtless, these bond lengths can only be compared between
similar compounds, since we should not expect to observe this
correlation between Brønsted acidity and acidic hydrogen bond
length in compounds that have completely different acidic groups. Fig. 3 Sulfonic acid-functionalized ILs reported in literature.
In the case of different acid groups, the differences in catalytic
5
activity cannot be explained by using only bond lengths as an These ILs were used as acidic catalysts for the acetylation of glycerol
indication of acidity.
at different conditions (80-120 °C, 6 h, 1:8 glycerol:acetic acid molar
By analyzing the bond lengths of the acidic hydrogens (N-H bond ratio, ~14% mol catalyst in relation to glycerol and 10 mL of
5
and Anion-H bond) of our catalysts (Fig. 2), it is clear that the cyclohexane). The reported results indicated that the ILs containing
-
-
change in anion and the addition of the nitro group drastically the anions HSO
4
and TsO exhibited better activities at 80 °C and
-
influence the bond lengths. First, by comparing the lengths in the 120 °C, respectively, whereas BF
had the worst performance
4
-
non-nitrated compounds we can observe that the N-H bond shrinks followed by H
2 4
PO at 80 and 120 °C. Even though the conditions
(
from 1.21 to 1.10 Å) with the substitution of the chloride by the were different, we believe that the same hypothesis used to explain
tosylate anion, which would indicate lower acidity. However, with the behavior of our catalysts can be applied to these compounds
the tosylate anion, the hydrogen is much more closely bonded to and possibly many others, since their activity seems to be
the anion than in the case of the chloride anion, so that effect of correlated to their acidity. A similar trend found in Fig. 2 regarding
getting closer to the anion may positively affect its acidity. Other the lengthening of the O-H (N-H in the case of Fig. 2) bond and
effects that will be discussed later can also account for this shortening of the anion-H bond can be observed in Fig. 4. The
difference in activity, since bond lengths cannot be used as a single intensity of the changes in bond lengths from one IL to the other is
indicative of activity and acidity.
considerably smaller than the ones in Fig. 2, since the only change
in the structure is the anion, which does not have such a strong
effect as the change in the structure of the cation.
Fig. 2 Bond lengths for the different imidazolium salts calculated in the
B3LYP-D3/6-31+G* level of theory. Increasing order of activity from the left Fig. 4 Bond lengths for the reported ILs calculated in the B3LYP-D3/6-31+G*
to the right.
level of theory. Increasing order of activity from the left to the right.
The electron withdrawing effect of the nitro group is very drastic,
since the acidic hydrogen is transferred from the cation to the
anion, as it can be observed in Fig. 1. This explains the lengthening
of the N-H bond when comparing nitrated to non-nitrated salts
Atomic charges. Atomic charges derived from the molecular
electrostatic potential, such as CHELPG charges, are numerical
descriptions for the changes observed in the potential maps in Fig.
13
(
from 1.21 in [Hmim]Cl to 1.84 Å in [NO
2
Hmim]Cl). One way of
1
. These charges can be easily calculated, at low computational
looking at this phenomenon, is that by nitrating these salts, the
cost, and are generally accepted as being more reliable than the
4
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 10
Pl Ne ae swe Jd oo u nr no at l ao df j Cu sh te mm ai sr tgr iyn s
View Article Online
DOI: 10.1039/C8NJ02520G
Journal Name
ARTICLE
1
4
conventional Mulliken charges for these compounds.
Our Fig. 6 CHELPG charges (in units of e) for the reported ILs calculated in the
B3LYP-D3/6-31+G* level of theory.
expectation is that the increase in the polarization of charges
around the acidic hydrogen can increase its acidity and,
consequently, its catalytic activity, since more polarized bonds are
likely more acidic. Observing these atomic charges in Fig. 5, we can
verify that [Hmim]Cl has the smallest charge in the acidic hydrogen
and the biggest charge in the N atom; therefore, its ꢀ between
charges (difference in absolute values between the charges in the
acidic hydrogen and the nitrogen to which it is bonded) is the
smallest (0.199). This ꢀ value increases with the substitution of the
chloride by the tosylate anion, and increases even more when
adding the nitro group to the cation.
