Understanding the Effect of Solvent Structure on Organic Reaction Outcomes When Using Ionic Liquid/Acetonitrile Mixtures

Article abstract of DOI:10.1021/acs.jpcb.6b11090

The rate constant for the reaction between hexan-1-amine and 4-methoxybenzaldehyde was determined in ionic liquids containing an imidazolium cation. The effect on the rate constant of increasing the length of the alkyl substituent on the cation was examin

Full text of DOI:10.1021/acs.jpcb.6b11090

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Article
Understanding the Effect of Solvent Structure on Organic Reaction
Outcomes When Using Ionic Liquid / Acetonitrile Mixtures
Sinead T Keaveney, Tamar L. Greaves, Danielle F Kennedy, and Jason Brian Harper
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b11090 • Publication Date (Web): 21 Nov 2016
Downloaded from http://pubs.acs.org on December 5, 2016
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Understanding the Effect of Solvent Structure on Organic
Reaction Outcomes when Using Ionic Liquid / Acetonitrile
Mixtures
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a
b
c
a,
Sinead T. Keaveney, Tamar L. Greaves, Danielle F. Kennedy and Jason B. Harper *
a
b
School of Chemistry, University of New South Wales, Sydney, NSW, 2052, Australia; School of Science, College of Sciꢀ
c
ence, Engineering and Health, RMIT University, Melbourne, VIC, 3001, Australia; CSIRO Manufacturing, Clayton, VIC,
3168, Australia.
ABSTRACT: The rate constant for the reaction between hexanꢀ1ꢀamine and 4ꢀmethoxybenzaldehyde was determined in ionic liqꢀ
uids containing an imidazolium cation. The effect on the rate constant of increasing the length of the alkyl substituent on the cation
was examined in a number of ionic liquid / acetonitrile mixtures. In general it was found that there was no significant effect of
changing the alkyl substituent on the rate constant of this process, suggesting that any nanoꢀdomains in these mixtures do not have a
significant effect on the outcome of this process. A series of SAXS/WAXS experiments were performed on mixtures of the ionic
liquid 1ꢀbutylꢀ3ꢀmethylimidazolium bis(trifluoromethanesulfonyl)imide ([Bmim][N(CF SO ) ]) and acetonitrile; this work indicatꢀ
3
2 2
ed that the main structural changes of the mixtures occur by about a 0.2 mole fraction of ionic liquid in the mixture (χ ). This reꢀ
IL
gion where the main changes in the solvent structuring occurs corresponds to the region where the main changes in rate constant
and activation parameters occur for S 2 and condensation reactions previously examined; this is the first time that such a correlaꢀ
N
tion has been observed. To examine the ordering of the solvent about the nucleophile hexanꢀ1ꢀamine, WAXS experiments were
performed on a number of [Bmim][N(CF SO ) ] / acetonitrile / hexanꢀ1ꢀamine mixtures, where it was found that some of the patꢀ
3
2 2
terns featured asymmetric peaks as well as additional peaks not observed in the [Bmim][N(CF SO ) ] / acetonitrile mixtures; this
3
2 2
suggests that the addition of hexanꢀ1ꢀamine to the mixture affects the bulk structure of the liquid. The SAXS/WAXS patterns of
mixtures of 1ꢀbutylꢀ2,3ꢀdimethylimidazolium bis(trifluoromethanesulfonyl)imide ([Bm im][N(CF SO ) ]) and acetonitrile were
2
3
2 2
also determined, with the results suggesting that [Bm im][N(CF SO ) ] is more ordered than [Bmim][N(CF SO ) ] due to an enꢀ
2
3
2 2
3
2 2
hancement in the shortꢀrange interactions.
are affected by the amount of ionic liquid in the reaction mixꢀ
ture. Typically there are nonꢀlinear changes in reaction outꢀ
come as the mole fraction of an ionic liquid in a molecular
solvent is varied, and the trend observed is different for differꢀ
INTRODUCTION
Ionic liquids are salts that generally contain bulky, charge
diffuse ions, and the weaker interactions between the compoꢀ
nents result in these species having lower melting points than
typical inorganic salts, with many ionic liquids molten at amꢀ
22, 29, 31ꢀ35
ent reaction types.
Previous work has proposed that the structuring of the solꢀ
1
ꢀ2
bient temperatures. These salts are being utilized in many
36ꢀ38
vent can also influence reaction outcome.
As the changes
3
ꢀ5
6ꢀ9
areas, including biomass processing; electrochemistry; as
in reaction outcome when using an ionic liquid solvent are
10ꢀ11
2, 12ꢀ14
lubricants;
and as solvents for organic processes.
13, 22, 28ꢀ29, 32, 34, 39ꢀ44
often dominated by entropic effects,
it is
Focusing on the latter, the application of ionic liquids is reꢀ
reasonable to suggest that ordering of the solvent itself is likeꢀ
ly to be important. As a representative example, in a study
ceiving significant attention due to their low flammability and
15ꢀ18
vapour pressure;
the ability to tune the physical and chemꢀ
examining the solvolysis of a picolinium salt in a number of
ical properties of the solvent by varying the constituent ions of
36ꢀ38
ionic liquid / molecular solvent mixtures
it was concluded
19ꢀ21
the salt;
and the potential to achieve different reaction rates
that the rate constant was affected by the ‘pseudoꢀ
encapsulation’ of reagents in the polar domain of the solvent.
It was suggested that this aggregation of the polar reagents in
the polar domain of the solvent increased the effective reagent
concentration, and hence the rate constant was increased. It
was proposed that for ionic liquids containing a longer alkyl
substituent on the cation there is a larger nonꢀpolar domain,
relative to an ionic liquid featuring a cation with a shorter alꢀ
kyl chain, resulting in a comparatively smaller polar domain
and an increased effective concentration of the polar reagents.
This resulted in larger rate constants in ionic liquids featuring
and selectivity when compared to typical molecular
1
2ꢀ13
solvents.
12ꢀ14, 22
There has been much experimental
and computaꢀ
work aimed at understanding the origin of the
23ꢀ27
tional
changes in reaction outcome when using an ionic liquid, relaꢀ
tive to molecular solvents. It has been repeatedly demonstrated
that understanding the interactions that exist between the comꢀ
ponent ions of ionic liquids and the species along the reaction
coordinate is essential to using these solvents rationally,
with the importance of knowing the magnitude of these interꢀ
1
2ꢀ13, 28
2
9ꢀ32
37
actions recently revealed.
It has also been demonstrated
a longer alkyl substituent.
that the rate constants and selectivities of organic processes
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2
9
Other work has suggested that the extent of ordering of the
solvent can affect reaction outcome through changing the enꢀ
tropy of activation of a process. This is exemplified by previꢀ
ous work examining both bimolecular nucleophilic substituꢀ
4); this is a suitable ‘parent’ ionic liquid as by simply inꢀ
creasing the chain length of the alkyl substituent on the imidꢀ
azolium cation, the structural heterogeneity of the solvent can
1
2
3
4
5
6
7
8
9
1
1
1
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6
45
be increased. If the rate constant of this process is affected by
pseudoꢀencapsulation of reagents in the different domains, it
would be expected that there would be an increase in the rate
constant on moving from an ionic liquid with a butyl substituꢀ
ent ([Bmim][N(CF SO ) ] 4) to one with a hexyl chain
([Hmim][N(CF SO ) ], 5), and then a further increase on goꢀ
ing to the octyl substituted system ([Omim][N(CF SO ) ], 6),
as the nonꢀpolar domains will get larger and hence the effecꢀ
tive concentration of the reagents in the polar domain will
increase.
2
2, 28, 34, 40ꢀ42, 44
tion (S 2) processes
and related condensation
N
29ꢀ30, 43
reactions;
it was found that the main interaction affectꢀ
ing reaction outcome was between the ionic liquid cation and
the nucleophile. Disruption of this interaction on forming the
transition state complex resulted in an enthalpic cost and a
more significant entropic benefit, resulting in an increased rate
constant relative to acetonitrile. When moving from low to
high mole fractions of ionic liquid in the reaction mixture (χIL),
3
2 2
3
2 2
3
2 2
0
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0
the entropic benefit due to the breaking of this cation – nucleꢀ
29, 34
ophile interaction remains comparable.
It was proposed
As mentioned earlier, it has been proposed that as the
amount of ionic liquid in the reaction mixture is increased, the
solvent becomes more structured and this causes a reduction in
that the entropic advantage of removing the coordination of
the nucleophile becomes less significant as the ordering in the
34
solvent itself increases with increasing χ , therefore the enꢀ
IL
the entropic effects observed in ionic liquid solvent effects.
2
9, 32, 34
tropic effect plateaus at higher χIL.
To further investigate this concept, the second section of this
manuscript focuses on examining the structuring of ionic liqꢀ
uid / molecular solvent mixtures through a series of Small and
Wide Angle Xꢀray Scattering (SAXS and WAXS, respectiveꢀ
Overall, the structuring of an ionic liquid solvent has been
proposed to affect reaction outcome through two mechanisms:
1
) the nanostructural heterogeneity of the solvent drives agꢀ
gregation of solutes into either the polar or nonꢀpolar
ly) measurements. As ionic liquids are often used in mixtures
37ꢀ38
22, 29, 31ꢀ35
domain;
2) the significant ordering of the ionic liquid reꢀ
with a molecular solvent,
it was of interest to examine
duces the extent of disorder on forming the transition state
how the structuring of the liquid changes, and if nanoꢀdomains
form, as the solvent composition is varied. A secondary motiꢀ
vation for this work was to investigate the source of the lowꢀ
ered rate constant at around χIL = 0.5 that has been seen in
plots of rate constant for the reaction between benzaldehyde 1
and hexanꢀ1ꢀamine 2 against the proportion of ionic liquid in
3
4
complex, changing the entropy of activation. To further inꢀ
vestigate these phenomena, it was firstly of interest to examine
the kinetics of a wellꢀstudied organic process in ionic liquids
that are known to form nanoꢀdomains. The reaction chosen
was the condensation reaction between benzaldehyde 1 and
hexanꢀ1ꢀamine 2, as this process has been examined previousꢀ
ly in a number of ionic liquids, allowing comparison of the
2
9
the reaction mixture. Such a ‘dip’ in the plots of k against
2
3
2
χIL has also been observed for some S 2 processes. Recent
N
29ꢀ30, 43
obtained rate data with this previous work.
work has demonstrated that there is a change in the hydrodyꢀ
namic boundary conditions when transitioning from ‘ionic
liquid dissolved in acetonitrile’ (χIL < 0.4) to ‘acetonitrile disꢀ
solved in ionic liquid’ (χIL > 0.4), which is proposed to affect
Scheme 1: The condensation reaction between benzalde-
hyde 1 and hexan-1-amine 2
the rate constant at χ ca. 0.4 and could account for these
IL
46
‘
dips’ observed. It is possible that changes in the structure of
the solvent also contribute to the decrease in the rate constant
that is observed at that point.
RESULTS AND DISCUSSION
Kinetic studies
As discussed in the Introduction, previous work has sugꢀ
gested that the partitioning of reagents into the polar and nonꢀ
polar domains of an ionic liquid solvent can affect reaction
outcome due to changes in the effective concentration of the
reagents in each domain. Considering this, it was of interest to
further examine the ionic liquid solvent effects on the condenꢀ
sation reaction between benzaldehyde 1 and hexanꢀ1ꢀamine 2,
to determine whether such an effect influences the rate conꢀ
stant of this process.
Figure 1: The ionic liquids [Bmim][N(CF SO ) ] 4,
3
2 2
[
Hmim][N(CF SO ) ] 5 and [Omim][N(CF SO ) ] 6
3
2 2
3
2 2
N
N
N
N
O
O
O
O
N
N
S
S
S
S
F3C
CF3
F3C
CF3
O O
O O
4
5
This concept was examined using the ionic liquids
[Hmim][N(CF SO ) ] 5 and [Omim][N(CF SO ) ] 6, as these
N
N
3
2 2
3
2 2
ionic liquids have been shown to have significant nanoscale
structural heterogeneities, with the extent of this nanostructurꢀ
ing being more significant in [Omim][N(CF SO ) ] 6 than
O
O
S
N
S
F3C
CF3
O O
3
2 2
45
[
Hmim][N(CF SO ) ] 5. This data can be compared with
3 2 2
6
previous work examining the effect of [Bmim][N(CF SO ) ] 4
3
2 2
on the rate constant of the reaction of species 1 and 2; imꢀ
The reaction between species 1 and 2 has been studied in
ionic liquid 1ꢀbutylꢀ3ꢀmethylimidazolium
([Bmim][N(CF SO ) ],
portantly, no significant degree of nanostructure has been obꢀ
45, 47
the
served in the ionic liquid 4.
As such, it could be speculated
bis(trifluoromethanesulfonyl)imide
that if the pseudoꢀencapsulation of reagents into the polar doꢀ
3
2 2
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crease in both activation parameters, relative to acetonitrile, is
mains of the ionic liquid solvent affects reaction outcome, the
1
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9
1
1
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5
5
6
rate constant of the process would be expected to increase on
observed, with the activation parameters in the ionic liquids 4
and 5 being the same within experimental uncertainty (Table
2). While the differences in the rate constant (Table 1) indicate
that there must be differences in the activation parameters for
these two ionic liquids, clearly the changes are too small to be
determined using this methodology. This demonstrates that the
moving from [Bmim][N(CF SO ) ] 4 to [Hmim][N(CF SO ) ]
3
2 2
3
2 2
5 to [Omim][N(CF SO ) ] 6.
3
2 2
Initially, the rate constants for this process were deterꢀ
mined in each of the ionic liquids 5 and 6, diluted only by
reagents,¥ to investigate any changes in the rate constant on
changing the salt from the comparatively ‘homogenous’
[Bmim][N(CF SO ) ] 4 to the more ‘heterogeneous’ ionic
effects of nanoꢀdomains and increased steric hindrance on the
+
[
Hmim] cation are not sufficient to change the activation paꢀ
3
2 2
rameters markedly.
liquids [Hmim][N(CF SO ) ] 5 and [Omim][N(CF SO ) ] 6
3
2 2
3
2 2
(
Table 1).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 2. The activation parameters for the reaction of ben-
zaldehye and hexan-1-amine (Scheme 1) in each of the ion-
ic liquids 4-6
Table 1. Bimolecular rate constants (k ) for the reaction of
2
benzaldehyde 1 and hexan-1-amine 2 (Scheme 1) in each of
the ionic liquids 4-6 at 8 °C
a
‡
ꢀ1 b
‡
ꢀ1
ꢀ1 b
solvent
∆H / kJ mol
∆S / J K mol
ꢀ272.1 ± 8.7
ꢀ235.2 ± 4.6
a
ꢀ3
ꢀ1 ꢀ1 b
29
solvent
[Bmim][N(CF SO ) ] 4 (χ = 0.97)
k / 10 L mol s
acetonitrile
27.2 ± 2.5
32.4 ± 1.3
2
2
9
6.80 ± 0.20
2.84 ± 0.10
5.39 ± 0.11
[Bmim][N(CF SO ) ] 4
3
2 2
IL
3
2 2
29
(χIL = 0.97)
[
[
Hmim][N(CF SO ) ] 5 (χ = 0.96)
3 2 2 IL
[
[
Hmim][N(CF SO ) ] 5
34.0 ± 1.5
28.7 ± 1.4
ꢀ237.2 ± 5.0
ꢀ250.4 ± 4.8
Omim][N(CF SO ) ] 6 (χ = 0.95)
3
2 2
3
2 2
IL
(
χIL = 0.96)
a
b
The ionic liquids are diluted only by the reagents 1 and 2.
Uncertainties quoted represent the standard deviation of three
Omim][N(CF SO ) ] 6
3
2 2
(χIL = 0.95)
replicates.
a
b
The ionic liquids are diluted only by the reagents 1 and 2.
Uncertainties quoted are from the fit of the linear regression.
It was found that the rate constant in each of the ionic liqꢀ
uids 4ꢀ6 were different (Table 1), indicating that changing the
length of the alkyl chain on the cation affects the rate constant
of the reaction between benzaldehyde 1 and hexanꢀ1ꢀamine 2.
When considering the ionic liquids 4ꢀ6 the only difference is
the length of the alkyl chain, as such these differences in the
rate constant could be due to: a) the pseudoꢀencapsulation of
reagents in the nanoꢀdomains of the solvent and/or; b) the inꢀ
creased steric hindrance on the cation as the alkyl chain is
increased reduces the extent of cation – nucleophile 2 interacꢀ
tions. Interestingly, there is no correlation between the alkyl
Interestingly, when using [Omim][N(CF SO ) ] 6 as the
3
2 2
solvent it was found that there was a decrease in both the enꢀ
thalpy and entropy of activation of the reaction, relative to
both [Bmim][N(CF SO ) ] 4 and [Hmim][N(CF SO ) ] 5 (Taꢀ
3
2 2
3
2 2
ble 2). This difference in the activation parameters for the
ionic liquids 6, relative to the salts 4 and 5, may arise from the
partitioning of the polar reagents 1 and 2 into the polar domain
in the ionic liquid 6. Alternatively, this decrease in the activaꢀ
tion parameters might be due to the increased alkyl chain
length hindering the charged centre on the cation. An increase
chain length and the observed changes in k , and the rate conꢀ
2
+
stant is highest in [Bmim][N(CF SO ) ] 4. This is the opposite
in the steric hindrance on the [Omim] cation, relative to both
3
2 2
+
+
of what would be expected based on a pseudoꢀencapsulation
model, as the existence of polar and nonꢀpolar domains is least
the [Bmim] and [Hmim] cations, would result in decreased
cation – nucleophile 2 interactions, and hence a reduction in
the enthalpic cost and entropic benefit that is associated with
this interaction. Such an effect has been demonstrated in preꢀ
vious work examining the effect of changing the steric hinꢀ
drance on the cation of the solvent for the reaction of species 1
and 2, although the effect of increasing the Nꢀalkyl chain
4
5
prevalent in the ionic liquid 4.