The effect that the anion exerts on the polarization of the O-H bond
becomes apparent when visualizing the molecular structure of
these ILs. In all cases, the anions are interacting closely with the
SO
Fig. 7). The anion appears to be polarizing the O-H bond, in a
similar manner to the nitro group on [NO Hmim][TsO], slightly
pulling electronic density away from the acidic hydrogen. All anions
3
H and the C2-H bond, which are sites with high positive charges
(
2
-
-
4
have this effect to some extent, but HSO and TsO appear to have
the strongest effect (largest value for ꢀ between charges).
Fig. 7 Top perspective of the molecular electrostatic potential map for
3 4
[HSO pmim][HSO ] (left) and its structure (right). Red color: negative
potential; blue color: positive potential. Isosurface value of 0.016803 and
grid from -0.09 to 0.09.
Evaluation of electronic descriptors. One may ask if in fact the
Brønsted acidity is the only property to be considered in these
cases. The well-known classical mechanism for acetylation reactions
involves an initial proton transfer from the catalyst to the substrate
and subsequent nucleophilic attack of the substrate by glycerol.
Doubtless, the proton transfer path is fundamental for the
Fig. 5 CHELPG charges (in units of e) for the different salts calculated in the
B3LYP-D3/6-31+G* level of theory.
This trend of increasing the ꢀ between charges is even larger if we
consider the effect of transference of hydrogen (N-H group) from
the nitrated cations to the anions. Since the acidic hydrogens are
more closely bonded to the anions in the nitrated compound, it is
occurrence of the reaction, but if the pK
a
value is the only important
aspect, it would be expected that the reactions employing strong
6
possible to calculate the ꢀ values using the charges in the Cl and O acids (H SO , TsOH, this work) should exhibit much better results
2
4
(
0
[
from the SO
.34 to 0.49 for [NO
NO Hmim][TsO]. Therefore, this transference of hydrogen caused the nitro-functionalized imidazolium salts, which include the
by the NO group increases even more the polarization of the bond concepts of Lewis and Pearson. Analyzing the energy differences
on the acidic hydrogen.
between the frontiers orbitals (HOMO and LUMO) of the involved
Fig. 6 depicts the same trend of increased charge polarization species, it is possible to define important properties, such as
observed in the previous reported SO
3
group in the tosylate). The values of ꢀ varies from than the nitro-imidazolium salts. In this context, we have tried to
2
Hmim]Cl and from 0.67 to 0.91 for explore additional explanations for the observed catalytic activity of
2
2
3
H-functionalized ILs from Fig. hardness (ñ), electronegativity (ꢀ) and electrophilicity ʚăʛ; the first
15-17
3
. The anions appear to have an effect in the charges of the acidic two can be used to determine electron flow tendencies (ꢀN)
group SO
3
H in the cation, although, it is not as accentuated as the between the reactive species. Single point energies can also be used
to calculate ion-pair binding energies (IPBE), which can quantify the
change in polarization induced by the nitro group.
1
8
strength of the interactions between cation and anion.
Hardness, electronegativity and electrophilicity. According to
1
5
Pearson,
compounds with a large energy gap between
HOMO/LUMO orbitals are hard, whereas those with a small energy
gap are soft. Observing the HOMO/LUMO gap values and the
hardness of the compounds (Table 2), it is probable that hard-hard
interactions prevail in these cases. Therefore, this is a case of
19
charge-controlled reactions. This may be the reason why these
kind of reactions are so well explained by charge distribution, in
contrast to soft-soft reactions that need to be analyzed using
frontier orbital densities. This may be an important factor if one
plans to use these kind of catalysts for other applications, since they
may not perform as well as expected for reactions involving soft
compounds. In our study, we could not find convincing evidence
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 6 of 10
View Article Online
DOI: 10.1039/C8NJ02520G
ARTICLE
Journal Name
that small changes in hardness impact the overall performance of acetylation of glycerol, since they do not correlate with activity, but
the catalysts, a broader sample of catalysts would be necessary to larger changes, such as those from the non-nitrated salts [Hmim]Cl
ascertain the importance of hardness values.
and [Hmim][TsO] to the nitrated ones, may be relevant (changes of
more than 1-1.5 eV). At this point, we cannot confirm if the increase
in electrophilicity is responsible for the increase in activity because
more data points would be required to evaluate the role of Lewis
acidity in the mechanism of this acid catalysis.
Table 2 Molecular descriptors for all compounds discussed in this work.
Calculated at B3LYP-D3/6-31+G* level of theory
Electronegativity ꢀ
Electrophilicity ă
Hardness ñ
Compound
Glycerol
(eV)
(eV)
(eV)
Electronic flow between reactive species. Another information that
the descriptors from Table 2 can provide are the ꢀN values
between reactive species. These ꢀN values are derived from
electronegativity and hardness of both acids and bases reacting, in
our case, the catalysts act as acids and the acetylating agents act as
bases. The traditional Fischer esterification mechanism involves the
transference of hydrogen from the acid catalyst to the carbonyl
group on the acetylating agent in order to increase the
electrophilicity of the carbon on the carbonyl group, making it
easier for the glycerol to perform a nucleophilic attack. We can also
analyze this mechanism using the Lewis acidity model. By this point
of view, there is an electron transfer from the oxygen of the
carbonyl of the acetylating agent to the catalyst. As we stated
before, in order to have electronic flow from the acetylating agent
to the catalyst, the catalysts need higher electronegativity values
when compared to the acetylating agent. ꢀN values (Table 3) can
provide information about the direction of the electronic flow and
the scale of the transference.
3.78945
4.01805
4.4637
4.4769
3.52505
3.63325
5.3437
4.92005
4.983
1.9836
2.0994
2.9712
3.3581
2.5981
2.6093
5.6502
5.4647
3.9105
3.6197
3.8451
3.3530
2.9842
2.3914
2.5296
2.5269
2.2149
3.1748
Acetic acid
Acetic anhydride
p-TSA
[
Hmim]Cl
Hmim][TsO]
NO Hmim]Cl
[
[
2
[
2
NO Hmim][TsO]
[
3 4
HSO -pmim][BF ]
[
HSO
3
-
pmim][H
2 4
PO ]
4.2858
4.1124
3.3098
3.3227
2.7748
2.5449
[
3
HSO -pmim][TsO]
Table 3 Electronic flow tendencies (in units of e) for all catalysts discussed in
this work. Calculated at B3LYP-D3/6-31+G* level of theory
[
HSO
3
-
Compound
ΔN from AcOH
ΔN from AA
pmim][HSO
4
]
4.31345
3.3392
2.7860
p-TSA
0.0336
0.0010
[
Hmim]Cl
Hmim][TsO]
NO Hmim]Cl
NO Hmim][TsO]
-0.0395
-0.0302
0.1040
-0.0817
-0.0706
0.0748
At this point, we cannot discard the possibility that Lewis acidity
plays a role in the mechanism of acetylation of glycerol. For this
reason, we will also explore concepts regarding the Lewis acidity of
these compounds presented in Table 2. For compounds to behave
like Lewis acids, they need higher values of electronegativity when
[
[
2
[
2
0.0744
0.0410
1
5
compared to the bases they are reacting with. The bases in this
case are acetic acid and acetic anhydride, since they will transfer
electrons to the salts in order to initiate the catalytic cycle of
glycerol acetylation. By this assumption, [Hmim]Cl and [Hmim][TsO]
should have low catalytic activity, since their electronegativity is
lower than acetic acid and acetic anhydride. On the other hand,
[HSO -pmim][BF ]
3
4
0.0687
0.0398
[HSO
3
-pmim][H
2 4
PO
]
0.0202
-0.0145
-0.0298
-0.0122
[
HSO
3
-pmim][TsO]
0.0074
2 2
[NO Hmim]Cl and [NO Hmim][TsO] are predicted to act as acids,
[
3
HSO -pmim][HSO
4
]
0.0223
since their electronegativity is higher than acetic acid and acetic
anhydride. This estimative is well supported by our experimental
evidences, as showed in Table 1.