To probe the microscopic origin of the differences in the
rate constant between these ionic liquids further, the activation
parameters of the reaction of species 1 and 2 were determined
in [Hmim][N(CF SO ) ]
5 and [Omim][N(CF SO ) ] 6
3
2 2
3 2 2
29
length was not examining in this previous work.
through temperatureꢀdependent kinetic analyses. These data
can be compared with the activation parameters in acetonitrile
and [Bmim][N(CF SO ) ] 4 which have been determined preꢀ
Overall, the lower activation parameters determined in
3
2 2
[Omim][N(CF SO ) ] 6, compared to the ionic liquids 4 and 5,
3
2 2
29
viously.
may arise from: a) pseudoꢀencapsulation of the reagents into
the polar domain; b) decreased cation – nucleophile 2 interacꢀ
It has been previously demonstrated that interactions beꢀ
tween the ionic liquid cation and the lone pair on the nucleoꢀ
phile 2 results in an increased enthalpy and entropy of activaꢀ
+
tion due to the steric bulk on [Omim] cation; or c) a combinaꢀ
tion of both effects. The extent to which each contribute is
difficult to attribute as both effects could affect the activation
parameters for the reaction of species 1 and 2.
2
9ꢀ30, 43
tion, relative to acetonitrile.
This cation – nucleophile 2
interaction deactivates the nucleophile 2 and thus increases the
enthalpy of activation. The increase in disorder that results
from the breaking of this interaction on forming the transition
state is the origin of the increased entropy of activation, relaꢀ
The principle outcome from the above work is that for the
reaction of species 1 and 2 there is not an increase in k on
2
moving to ionic liquids featuring longer alkyl chains, as might
29
tive to acetonitrile. In [Bmim][N(CF SO ) ] 4 the change in
have been expected if the reagents were being preferentially
3
2 2
37ꢀ38
the entropy of activation was more significant than the change
in the enthalpy of activation, resulting in an entropically drivꢀ
localised in the polar domain of the ionic liquid.
As such, it
is reasonable to suggest that the pseudoꢀencapsulation of the
reagents 1 and 2 into polar domains does not significantly
29
en rate increase. For [Hmim][N(CF SO ) ] 5, a similar inꢀ
3
2 2
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affect the rate constant of this process when using the ionic
liquids 4ꢀ6.
χIL ca. 0.6 (Chart 1). There is also a decrease in k on moving
to the highest mole fraction of the salt 6 (χIL ca. 0.95), altꢀ
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
hough this decrease in k is not as significant as that seen for
As it has been widely shown that the amount of ionic liqꢀ
2
2
2,
[Hmim][N(CF SO ) ] 5.
uid present in the reaction mixture affects reaction outcome,
3
2 2
2
9, 31ꢀ35
it was also of interest to determine the rate constant for
As the only difference between the ionic liquids 4, 5 and 6
is the length of the alkyl chain, the origin of the differences in
the trends shown in Chart 1 are likely due to either: a) the exꢀ
istence of nanoꢀdomains in the different ionic liquid / acetoniꢀ
trile mixtures; b) differences in the extent of ordering of the
solvent with increasing χIL for each ionic liquid, which could
affect the changes in the entropy of activation; and/or c)
changes in the cation – amine 2 interaction due to the different
steric hindrance on the cation. It is clear that changing the
length of the alkyl chain on the cation affects the rate constant
of the reaction between benzaldehyde 1 and hexanꢀ1ꢀamine 2.
However there are no clear correlations between the alkyl
chain length and the observed changes in the rate constant. As
such, understanding the exact origin of these differences in the
plots of the rate constant against ionic liquid concentration is
difficult, especially as it is unknown if nanoꢀdomains exist in
the ionic liquid / acetonitrile mixtures; this will be discussed
further in the following section.
the reaction of species 1 and 2 in mixtures of either
[Hmim][N(CF SO ) ] 5 or [Omim][N(CF SO ) ] 6 and aceꢀ
3
2 2
3
2 2
tonitrile. In previous work examining [Bmim][N(CF SO ) ] 4 /
3
2 2
acetonitrile mixtures it was found that there is a gradual inꢀ
crease in k as the amount of ionic liquid in the reaction mixꢀ
2
ture was increased, with the most significant increase occurꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ring between χIL = 0 and 0.2 and a slight decrease in k at χIL
2
ca. 0.5. In this current work it was of interest to determine
whether there is a similar trend in k with changing χIL for the
2
ionic liquids 5 and 6, and whether there are any ‘dips’ in these
29
plots as was seen for the [Bmim][N(CF SO ) ] 4 case. As
3
2 2
one potential source of this ‘dip’ observed is that χIL ca. 0.5 is
the solvent composition at which the mixture becomes more
structured, it might be expected that this effect would be more
significant in the ionic liquids 5 and 6 as they have more proꢀ
4
5
nounced nanoꢀstructuring.
When using the ionic liquid [Hmim][N(CF SO ) ] 5, there
3
2 2
were a few key differences in the plot of k against χ relative
2
IL
SAXS and WAXS measurements
to the salt 4 (Chart 1); these were: a) the magnitude of the
solvent effect for [Hmim][N(CF SO ) ] 5 is smaller in the
Considering the number of studies that use ionic liquid /
molecular solvent mixtures, it was of interest to gain a better
understanding of how the properties of the solvent change
when using different proportions of an ionic liquid in a molecꢀ
ular solvent. In the kinetic studies presented above for the
reaction of species 1 and 2, it was difficult to attribute the
origin of the differences in the plots of k against χ for the
3
2 2
range χ = 0 to ca. 0.5, relative to the trend seen for the salt 4,
IL
with the trend in k comparable for the ionic liquids 4 and 5
2
between χIL ca. 0.5 and ca. 0.8; b) for [Hmim][N(CF SO ) ] 5
3
2 2
there is not a significant dip in the mole fraction dependence
plot at χIL ca. 0.5; and c) the rate constant at χIL ca. 0.95 is
2
IL
significantly lower in [Hmim][N(CF SO ) ]
5 than in
3
2 2
ionic liquids 4ꢀ6. To further investigate the role that solvent
structure may have on reaction outcome, we examined the
structure of ionic liquid / molecular solvent mixtures to probe
whether there is a correlation between solvent structuring and
the rate constant for the reaction between species 1 and 2. As
[
Bmim][N(CF SO ) ] 4, as discussed earlier.
3
2 2
Chart 1: The dependence of k of the reaction of species 1
2
and 2 (Scheme 1) as the mole fraction of either
[Bmim][N(CF SO ) ] 4 (black), [Hmim][N(CF SO ) ] 5
mixtures of [Bmim][N(CF
monly used, and there is an anomalous decrease in k
0.5 that may be due to changes in solvent structure, it was
concluded that this system was of most interest to investigate.
SO )
] 4 and acetonitrile are comꢀ
at χIL ca.
3
2 2
3
2 2
3
2 2
(red) or [Omim][N(CF SO ) ] 6 (blue) in acetonitrile was
2
3
2 2
a
29
varied, at 8.1°C.
8
.E-03
Useful methods to examine the bulk structure of liquids
include Small and Wide Angle Xꢀray Scattering
7.E-03
(SAXS/WAXS) experiments, where the information obtained
6
5
4
3
.E-03
.E-03
.E-03
.E-03
from these measurements is interpreted based on Bragg's Law
(Equation 1).
2.E-03
1.E-03
0
.E+00
SAXS/WAXS techniques have been applied to investigate
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
4
8ꢀ51
the extent of nanoscale organisation in many ionic liquids.
Mole fraction of ionic liquid
45,
This previous work has mainly focused on neat ionic liquids,
a
47ꢀ48, 52ꢀ62
Uncertainties are reported as the standard deviation of three repꢀ
but there have also been investigations on mixtures of
licates. It should be noted that some of the error bars fall within
the size of the marker.
ionic liquids and molecular solvents, although they have mainꢀ
63ꢀ65
ly involved protic ionic liquids.