As it was discussed before, since we are dealing with hard-hard
interactions, the flow of electron is very limited, varying from 0.001
to 0.10 electrons. The most interesting observation we made is that
the non-nitrated salts [Hmim]Cl and [Hmim][TsO] have negative
values for both ꢀN (from AcOH and AA), which indicates that the
electronic flow tendencies are reversed. Hence, they are acting like
Lewis bases, and not like acids. The nitro-functionalized
imidazolium salts act like acids when reacting with AcOH and AA,
whereas the negative ꢀN values from AA observed for the reported
3
Considering the already reported SO H-functionalized ILs (Fig. 3),
we can confirm that all of them should behave as acids when
reacting with AcOH, but this trend could change in a reaction with
AA, since the only catalyst with electronegativity higher than it, is
3 4
[HSO -pmim][BF ]. Then, it is reasonable to suppose that the Lewis
acidity of the compounds is not involved in the reaction
mechanism, only their Brønsted acidity.
Electrophilicity can be considered as a thermodynamical descriptor
related to the strength of the Lewis acidic character of a molecule,
1
7
3 3 2 4 3
SO H-functionalized ILs [HSO -pmim][H PO ], [HSO -pmim][TsO]
where higher values indicate a stronger Lewis acid. It should not
be confused with experimental measures of electrophilicity, since
those are kinetic measurements. It is not clear whether small
changes in the value of this descriptor are relevant in the
and [HSO -pmim][HSO ] suggest a lower activity in the reaction with
3
4
3 4
AA when compared to [HSO -pmim][BF ]. Notably, it is difficult to
define how significant are these Lewis and Pearson acidities in
This journal is © The Royal Society of Chemistry 20xx
6
| J. Name., 2012, 00, 1-3
Please do not adjust margins

Page 7 of 10
Pl Ne ae swe Jdoo u nr no at l ao df j Cu sh te mm ai sr tgr iyn s
View Article Online
DOI: 10.1039/C8NJ02520G
Journal Name
ARTICLE
[
HSO
3
-pmim][TsO]
110.0844
43.7090
relation to the more broadly accepted Brønsted acidity in these acid
reactions.
[
HSO
3
-pmim][HSO ]
4
Ion pairing effects. The effect that the strength of interactions
between anion and cation of the imidazolium salts could have on
the catalyst activity is another aspect to be considered. In this
context, the ion-pair binding energies (IPBE) were calculated (Table
The intention with these particular theoretical studies is to highlight
the possible existence of other acid-base aspects in the acetylation
of glycerol promoted by imidazolium salts. Possibly the argument
that imidazolium salts are acting as simple proton donator is the
preferable manner to explain the activities in acidic reactions, but
the influence of other parameters may be investigated in order to
help the understanding of the catalytic behavior of these 3D
supramolecular organized compounds.
4). Low values indicate stronger interaction between the ions and
high values indicate weaker interactions. It is expected that ionic
pairs with strong interaction may trap the acidic hydrogens,
decreasing the Brønsted acidity of the salt. Therefore, high IPBE
values should be related to a superior acid activity. Comparing
[Hmim]Cl with [Hmim][TsO] it is apparent that the chloride anion is
more strongly bound to the anion, and this effect may reduce the
acidity of the chloride salts. This appears to be the case for nitrated
2 2
salts ([NO Hmim]Cl and [NO Hmim][TsO]) as well. The nitro group
also appears to have an effect on the IPBE; it increases the Conclusions
interaction between anions, resulting in a lower IPBE when
In summary, we have demonstrated the use of nitro-functionalized
compared to the non-nitrated salts. This indicates that the nitro
group should decrease the acidity of these compounds, but the
inverse is observed due to more relevant electronic effects.
Consequently, these results demonstrate the importance of not
analysing IPBE isolated from other descriptors in an attempt to
explain the acidity of these compounds. This descriptor, as most
others, needs to be considered in conjunction with other
descriptors to accurately characterize the acidity of these
compounds.
imidazolium salts as efficient catalysts for the acetylation of glycerol
under relatively mild conditions. The results demonstrate that the
introduction of an electron-withdrawing group in the imidazolium
cation affects the acidity/activity of the salt, which is sufficient to
provide excellent performances for the acetylation of glycerol.
Indeed, high conversions and selectivities were obtained in the
reaction employing AA or AcOH, approaching the results obtained
2
by a classical strong organic acid, TsOH.H O. Moreover, the reaction
could be carried out without the use of solvents, such as toluene
typically used for the azeotropic distillation of water. Our system is
a new approach to increase the Brønsted acidity of this class of
imidazolium salts, where the electronic properties can be tuned by
the simple inclusion of a functional group in the imidazolium ring.
The present method seems to be very promising due to the
preparation of a simpler and cheaper catalyst, when compared to
For the known SO
HSO -pmim][TsO] would be much more active than [HSO
pmim][HSO ], since there is a considerable difference in IPBE
between the two compounds. However, the IL [HSO -pmim][TsO] is
3
H-functionalized ILs, we should expect that the IL
[
3
3
-
4
3
the slightly more active catalyst above 120 °C (only 2% more
selectivity towards triacetin). Hence, the IPBE does not appear to be
capable of accounting for the differences in activity between the
more sophisticated and expensive SO H-based ILs. Notably, the
3
3
SO H-functionalized ILs. Although the coordination properties of
claim of this study is not to make a direct comparison of the
properties and activities of the nitro-functionalized imidazolium
salts with other acidic catalysts already described in the literature,
but open possibilities for the creation of new and improved acidic
the anions in the 3D structural organization of the imidazolium salt
are important aspects to be considered, the use of the IPBE concept
isolated from other descriptors may not be accurate to the reality
of these ionic compounds (the ions can easily separate in solution),
which makes the hypothesis that acidic hydrogens can be trapped
due to strong ionic interactions less likely. Therefore, more studies
need to be conducted to determine if there is a correlation
between IPBE and acidity. Moreover, other possible effect of the
tosylate anion, in contrast to the chloride, may be the stabilization
of the transition states by π-π stacking interaction with the
imidazolium cation.
3
imidazolium compounds that are not based on the usual SO H
functionalization, but instead, rely on increasing the natural acidity
of the N-H group. It is preferable that the Brønsted acidity is the
main factor to be considered in the development of these kind of
acidic catalysts, but more studies need to be conducted in order to
confirm the influence of other parameters such as hardness,
electrophilicity and electronegativity in the acid activity of
imidazolium salts. In fact, we have confidence that this work will
serve as a starting point to a more comprehensive study of the
properties that govern the activity of these imidazolium
compounds. We also expect that these findings can be useful for
the design of new acidic imidazolium salts/ionic liquids with specific
structural features for several acid-promoted applications.
Table
4 Ion-pair binding energies (IPBE). Calculated at TPSS-D3/6-
3
11++G(3df,3dp) level of theory and counterpoise corrected
Compound
Hmim]Cl
Hmim][TsO]
NO Hmim]Cl
IPBE (kcal/mol)
[
-43.4852
44.1177
-55.0230
33.5756
45.1772
44.0702
[
Experimental
[
2
General information
[
2
NO Hmim][TsO]
All reagents were purchased from various commercial suppliers and
were used without further purification, with the exception of 1-
[
HSO
3
-pmim][BF
4
]
1
13
C
methylimidazole that was purified by distillation. All H and
NMR spectra were acquired using a Varian MR-400 (400 MHz and
01 MHz, respectively), with the exception of the 1-methyl-4-
[
3 2 4
HSO -pmim][H PO ]
1
nitroimidazole spectra, which were acquired with a Bruker Avance
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 8 of 10
View Article Online
DOI: 10.1039/C8NJ02520G
ARTICLE
Journal Name
III HD 400 MHz (HMBC) and a Bruker Ascend 400 MHz (HSQC). All precipitating after being washed with ethyl ether several times. All
FTIR spectra were acquired using a Bruker Alpha with a Platinum salts were washed with ethyl ether several times and then dried up
ATR module. Melting points were acquired using a Buchi B-540 with by vacuum at 60 °C for several hours. The salts were stored under
sealed capillary glass tubes.