Previous SAXS/WAXS
measurements of neat ionic liquids have shown that there are
three main peaks that tend to be observed in their scattering
ꢀ
1
When using [Omim][N(CF SO ) ] 6 as the ionic liquid, the
3
2 2
patterns: at q ca. 0.2 ꢀ 0.6 Å corresponding to polarity alterꢀ
changes in k for the reaction shown in Scheme 1 with increasꢀ
2
nation and characteristic of nanoscale structural heterogeneity;
ꢀ
1
ing χIL are similar to that seen for [Hmim][N(CF SO ) ] 5,
3
2 2
at q ca. 0.9 Å due to charge alternation of the ionic compoꢀ
ꢀ1
although for the salt 6 there appears to be two ‘dips’ in the
plot, one in the region χIL ca. 0.3 to χIL ca. 0.4, and another at
nents; and at q ca. 1.5 Å due to correlations between neighꢀ
ACS Paragon Plus Environment

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The Journal of Physical Chemistry
creases in the amount of ionic liquid 4 in the mixture, from χIL
bouring atoms, both intraꢀ and interꢀmolecularly (often termed
4
5, 52, 60, 66
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
‘adjacency correlations’).
= 0.21 to 1, result in minimal changes in the position of this
lower q peak, although it does become narrower and increases
in intensity (Chart S1); the narrowing of the peak suggests that
For neat [Bmim][N(CF SO ) ] 4 it has been shown that
3
2 2
there is not a significant peak at low q values, suggesting that
there is not a significant degree of nanostructure in this ionic
liquid.
this structuring is becoming more defined at higher χ . The
IL
4
5, 47
change in the peak position of the lower q peak can be most
clearly seen by plotting qmax against χIL (Chart 3), noting that
for mixtures containing very low χ , that the value of q
Only when moving to ionic liquids that feature a
45
longer alkyl chain (six or more carbon atoms ) on the imidazꢀ
olium cation does a prominent low q peak appear. This is a
result of the longer alkyl chains driving the formation of polar
and nonꢀpolar domains, as discussed earlier. It was shown that
IL
max
could not be accurately determined as the peak is not well
defined.
ꢀ
1
[
Bmim][N(CF SO ) ] 4 does have a peak at q ca. 0.9 Å , due
3 2 2
ꢀ1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
to charge alternation, and a peak at q ca. 1.4 Å , due to correꢀ
Chart 3: The position of qmax for the low q peak as the mole
4
5, 47
lations between neighbouring atoms.
previous studies examining the structure of mixtures of
Bmim][N(CF SO ) ] 4 and acetonitrile.
There have been no
fraction of [Bmim][N(CF SO ) ] 4 in acetonitrile was var-
3
2 2
a
ied.
[
3
2 2
The SAXS/WAXS patterns of mixtures containing differꢀ
ent proportions of [Bmim][N(CF SO ) ] 4 in acetonitrile were
1
3
2 2
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
measured. Initially the focus was on examining the changes in
solvent structure when moving from acetonitrile to mixtures
containing small concentrations of the ionic liquid 4, to deterꢀ
mine whether there was aggregation of the ions at very low
mole fractions. The results of these experiments are presented
as the peak intensity (I) plotted against the scattering vector
(q) (Chart 2).
0.1
0
Chart 2: SAXS/WAXS data for the [Bmim][N(CF SO ) ] 4
3
2 2
0
0.2
0.4
0.6
0.8
1
/
(
(
acetonitrile mixtures at: χIL = 0 (dark blue); χIL = 0.004
brown); χIL = 0.009 (red); χIL = 0.019 (green); χIL = 0.043
purple); χIL = 0.071 (orange), χIL = 0.106 (black) and χIL
Mole fraction [Bmim][N(CF SO ) ] 4
3 2 2
a
=
The low q peak data for χIL < 0.043 could not be determined as
the peak was not well defined, and the value of qmax at χIL = 0.043
is an estimate as the peak is broad. Uncertainties are reported as ±
0
.211 (light blue). The arrow shows the direction of in-
creasing
χ .
IL
ꢀ
1
0.02 Å , which is an estimate of the uncertainty associated with
the fitting method used to determine qmax
1
200
.
1
000
8
6
4
2
00
00
00
00
0
The most significant change in the position of qmax occurs
at lower mole fractions of [Bmim][N(CF SO ) ] 4 in acetoniꢀ
3
2 2
trile (χIL < 0.2), and with increasing amounts of the salt 4 in
the mixture there is little change in the position of this peak
(Chart 3). This indicates that for χIL > 0.2 there is little change
in the electrostatic interactions in the solvent mixture, and that
the length scale of this intermediate range structuring, due to
charge alternation of the ions in the solvent mixture, remains
similar when moving from mixtures containing χIL ca. 0.2 to
0
0.5
1
1.5
q / Å-1
2
2.5
3
3.5
those containing χ ca. 0.9.
IL
Interestingly, it was found that when moving from neat acꢀ
etonitrile to mixtures containing low mole fractions of
Overall, the changes in the peak position of the low q peak
[
(
Bmim][N(CF SO ) ] 4 in acetonitrile a low q feature appears
suggest that at lower concentrations of [Bmim][N(CF SO ) ] 4
3 2 2
3
2 2
ꢀ
1
q ca. 0.4 Å ). This feature becomes larger with increasing
in acetonitrile the component ions cluster together, and then
with increasing amounts of the ionic liquid 4 this clustering
becomes less significant, and charge alternation starts to beꢀ
come more significant. That is, at very low values of χIL (<
0.05) there are ionic liquid aggregates, and at higher values of
amounts of [Bmim][N(CF SO ) ] 4 up to χ ca. 0.04, and
3
2 2
IL
above χIL ca. 0.04 this low q peaks shifts to higher q values
(Chart 2). This low q feature corresponds to longꢀrange strucꢀ
tural heterogeneities in the liquid, and the increasing intensity
of this peak suggests that at very low concentrations of
χ
IL these aggregates break down and general coulombic, inꢀ
termediate length scale interactions dominate. This is imꢀ
portant as it suggests that for mixtures of
Bmim][N(CF SO ) ] 4 and acetonitrile, structural heterogeneꢀ
ities of significant scale do not form for mixtures containing
χIL > 0.04. Hence it is unlikely that there is trapping of the
reagents in either polar or nonꢀpolar domains in the
[
Bmim][N(CF SO ) ] 4 in acetonitrile (χ < 0.05) the ionic
3 2 2 IL
liquid components are aggregating.
[
3
2 2
In the range χIL = 0.04 to 0.21, the peak being considered
ꢀ1
shifts from q ca. 0.4 to 0.78 Å and becomes narrower (Chart
). This shift likely arises from a progression from larger,
2
more diffuse clusters in mixtures containing the lower χIL to
smaller, more defined clusters in mixtures containing higher
χ , with the peak then lying in the region where peaks due to
charge alternation are generally found.
[
Bmim][N(CF SO ) ] 4 / acetonitrile solvent mixtures.
3 2 2
The SAXS/WAXS pattern of neat acetonitrile has a single,
ꢀ1
large peak at q = 1.7 Å , due to the shortꢀrange interactions in
IL
45, 52, 60, 66
Further inꢀ
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 6 of 15
the liquid (Chart 2). With increasing amounts of
[Bmim][N(CF SO ) ] 4 added to the solvent mixture this peak
efit to be comparable and therefore there is little change k2
above χ ca. 0.2.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
2 2
IL
shifts to lower q values (Charts 2 and S1). This trend is best
visualised by plotting qmax against χIL (Chart 4), and indicates
that the nature of the shortꢀrange interactions is becoming less
like those in acetonitrile when more ionic liquid 4 is added to
the solvent mixture. This wasn’t too surprising, as when addꢀ
ing ions to the mixture it would be expected that the solvent
structuring would become less like the structuring in acetoniꢀ
trile and more like the neat ionic liquid 4.
Some previous work that is worth considering here is a
study examining the structuring of mixtures of protic ionic
64
liquids and water. In those cases there was a significant
ꢀ
1
downward shift in the high q peak (q = 1.4ꢀ1.7 Å ) when
moving from neat water to χIL ca. 0.3, with little change in the
position of q when moving to higher mole fractions of ionic
max
64
liquid; this is essentially the same trend as that seen in this
current work. In that previous work the change in the peak
position of the high q peak was attributed to a shift from water
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
–
water and water – ionic liquid interactions dominating at
Chart 4: The position of qmax for the high q peak as the
lower χ , to ionic liquid – ionic liquid interactions becoming
IL
64
mole fraction of [Bmim][N(CF SO ) ] 4 in acetonitrile was
3
2 2
more prevalent at higher χIL. The point at which the change
in the position of qmax became negligible (χIL ca. 0.3) was said
to be the point where the intermediate range ionic liquid –
ionic liquid interactions dominate, and hence there was little
change in the shortꢀrange interactions above this point. Such a
trend was also seen in another study on mixtures of an ammoꢀ
varied.
1
1
1
1
.75
.7
.65
.6
.55
.5
.45
.4
.35
.3
1
63
nium based ionic liquid and water.