2 2
argon. Melting points for [NO Hmim]Cl and [NO Hmim][TsO] were
1
35.5-136.1 °C (decomposition occurred as well as melting) and
1
13
Synthesis of compounds
119.6-120.4 °C, respectively. H and C-NMR analyses of the
compounds are shown in SI (Fig. S3-S10). FTIR spectra of
[
NO
2 2
Hmim][TsO] and [NO Hmim]Cl are presented in Fig. S11 and
4
(5)-nitroimidazole. This method was based on a previously
2
0
S12, respectively. Most salts remained apparently unmodified after
weeks of storage, with the exception of [NO
from an off-white color to slightly green.
published work. Nitric acid (86 mL, 68%, 0.93 mol) was added
dropwise to imidazole (10 g, 0.15 mol) in a round-bottom flask with
a magnetic stir bar over an ice bath. After adding all the nitric acid,
the solution was stirred for a couple of minutes until it cooled down
2
Hmim]Cl that changed
to the ice bath temperature. Then, sulfuric acid (41 mL, 98%, 0.41 General procedure for the acetylation of glycerol. In a Schlenk flask
mol) was added in the same way as the nitric acid, the solution was with a magnetic stir bar were added glycerol (0.4878 g, 5.3 mmol),
then stirred over an ice bath for 30 min. The reaction mixture was catalyst and acetylating agent (2 mL, 21.2 mmol for acetic
then refluxed overnight and then allowed to cool down to room anhydride or 2.43 mL, 42.4 mmol for acetic acid). After the addition
temperature. The reaction mixture was submitted to an ultrasonic of the acetylating agent, the Schlenk flask was immediately placed
bath and compressed air was bubbled through it to decrease the in a pre-heated oil bath with magnetic stirring, covering most of the
amount of nitrogen oxides in solution, which gives it an orange flask in oil. For entries 2, 3, 5, 6, 8, 10, 13 and 18 in Table 1, right
color. The reaction mixture was then cooled down in the fridge and after the end of the reaction, the whole reaction mixture was
roughly 150 g of crushed ice was added to the solution. The white transferred to a Buchi Kugelrohr glass oven. The acetylating agent
precipitate formed was filtered and washed with distilled water and was removed under vacuum at 75 °C for 10 to 15 min. All entries
dried under vacuum at 60 °C to obtain off-white crystals of 4(5)- then followed the same procedure; right after the end of the
nitroimidazole. δ
H, s, CH), 7.84 (1 H, s, CH). δ
H
(400 MHz, DMSO-d
6
) 13.23 (1 H, br s, NH), 8.31 (1 reaction or distillation on the glass oven, 300 mg of reaction
): 147.6 (C4-NO
), mixture were dissolved in DMSO-d to a volume of roughly 0.6 mL,
C
(101 MHz, DMSO-d
6
2
6
1
3
135.9 (C2), 119.1 (C5).
placed in an NMR glass tube and then analyzed by C-qNMR
spectroscopy.
1
-Methyl-4-nitroimidazole. This method was based on a previously
2
1
13
published paper. 4(5)-Nitroimidazole (10 g, 88.4 mmol) and General procedure for
C-qNMR spectroscopy. Sample
potassium carbonate (18.35 g, 0.13 mol) were dispersed in 150 mL preparation was done as follows: a fraction of the reaction mixture
of acetonitrile. Iodomethane (6.6 mL, 106 mmol) dissolved in 50 mL with or without acetylating agent was transferred to an Eppendorf
of acetonitrile was added dropwise to the reaction mixture with type tube, approximately 300 mg of sample was used for each
magnetic stirring over an ice bath. After all the iodomethane analysis. This sample amount was then dissolved in DMSO-d
6
solution was added, the solution was stirred for 30 min over an ice (stored on 4Å molecular sieves) to a volume of about 0.6 mL and
bath. The reaction mixture was then heated to 65 °C overnight. An then transferred to an appropriate NMR tube. All spectra were
Allihn type condenser attached to a cooling system was used to acquired in the same way, with a temperature of 25 °C, no spin, 240
avoid solvent and reactant losses. The reaction mixture was filtered scans, relaxation delay of 25 s, pulse angle of 90°, no NOE
and the solid was washed with a small amount of acetonitrile and enhancement, spectral window from 58 to 78 ppm and 8192 data
discarded. The acetonitrile from this washing was added to the rest points. Auto lock and gradient shimming were used before
of the reaction mixture. All the acetonitrile was then removed acquiring. Mestrenova v6.0.2 – 5475 was used for data handling. All
under vacuum using a rotary evaporator, the product was extracted spectra were treated in the same way, zero filling to 16k,
from the solid reaction mixture using a mixture of dichloromethane apodization was made by using the exponential function set to 1.50
and methanol (95:5 v/v). The solvents were removed under vacuum Hz and the convolution difference function set to the standard
using a rotary evaporator. The slightly yellow solid that remained values of the software. Automatic phase and baseline correction
was recrystallized from 2-propanol. The product was an off-white were used. To decrease the error when integrating the area of the
solid. δ
H), 3.76 (3 H, s, Me). δ
C2), 122.6 (C5), 34.2 (Me). HMBC and HSQC analyses are shown in Since the GSD tool generates
H
(400 MHz, DMSO-d
(101 MHz, DMSO-d
6
): 8.35 (1 H, s, C5-H), 7.80 (1 H, s, C2- peaks, the global spectral deconvolution (GSD) tool was used,
): 146.9 (C4-NO
), 138.1 refinement level set to 20 fitting cycles and resolution set to 0.10.
synthetic spectrum by
C
6
2
(
a
the SI (Fig. S1 and S2, respectively).
deconvolution, the software can display a table with the areas of all
peaks; these areas were used to calculate the relative amounts of
acetins in the mixtures. The chemical shifts were corrected by
setting the DMSO peak to 39.52 ppm; since the spectra were all
acquired between 58 and 78 ppm, the chemical shift corrections
were made by setting one full spectra by the DMSO peak and then
using the acetin peaks to correct all other spectra.
General procedure for the synthesis of the imidazolium salts. In a
Schlenk flask with a magnetic stir bar over an ice bath were
dissolved or dispersed imidazole compounds (4(5)-Nitroimidazole,
1-Methylimidazole or Imidazole) in acetonitrile (roughly 5 mL for
every gram of imidazole compound) under argon. To the mixture
were added dropwise equal molar amounts of the acids (pure
hydrochloric acid 37% and a p-toluenesulfonic acid monohydrate
dispersion in a small amount of acetonitrile), the mixture was then Theoretical calculations. The theoretical calculations were
stirred for 30 min under these conditions. Then, the mixture was performed for the four imidazolium salts, four ionic liquids, p-
heated up to 60 °C for 24 h. After that, the acetonitrile was toluenesulfonic acid, glycerol, acetic anhydride and acetic acid
removed by vacuum at 60 °C, all salts precipitated with the optimized structures. The structures were firstly optimized using
exception of [Hmim]Cl, which remained as a viscous liquid until the standard settings for the classical AMBER force field as
8
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 10
Pl Ne ae swe Jdoo u nr no at l ao df j Cu sh te mm ai sr tgr iyn s
View Article Online
DOI: 10.1039/C8NJ02520G
Journal Name
ARTICLE
22
implemented in the GABEDIT software, secondly at the Hartree- 11. L. Zhou, E. Al-Zaini and A. A. Adesina, Fuel, 2013, 103, 617-625.
Fock/6-31+G* level of theory and, finally, at the B3LYP functional 12. C. Chiappe, S. Rajamani and F. D'Andrea, Green Chem., 2013,
2
3
using 6-31+G* basis set with the D3BJ correction in the ORCA
15, 137-143.
.0.1.2 software. IPBE were calculated using structures optimized 13. C. M. Breneman and K. B. Wiberg, J. Comput. Chem., 1990, 11,
in the TPSS-D3/6-311++G(3df,3dp) level, using structures from 361-373.
B3LYP-D3 as inputs. The resolution of identity approximation (Split- 14. J. Rigby and E. I. Izgorodina, Phys. Chem. Chem. Phys., 2013, 15,
RI-J as implemented in ORCA) was used in the TPSS-D3 calculations 1632-1646.
in order to speed up the process. IPBE were also counterpoise 15. R. G. Pearson, Coord. Chem. Rev., 1990, 100, 403-425.