Applying this argument to the work completed here sugꢀ
gests that for the [Bmim][N(CF SO ) ] 4 / acetonitrile mixꢀ
1
1
3
2 2
tures with χIL above ca. 0.2, general ionic liquid – ionic liquid
interactions become significant and there is no further change
in the ordering within the solvent mixture. It is possible that in
the region χIL 0 to ca. 0.2, where the main changes in the
shortꢀrange acetonitrile – acetonitrile and ionic liquid – aceꢀ
tonitrile interactions occur, that the main changes in the ionic
1
1
1
0
0.2
0.4
0.6
0.8
1
Mole fraction [Bmim][N(CF SO ) ] 4
3
2 2
liquid – solute interactions also occur. When moving to χIL
>
a
ꢀ1
Uncertainties are reported as ± 0.02 Å , which is an estimate of
the uncertainty associated with the fitting method used to deterꢀ
mine qmax
0.2, where there was little change in the shortꢀrange interacꢀ
tions in the solvent mixture, there might also be expected to be
little change in the ionic liquid – solute interactions. This conꢀ
cept is consistent with the minimal changes in the rate conꢀ
.
2
2, 32, 34
stants and activation parameters for both the S 2
and
N
29
The change in the position of the higher q peak (Chart 4)
suggests that the shortꢀrange ordering, due to correlations such
as van der Waals' interactions between alkyl chains, exists
over a larger length scale when moving to higher concentraꢀ
tions of the salt 4 in acetonitrile. The main changes in the posiꢀ
tion of this high q peak occur when moving from neat acetoniꢀ
condensation reactions considered in these systems above χIL
ca. 0.2.
In combination, the SAXS/WAXS studies and the kinetic
22, 34
29
analyses on the SN2
and condensation reactions suggest
that for the [Bmim][N(CF SO ) ] 4 / acetonitrile mixtures, the
3
2 2
region where the main changes in the structuring of the solvent
occurs, corresponds to the region where the main changes in
rate constant occur. This correlation likely arises from both: a)
trile to χ ca. 0.2, in a comparable fashion to that observed for
IL
the lower q peak (Chart 3).
The trends shown in Charts 3 and 4 indicate that the main
structural changes of the mixtures occur before χIL ca. 0.2.
This is important, as it suggests that by χIL ca. 0.2 the
a reduction in the entropic effects observed above χ ca. 0.2 as
IL
the solvent becomes more structured, and hence the entropic
advantage of breaking the cation – nucleophile interaction
changes less significantly with proportion of ionic liquid; and
b) the main changes in the shortꢀrange ionic liquid – nucleoꢀ
[Bmim][N(CF SO ) ] 4 / acetonitrile mixtures are comparable
3
2 2
to the neat ionic liquid 4, according to the SAXS/WAXS data.
Such an effect is of particular interest in this work as for both
phile interactions also occurring between χ = 0 and ca. 0.2.
IL
2
2, 32, 34
an S 2 reaction
and the condensation reaction between
N
29
The mixtures considered so far have only contained the
ionic liquid 4 and the coꢀsolvent acetonitrile, but it was also of
interest to see how the bulk structure of the liquid is affected
when a reagent is dissolved into the solvent. Ideally, measꢀ
urements of the bulk structure of systems containing both benꢀ
zaldehyde 1 and hexanꢀ1ꢀamine 2 dissolved in the solvent
mixture (preferably at the concentrations used in reactivity
studies) would be carried out, but both being present would
result in reaction during the measurements so this was deemed
impractical. Considering the importance of the interaction
species 1 and 2 examined previously, the main changes in
the rate constant and activation parameters in
[
Bmim][N(CF SO ) ] 4 / acetonitrile mixtures occurred in the
3 2 2
region χ = 0 to ca. 0.2ꢀ0.3. For those cases, the increased rate
IL
constant in the ionic liquid was rationalized to be due to interꢀ
actions between the ionic liquid cation and the nucleophile,
where breaking of this interaction on forming the transition
state resulted in an enthalpic cost and a more significant enꢀ
tropic benefit, relative to acetonitrile. This current work supꢀ
ports the idea that this trend in k (for example, Chart 1) arises
2
between the ionic liquid cation and the nucleophile 2 demonꢀ
from a reduction in the entropic effect observed at higher χ as
29ꢀ30, 43
IL
strated previously,
it was concluded that inclusion of this
the solvent becomes more structured. That is, the ordering of
the solvent limits the extent of disorder that can occur on
forming the transition state complex, causing the entropic benꢀ
reagent in the solvent mixture was of more interest, particularꢀ
ACS Paragon Plus Environment

Page 7 of 15
The Journal of Physical Chemistry
ly to determine if its inclusion would result in changes in the
bulk structure of the mixture.
comparison is that between the measured scattering pattern at
χIL = 0.39, which is similar to the WAXS data for the simple
[Bmim][N(CF SO ) ] 4 and acetonitrile mixtures described
earlier, to the data at χIL = 0.36 (Chart 6). With only a slight
decrease in the ionic liquid 4 concentration, there is a signifiꢀ
cant change in the scattering pattern, with a much more comꢀ
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
To investigate the structuring of these ternary mixtures, the
3
2 2
WAXS patterns of
a
number of mixtures of
[
Bmim][N(CF SO ) ] 4 and acetonitrile containing the nucleoꢀ
3
2 2
ꢀ
1
phile hexanꢀ1ꢀamine 2 (ca. 0.1 mol L , χ ca. 0.03) were measꢀ
ured. As the above section demonstrated that there was no low
q peak above χIL ca. 0.04, only the higher q region was examꢀ
ined in this section. The results of the WAXS experiments are
once again presented as the peak intensity (I) plotted against
the structure factor (q) (Charts S2ꢀ4).
plex pattern measured at χ = 0.36 and a new peak formed at q
IL
ꢀ
1
ca. 1.05 Å .
Chart 6: WAXS data for mixtures of [Bmim][N(CF SO ) ]
3
2 2
-1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4, acetonitrile and hexan-1-amine 2 (ca. 0.1 mol L , χ ca.
0.03) at χIL = 0.36 (black) where no new peak is observed,
and at χIL = 0.22 (blue) and χIL = 0.39 (orange) where an
The scattering pattern of neat hexanꢀ1ꢀamine 2 has a single
ꢀ
1
peak at q ca. 1.4 Å , which is in a similar position as the high
q peak observed for [Bmim][N(CF SO ) ] 4, while acetonitrile
-
1
3
2 2
additional peak in the region of q = 0.95 to 1.2 Å is pre-
ꢀ
1
a
has one broad peak at q ca. 1.5ꢀ2 Å (Chart 5). Generally the
sent.
determined WAXS patterns for the mixtures of
[
(
Bmim][N(CF SO ) ] 4, acetonitrile and hexanꢀ1ꢀamine 2
Charts S2ꢀ4) were found to have similar peaks at low and
10000
8000
6000
4000
2000
0
3
2 2
high q values to those seen for the [Bmim][N(CF SO ) ] 4 and
3
2 2
acetonitrile mixtures (Chart 2 and S1). Interestingly, the data
for the mixtures containing the amine 2 are much more comꢀ
plicated than those for the mixtures of just
[
Bmim][N(CF SO ) ] 4 and acetonitrile, with some of the patꢀ
3 2 2
terns featuring asymmetric peaks as well as additional peaks
not observed previously in simple mixtures of salt 4 and aceꢀ
tonitrile. The determined scattering patterns appear to be the
sum of several distinct contributions and hence result in a
spectrum with poorly defined peaks. As such, simple peak
0
.5
1
1.5
q / Å-1
2
fitting to determine q was not valid. The peak observed at q
max
ꢀ
1
ca. 1.4 Å in neat hexanꢀ1ꢀamine 2 may be present in the mixꢀ
tures, and could contribute, to some extent, to the complexity
of the peak shapes observed.
a
The discontinuities observed at q ca. 1.4 are an artifact from the
detector and should be ignored.
At this point it is difficult to comment further on the
source of this anomalous behavior observed for the mixtures
of [Bmim][N(CF SO ) ] 4, acetonitrile and hexanꢀ1ꢀamine 2.
Chart 5: WAXS data for neat [Bmim][N(CF SO ) ] 4
3
2 2
a
(black), acetonitrile (green) and hexan-1-amine 2 (purple).
3
2 2
Despite the relatively limited understanding of these ternary
mixtures, the behavior observed clearly demonstrates that adꢀ
dition of hexanꢀ1ꢀamine 2 to the [Bmim][N(CF SO ) ] 4 /
9000
8000
7000
6000
3
2 2
acetonitrile mixture affects the bulk structure of the liquid. The
formation of an additional peak for only some of the mole
fractions examined suggests that predicting how the liquid
structure changes on addition of the nucleophile 2 is nonꢀ
trivial, and these changes in the interactions within the mixꢀ
tures may be the origin of the ‘dips’ in the mole fraction deꢀ
pendence plots that have been observed previously.
5000
4000
3000
2000
1000
0
Another ionic liquid that is of particular interest to investiꢀ
0.5
1
1.5
q / Å-1
2
gate in terms of its structure is [Bm im][N(CF SO ) ] 7 (Figꢀ
2
3
2 2
ure 2). The effect of this ionic liquid on the rate constant and
activation parameters for the reaction of either benzaldehyde 1
or 4ꢀmethoxybenzaldehyde and hexanꢀ1ꢀamine 2 has been
a
The discontinuities observed at q ca. 1.4 are an artifact from the
detector and should be ignored.
29
previously investigated. It was generally found that the ionic
For some, but not all, mixtures containing the ionic liquid
liquid 7 affected reaction outcome in a similar manner to when
4
, acetonitrile and the amine 2 there is a new peak present in
[Bmim][N(CF SO ) ] 4 was used, with identical changes in
3 2 2
ꢀ
1
the range q = 0.95 to 1.2 Å (Charts S2ꢀ4). It would seem
reasonable to conclude (given other data) that this new peak is
due to correlations between the ionic liquid 4, particularly the
cation, and hexanꢀ1ꢀamine 2. However, it is difficult to attribꢀ
ute the exact nature of this scattering peak as it is only visible
in the WAXS data for some proportions of the salt 4 in the
mixture. Further, there appears to be no pattern as to when this
additional peak is observed. A particularly demonstrative
the rate constant for the reaction between species 1 and 2 as
the proportion of the ionic liquid in the reaction mixture was
29
increased. The only difference in the solvent effects observed
for ionic liquids 4 and 7 is that the activation parameters deꢀ
termined for the reaction between 4ꢀmethoxybenzaldehyde
and the amine 2 were slightly lower in [Bm
7 than [Bmim][N(CF SO ] 4. This difference in the activaꢀ
tion parameters was attributed to the increased steric bulk on
im][N(CF SO ) ]
2
3 2 2
)
2 2
3
ACS Paragon Plus Environment

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Page 8 of 15
the cation of the salt 7, resulting in reduced cation – hexanꢀ1ꢀ
amine 2 interactions, and is consistent with previous observaꢀ
tions on related processes.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Chart 7: SAXS/WAXS data for [Bmim][N(CF SO ) ] 4 in
3
2 2
2
8
acetonitrile at χIL = 0.90 (green) and [Bm im][N(CF SO ) ]
2
3
2 2
a
7
in acetonitrile at χIL = 0.90 (purple).
Figure 2: The ionic liquid [Bm im][N(CF SO ) ] 7
2
3
2 2
6000
5000
4000
N
N
O
O
N
S
S
3000
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
F3C
CF3
O O
2
000
000
0
7
1
While the solvent effects of the ionic liquid 7 on the conꢀ
densation reactions examined previously were relatively
straightforward, this ionic liquid has been previously shown to
exhibit some unexpected behaviour. The ionic liquid 7 has a
methyl group at the C2 position, rather than the acidic proton
that is present in [Bmim][N(CF SO ) ] 4. Intuitively, removꢀ
0
0.5
1
1.5
q / Å-1
2
2.5
3
a
The sharp peak at q ca. 0.4 is a scattering contribution due to the
Kapton tape used to secure the samples and should be ignored.
3
2 2
ing this acidic proton would be expected to result in a decrease
In terms of peak positions, the low q charge alternation
peak has qmax at a similar position for both ionic liquids, sugꢀ
gesting that the ordering due to charge alternation is comparaꢀ
ble for these two ionic liquids. Conversely, qmax for the high q
+
in the extent of hydrogen bonding of the [Bm im] cation with
2
ꢀ
+
the [N(CF SO ) ] anion, relative to the [Bmim] cation, and
3
2 2
therefore result in [Bm im][N(CF SO ) ] 7 having a lower
2
3
2 2
viscosity and melting point than [Bmim][N(CF SO ) ] 4. Inꢀ
peak is shifted to
a
slightly lower
q
value for
SO
3
2 2
terestingly, the trend is the opposite to what would have been
[Bm im][N(CF SO ] 7, compared to [Bmim][N(CF
2
3
2
)
2
3
) ]
2 2
expected, with [Bm im][N(CF SO ) ] 7 having a higher visꢀ
4. This shift to lower q indicates that the lengthꢀscale of the
ordering is slightly longer for the methylated case 7 than the
parent 4. The increased low q peak intensity suggests that corꢀ
relations between alkyl chains and other van der Waals' interꢀ
2
3
2 2
6
7ꢀ68
cosity and melting point than the ionic liquid 4.
The origin
of this phenomenon has been attributed to restricted movement
of the ions of [Bm im][N(CF SO ) ] 7, relative to
2
3
2 2
[
Bmim][N(CF SO ) ] 4, due to the high potential energy barriꢀ
actions exist over a larger distance for [Bm im][N(CF SO ) ]
2 3 2 2
3
2 2
69
ers for the different conformers, and a reduction in the numꢀ
ber of stable ionꢀpair conformers (reducing the entropy relaꢀ
tive to the salt 4).
ume for the ionic liquid 7 and longer lived ion cages in this
salt, relative to the ionic liquid 4, has also been suggested to
7, relative to the salt 4, which is in agreement with the inꢀ
creased ordering of this ionic liquid that has been proposed
70ꢀ71
69ꢀ71
A decrease in the unoccupied free volꢀ
previously.
7
2
There are two main conclusions from the discussion
above: a) the high q peak position shifts to lower q values in
73
contribute to the viscosity differences.
[
Bm im][N(CF SO ) ] 7 relative to to [Bmim][N(CF SO ) ] 4;
2 3 2 2 3 2 2
Considering this behaviour of [Bm im][N(CF SO ) ] 7, it
and b) the position of the low q peak is the same for both ionic
liquids 7 and 4. These observations suggest that the increased
2
3
2 2
was of interest to investigate the structuring of this solvent. As
such, the SAXS/WAXS patterns of a number of mixtures of
the ionic liquid 7 and acetonitrile were measured, to investiꢀ
gate any differences in the ordering of [Bmim][N(CF SO ) ] 4
ordering in [Bm im][N(CF
[Bmim][N(CF SO ) ] 4 is due to changes in the shortꢀrange
2 2
SO
)
2
]
7
compared with
2
3
2
3
correlations rather than changes in the ordering due to charge
alternation.
3
2 2
and [Bm im][N(CF SO ) ] 7. To begin with, the
2
3
2 2
SAXS/WAXS patterns of samples containing a high mole
fraction (χIL = 0.90) of either [Bm im][N(CF SO ) ] 7 or
SAXS/WAXS patterns were measured across a range of
2
3
2 2
different mole fractions of [Bm im][N(CF SO ) ] 7 in acetoniꢀ
2
3
2 2
[
Bmim][N(CF SO ) ] 4 in acetonitrile were compared (Chart
3 2 2
trile to investigate how the bulk structure of the liquid changes
as the amount of the ionic liquid 7 in the mixture is varied.
The results of these experiments are presented as the intensity
7
). It should be noted that this mole fraction was chosen as
when using these ionic liquids as solvents they will always be
diluted by reagents, hence, comparing data at χIL ca. 0.9 is of
interest.
(I) plotted against the structure factor (q) for χ = 0.05 to 0.28
IL
(Chart 8) and χIL = 0.32 to 0.90 (Charts S5ꢀ7; included in the
It was found that for both samples there are two distinct
ESI as there was little change in the SAXS/WAXS patterns
above χIL ca. 0.3).
ꢀ
1
ꢀ1
peaks, one at q ca. 0.8 Å and q ca. 1.3 Å (Chart 7); these
correspond to charge alternation and shortꢀrange correlations,
respectively, as described earlier. Interestingly, the peaks for
It was observed that with increasing χIL the low q peak in
the SAXS/WAXS data shifted to higher q values (Chart 8 and
Charts S5ꢀ7), which was analogous to what was observed for
mixtures of acetonitrile and [Bmim][N(CF SO ) ] 4. This efꢀ
the [Bmim][N(CF SO ) ] 4 case are much sharper than those
3
2 2
for [Bm im][N(CF SO ) ] 7, suggesting that the intermediate
2
3
2 2
3
2 2
and shortꢀrange ordering is more defined in the ionic liquid 4
fect for mixtures of acetonitrile and [Bm im][N(CF SO ) ] 7 is
2
3
2 2
ꢀ
1
case. The peak at q ca. 1.3 Å in [Bm im][N(CF SO ) ] 7 is
2
3
2 2
best seen by plotting the positions of qmax for the low q peak
against χIL (Chart 9). The shift of the lower q peak to higher q
values with increasing amounts of the ionic liquid 7 indicates
that at very low mole fractions of [Bm im][N(CF SO ) ] 7 in
particularly broad, which indicates that there are a variety of
shortꢀrange correlations that exist on similar, but slightly difꢀ
ferent, length scales.