2
4
4
2
5
corrected using a previous procedure. Tight conversion settings 16. R. G. Parr, L. v. Szentpaly and S. Liu, J. Am. Chem. Soc., 1999,
were used in all calculations as implemented in ORCA. The initial 121, 1922-1924.
guess of the molecular orbitals stem from the PMODEL method, as 17. P. K. Chattaraj, S. Giri and S. Duley, Chem. Rev., 2011, 111,
implemented in ORCA. Vibrational frequency calculations were PR43-PR75.
performed in the final optimized structures to certify the minimum 18. S. Zahn, D. R. MacFarlane and E. I. Izgorodina, Phys. Chem.
local state in the potential energy surface. Most of the final Chem. Phys., 2013, 15, 13664-13675.
structures have no imaginary frequencies associated to them, with 19. G. Klopman, J. Am. Chem. Soc., 1968, 90, 223-234.
the exception of [Hmim][TsO] at the B3LYP-D3/6-31+G* level, which 20. B. S. Samant and M. G. Sukhthankar, Bioorg. Med. Chem. Lett.,
-
1
has a imaginary frequency at 5.89 cm , but since this is such a small
value, the structure was considered to be valid. The frontier orbital 21. Q. Su, S. Ioannidis, C. Chuaqui, L. Almeida, M. Alimzhanov, G.
2011, 21, 1015-1018.
energies, CHELPG charges and bond distances for the reaction
descriptors were calculated in single point runs at B3LYP-D3/6-
Bebernitz, K. Bell, M. Block, T. Howard, S. Huang, D. Huszar, J. A.
Read, C. R. Costa, J. Shi, M. Su, M. Ye and M. Zinda, J. Med.
Chem., 2014, 57, 144-158.
31+G* level of theory in ORCA. Molecular electrostatic potential
maps (MEPs) of total electronic densities using the partial charges 22. A. R. Allouche, J. Comput. Chem., 2011, 32, 174-182.
were calculated at the B3LYP-D3/6-31+G* level using the Gabedit 23. S. Grimme, S. Ehrlich and L. Goerigk, J. Comput. Chem., 2011,
software. The isosurface value used was 0.016803 with grid values
32, 1456-1465.
from -0.09 to 0.09. Details on the equations used to calculate the 24. F. Neese, WIREs Comput. Mol. Sci., 2018, 8, e1327.
descriptors, as well as all optimized structures can be found in the 25. S. F. Boys and F. Bernardi, Mol. Phys., 1970, 19, 553-566.
supporting information.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors thank the following Brazilian agencies for financial
support: CAPES, CNPq (449758/2014-1) and FAPERGS (16/2551-
0000373-4).
Notes and references
1
2
3
4
5
6
7
8
9
1
.
.
.
.
.
.
.
.
.
P. S. Kong, M. K. Aroua, W. M. A. W. Daud, H. V. Lee, P. Cognet
and Y. Peres, RSC Adv., 2016, 6, 68885-68905.
S. Bagheri, N. M. Julkapli and W. A. Yehye, Ren. Sust. Energ.
Rev., 2015, 41, 113-127.
X. Liao, S. G. Wang, X. Xiang, Y. Zhu, X. She and Y. Li, Fuel Proc.
Technol., 2012, 96, 74-79.
M. Y. Huang, X. X. Han, C. T. Hung, J. C. Lin, P. H. Wu, J. C. Wu
and S. B. Liu, J. Catal., 2014, 320, 42-51.
L. Li, S. T. Yu, C. X. Xie, F. S. Liu and H. J. Li, J. Chem. Technol.
Biotechnol., 2009, 84, 1649-1652.
X. Liu, H. Ma, Y. Wu, C. Wang, M. Yang, P. Yan and U. Welz-
Biermann, Green Chem., 2011, 13, 697-701.
A. C. Cole, J. L. Jensen, I. Ntai, K. L. T. Tran, K. J. Weaver, D. C.
Forbes and J. H. Davis, J. Am. Chem. Soc., 2002, 124, 5962-5963.
A. Casas, M. J. Ramos, A. Perez, A. Simon, C. Lucas-Torres and A.
Moreno, Fuel, 2012, 92, 180-186.
S. Sandesh, P. Manjunathan, A. B. Halgeri and G. V. Shanbhag,
RSC Adv., 2015, 5, 104354-104362.
0. L. N. Silva, V. L. C. Gonçalves and C. J. A. Mota, Catal. Commun.,
010, 11, 1036-1039.
2
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins

New Journal of Chemistry
Page 10 of 10
View Article Online
DOI: 10.1039/C8NJ02520G
Table of Contents
A simple and inexpensive nitration of the imidazolium cation can considerably increase
its acidic activity in the acetylation of glycerol.