2
3
2 2
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that observed for both the low q peak (Chart 9) and the high q
acetonitrile there is clustering of the ionic liquid components.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
With increasing χ this peak shifts into the region where peaks
peak (Chart 4) for the salt 4. This suggests that for the salt 7
the shortꢀrange interactions present at lower χIL persist on
IL
due to intermediate range ordering are expected, indicating
that ordering due to charge alternation becomes prominent for
χIL > ca. 0.2.
moving to higher χ , and that there is a more gradual transiꢀ
IL
tion of the short range interactions from being more ‘acetoniꢀ
trileꢀlike’ to more ‘ionic liquidꢀlike’ with increasing χ . Addiꢀ
IL
tionally, the high q peak is much more broad across the mixꢀ
tures (Chart 8 and Charts S5ꢀ7) than that observed for
Chart 8: The determined SAXS/WAXS patterns for the
mixtures of [Bm im][N(CF SO ) ] 7 and acetonitrile, at χ
2
3
2 2
IL
[
Bmim][N(CF SO ) ] 4 (Charts 2 and S1), further suggesting
3 2 2
=
0.050 (black), χIL = 0.103 (blue), χIL = 0.151 (red), χIL
=
that the shortꢀrange correlations in these mixtures exist on
similar, but slightly different, length scales.
0
.202 (green), χIL = 0.241 (purple) and χIL = 0.280 (orange).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4500
4000
Chart 10: The position of qmax for the high q peak as the
mole fraction of [Bm im][N(CF SO ) ] 7 in acetonitrile was
3500
2
3
2 2
varied.
3000
2500
1.65
2000
1500
1000
1.6
1.55
1.5
500
0
1.45
0
0.5
1
1.5
q / Å-1
2
2.5
3
1.4
1
.35
1.3
.25
a
The sharp peak at q ca. 0.4 is a scattering contribution due to the
Kapton tape used to secure the samples and should be ignored.
1
0
0.2
0.4
0.6
0.8
1
Mole fraction [Bm im][N(CF SO ) ] 7
2
3
2 2
Chart 9: The position of qmax for the low q peak as the mole
fraction of [Bm im][N(CF SO ) ] 7 in acetonitrile was var-
a
ꢀ1
2
3
2 2
a
Uncertainties are reported as ± 0.02 Å , which is an estimate of
the uncertainty associated with the fitting method used to deterꢀ
mine qmax
ied.
.
1
.9
.8
.7
.6
.5
0
0
0
0
0
Overall, the trends in both the low and high q peak posiꢀ
tion with increasing χ for the salt 7 are similar to that seen for
IL
the [Bmim][N(CF SO ) ] 4 / acetonitrile mixtures examined
3
2 2
earlier. Once again, this indicates that the main changes in the
0.4
structure of the solvent occur at lower χ . The trends observed
IL
0.3
0.2
0.1
0
for [Bm im][N(CF SO ) ] 7 suggests that for χ < ca. 0.3ꢀ0.4
2
3
2 2
IL
acetonitrile
–
acetonitrile
and
acetonitrile
–
[Bm im][N(CF SO ) ] 7 interactions dominate (more acetoniꢀ
2
3
2 2
trileꢀlike), and for χIL > ca. 0.3ꢀ0.4 electrostatic interactions
dominate (more ionic liquidꢀlike).
0
0.2
0.4
0.6
0.8
1
Mole fraction [Bm im][N(CF SO ) ] 7
2
3
2 2
As was discussed earlier, the range of χIL where the main
changes in the solvent structure occurs corresponds to the
range of χIL where the main changes in reaction outcome ocꢀ
cur. This is demonstrated by kinetic analyses on the reaction
between species 1 and 2 in the ionic liquid 7, where the most
significant changes in the rate constant and activation parameꢀ
a
ꢀ1
Uncertainties are reported as ± 0.02 Å , which is an estimate of
the uncertainty associated with the fitting method used to deterꢀ
mine qmax
.
The change in the position of the high q peak is now going
ters were found to occur between χ = 0 and ca. 0.3, with negꢀ
IL
to be considered; it was found that with increasing χ the high
ligible changes in the activation parameters when going to
IL
29
q peak shifted to slightly lower q values (Chart 10). This trend
suggests that the shortꢀrange correlations (such as van der
Waals' interactions between alkyl chains) become more promꢀ
inent and exist over a larger length scale when moving to
higher concentrations of the salt 7. The main changes in the
from χIL ca. 0.3 to χIL ca. 0.9. Once again, this effect likely
arises from the main both: a) reduced entropic effects above
χIL ca. 0.3 as the solvent becomes more structured, and hence
the entropic advantage of breaking the cation – nucleophile 2
interaction changes less significantly with proportion of ionic
liquid; and b) the main changes in the shortꢀrange correlations
between the ionic liquid 7 and hexanꢀ1ꢀamine 2 occur between
χIL = 0 and ca. 0.3.
peak position of the high q peak occur between χ = 0 and ca.
IL
0
.3ꢀ0.4, which indicates that the main changes in the shortꢀ
range correlations of the [Bm im][N(CF SO ) ] 7 / acetonitrile
2
3
2 2
mixtures occurs at χ < ca. 0.4. Interestingly, for the salt 7 the
IL
decrease in qmax is much more gradual with increasing χ , than
CONCLUSIONS
IL
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It was found that for the reaction between benzaldehyde 1 certain ionic liquid / molecular solvent mixture on organic
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
and hexanꢀ1ꢀamine 2 there is no correlation between the alkyl
chain length on the 1ꢀalkylꢀ3ꢀmethylimidazolium cation of an
ionic liquid and the rate constant in that ionic liquid. Additionꢀ
ally, the rate constant was highest in [Bmim][N(CF SO ) ] 4,
reaction outcomes it is essential to have a good understanding
of the origin of the nonꢀlinear changes in the rate constant as
the solvent composition is varied. This current work highlights
the importance of considering the bulk structure of the reacꢀ
tion mixture in this sense.
3
2 2
the opposite of what would expected from a pseudoꢀ
3
7ꢀ38
encapsulation model.
This suggests that the trapping of the
reagents 1 and 2 in polar domains is not the only origin of the
changes in the rate constant when using the ionic liquids 4ꢀ6.
EXPERIMENTAL
Materials
When
moving
from
[Bmim][N(CF SO ) ]
4
to
3
2 2
Benzaldehyde 1 and hexanꢀ1ꢀamine 2 were commercially
available, and each was distilled under reduced pressure then
stored over molecular sieves at 253 K prior to use. Analytical
grade deuterated acetonitrile was dried over molecular sieves
for at least 48 h prior to use. The ionic liquids 4-7 were preꢀ
[
Omim][N(CF SO ) ] 6 there was a decrease in the activation
3
2 2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
parameters, which could be arise from: a) pseudoꢀ
encapsulation of the reagents into the polar domain; b) deꢀ
creased cation – nucleophile 2 interaction due to the steric
+
bulk on [Omim] cation; or c) a combination of both factors.
74ꢀ76
pared with reference to literature methods
by first treating
At this point it is difficult to comment on the exact origin of
the corresponding imidazole with either butyl bromide or butyl
chloride to afford the intermediate halide salt, which was then
treated with lithium bis(trifluoromethanesulfonyl)imide to
give the required ionic liquid. All ionic liquids were dried to
constant weight at 70°C under reduced pressure immediately
before use, and were found to have <0.1% water using Karl
Fischer titration methodology.
the reduced activation parameters in [Omim][N(CF SO ) ] 6,
3
2 2
relative to the ionic liquid 4 and 5.
The SAXS/WAXS measurements
Bmim][N(CF SO ) ] acetonitrile
B mim][N(CF SO ) ] 7 / acetonitrile mixtures indicated that
of
both
and
[
[
4
/
3
2 2
2 2
2
3
the main changes in solvent structure occur between χIL = 0
and χ ca. 0.2ꢀ0.3, with little change in structure on moving to
IL
higher χ . This work, in combination with previous kinetic
Kinetic studies
IL
22, 32, 34
29
analyses on SN2
and condensation reactions, highlights
Kinetic analyses were carried out in solutions containing
that the region where the main changes in the solvent structurꢀ
ing occurs, corresponds to the region where the main changes
in rate constant and activation parameters occur; this is the
first time that such a correlation has been observed. It is proꢀ
posed that this correlation arises from two factors; firstly,
above χIL ca. 0.2ꢀ0.3 there is limited increase in disorder on
forming the transition state, as the solvent has become more
structured as the proportion of the ionic liquid in the reaction
mixture increases. This results in the entropic advantage of
breaking the cation – nucleophile interaction reaching a maxꢀ
imum at χIL ca. 0.2ꢀ0.3 and, as this is an entropically driven
ꢀ1
the benzaldehyde 1 (ca. 0.02 mol L ) and the amine 2 (ca.
ꢀ1
0.25 mol L ) at a given temperature and specific mole fraction
of the ionic liquid, with the remaining solvent being made up
by deuterated acetonitrile. For all reactions the prepared NMR
samples were stored in liquid nitrogen prior to analysis. In
each case the reaction was monitored in situ using H NMR
spectroscopy with the spectrometer being set to the desired
1
1
temperature for the duration of the reaction. H NMR kinetics
experiments were carried out on either a Bruker Avance III
4
00, Bruker Avance III 500 or Bruker Avance III 600 specꢀ
trometer with either a BBFO or TBI probe using ca. 0.5 mL of
reaction mixture in a 5 mm NMR tube. Results were shown to
be reproducible between the different spectrometers.
solvent effect, there is limited further increase in k above this
2
6
3ꢀ64
point. Secondly, considering previous work,
it is reasonaꢀ
ble to suggest that the main changes in the shortꢀrange ionic
liquid – nucleophile interactions also occurs between χIL = 0
and ca. 0.2ꢀ0.3, and hence the main solvent effects are obꢀ
served in this region.
All reactions were followed until more than 95% of the
starting material 1 was consumed, and all kinetic analyses
were performed in triplicate. NMR spectra were processed
using the MestReNova 7.1.1 software. The pseudoꢀfirst order
rate constants were calculated using integrations of the aldeꢀ
hyde signal in the starting material 1 at δ ca. 10.0, obtained
from the processed H NMR spectra, by fitting the natural
logarithm of the integrations to a linear function using the
Microsoft Excel 14.4.3 LINEST function. Bimolecular rate
On examination of the WAXS patterns of
[Bmim][N(CF SO ) ] 4 / acetonitrile / hexanꢀ1ꢀamine 2 mixꢀ
3
2 2
tures it was found that some mixtures featured asymmetric
peaks as well as additional peaks not observed in the
1
[
Bmim][N(CF SO ) ] 4 / acetonitrile mixtures. This suggests
3 2 2
that the addition of hexanꢀ1ꢀamine 2 affects the bulk structure
of the mixture. Interestingly, there was no pattern to when this
additional peak is observed, suggesting that predicting how the
liquid structure changes on addition of the nucleophile 2 to the
constants (k ) were obtained from the pseudoꢀfirst order conꢀ
2
stants by dividing by the initial amine 2 concentration in the
reaction mixture.
Where appropriate, the activation parameters were then deꢀ
termined through fitting the obtained data using the Microsoft
Excel 14.4.3 LINEST function to the bimolecular Eyring
[
Bmim][N(CF SO ) ] 4 / acetonitrile is difficult.
3 2 2
Overall, using an ionic liquid diluted by a molecular solꢀ
vent is beneficial, both practically (e.g. cheaper and less visꢀ
cous) and from a solvent effect standpoint (similar, or greater,
rate enhancement than at higher proportions of the ionic liqꢀ
uid). This current work demonstrates that while the trend in
77
equation. Tables containing the exact mole fraction, amine 2
concentration, temperature and rate constants for all the sysꢀ
tems described above can be found in the Electronic Supportꢀ
ing Information.
the rate constant with changing χ is mainly due to changes in
IL
SAXS and WAXS measurements
the solvent – solute (and solvent – transition state) interactions
along the reaction coordinate, subtle changes in the ordering of
the solvent can influence the entropic effects associated with
these interactions. In order to be able to predict the effect of a
The small and wide angle Xꢀray scattering
(SAXS/WAXS) experiments were performed on the
SAXS/WAXS beamline at the Australian Synchrotron,
ACS Paragon Plus Environment

Page 11 of 15
The Journal of Physical Chemistry
Victoria, Australia. The
q
range for the SAXS experiments
(6) Zhang, S.; Sun, J.; Zhang, X.; Xin, J.; Miao, Q.; Wang, J., Ionic
Liquidꢀbased Green Processes for Energy Production. Chem. Soc. Rev.
2014, 43, 7838ꢀ7869.
-1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
was 0.02 to 0.62 Å , and the range for the WAXS experꢀ
q
-
1
iments was 0.54 to 3 Å . The experiments were performed
using a one second exposure time at 25 °C. The temperaꢀ
ture of the sample holder was regulated using a Huber reꢀ
circulating water bath. The samples were loaded into 1.5
mm capillaries, which were sealed with silicon rubber to
prevent water absorption from the atmosphere. Samples
were housed in a 60 sample capillary holder, which was
mounted on a three axis sample stage, and the contribution
from an empty capillary was subtracted from the scattering
profiles. The data acquired was integrated from two dimenꢀ
sions to one dimension and, where relevant, the SAXS and
WAXS data were carefully combined to obtain scattering
(
7) Buzzeo, M. C., Evans, Russell G., Compton, Richard G., Nonꢀ
Haloaluminate RoomꢀTemperature Ionic Liquids in Electrochemistry—A
Review. ChemPhysChem 2004, 5, 1106.
(8) MacFarlane, D. R.; Forsyth, M.; Howlett, P. C.; Pringle, J. M.; Sun,
J.; Annat, G.; Neil, W.; Izgorodina, E. I., Ionic Liquids in Electrochemical
Devices and Processes: Managing Interfacial Electrochemistry. Acc.
Chem. Res. 2007, 40, 1165ꢀ1173.
(9) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B.,
Ionic Liquid Materials for the Electrochemical Challenges of the Future.
Nat. Mater. 2009, 8, 621ꢀ629.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
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(
ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Author
(18) Wu, B.; Liu, W.; Zhang, Y.; Wang, H., Do We Understand the
Recyclability of Ionic Liquids? Chem. Eur. J. 2009, 15, 1804ꢀ1810.
(19) Hawker, R. R.; Haines, R. S.; Harper, J. B., Variation of the Cation
of Ionic Liquids: The Effects on Their Physicochemical Properties and
Reaction Outcome. In Targets in Heterocyclic Systems, Noto, R., Ed.
*
2
Email: j.harper@unsw.edu.au; Fax: +61 2 9385 6141; Tel: +61
9385 4692
2
014; Vol. 18, pp 141ꢀ213.
20) Clough, M. T.; Crick, C. R.; Grasvik, J.; Hunt, P. A.; Niedermeyer,
Notes
¥ As the amount of ionic liquid present in the reaction
mixture has been widely shown to affect reaction outcome, it
(
H.; Welton, T.; Whitaker, O. P., A Physicochemical Investigation of Ionic
Liquid Mixtures. Chem. Sci. 2015, 6, 1101ꢀ1114.
(21) Tang, S.; Baker, G. A.; Zhao, H., Etherꢀ and Alcoholꢀfunctionalized
Task Specific Ionic Liquids: Attractive Properties and Applications.
Chem. Soc. Rev. 2012, 41, 4030ꢀ4066.
(22) Yau, H. M.; Keaveney, S. T.; Butler, B. J.; Tanner, E. E. L.;
Guerry, M. S.; George, S. R. D.; Dunn, M. H.; Croft, A. K.; Harper, J. B.,
Towards SolventꢀControlled Reactivity in Ionic Liquids. Pure Appl.
Chem. 2013, 85, 1979ꢀ1990.
is important to know the χ for each case being considered.
IL
ACKNOWLEDGMENTS
STK acknowledges the support of the Australian government
through the receipt of an Australian Postgraduate Award. JBH
acknowledges financial support from the Australian Research
Council Discovery Project Funding Scheme (Project
DP130102331). The authors would like to acknowledge the NMR
Facility within the Mark Wainwright Analytical Centre at the
University of New South Wales for NMR support. The scattering
measurements were performed on the SAXS/WAXS beamline at
the Australian Synchrotron, Victoria, Australia.
(23) Keaveney, S. T.; Harper, J. B.; Croft, A. K., Computational
Approaches to Understanding Reaction Outcomes of Organic Processes in
Ionic Liquids. RSC Adv. 2015, 5, 35709ꢀ35729.
(24) Hubbard, C. D.; Illner, P.; van Eldik, R., Understanding Chemical
Reaction Mechanisms in Ionic Liquids: Successes and Challenges. Chem.
Soc. Rev. 2011, 40, 272ꢀ290.
(25) Chiappe, C.; Pomelli, C. S., Computational Studies on Organic
Reactivity in Ionic Liquids. Phys. Chem. Chem. Phys. 2013, 15, 412ꢀ423.
(26) Zahn, S.; Brehm, M.; Brüssel, M.; Hollóczki, O.; Kohagen, M.;
Lehmann, S.; Malberg, F.; Pensado, A. S.; Schöppke, M.; Weber, H. et al.,
Understanding Ionic Liquids from Theoretical Methods. J. Mol. Liq. 2014,
192, 71ꢀ76.
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