Table 2 Arrhenius activation energies for the reaction of methyl
ion towards methyl p-nitrobenzenesulfonate have been deter-
p-nitrobenzenesulfonate with chloride or bromide in [bmim][BF4]
mined and show a trend different to that observed in another
class of ionic liquid. Activation energies for the reaction of
chloride and of bromide with the substrate have been found to
be near identical to each other, and similar to the activation
energy for the reaction of the free solvated ion with the same
substrate in dichloromethane.
This work will be continued by the investigation of the
effect of ionic liquid anion (both coordinating and non-
coordinating); the effect of the imidazolium cation; and the use
of neutral nucleophiles.
3
Ϫ1
Ϫ1
[
Halide]/M
bmim]Cl
[1] /mM
0
10 kobs/s
T /K
∆E /kJ mol
ln A
a
[
0
0
0
0
.118
.118
.120
.119
0.307
0.299
0.303
0.294
3.32 (0.15)
3.99 (0.07)
6.53 (0.04)
9.20 (0.15)
297.6
301.2
306.8
311.5
58.2
(3.5)
17.8
(1.4)
[
bmim]Br
0
0
0
0
.118
.119
.119
.120
0.299
0.294
0.305
0.299
3.00 (0.04)
3.73 (0.04)
4.72 (0.03)
9.69 (0.06)
297.6
301.2
305.8
312.5
60.0
(8.4)
18.4
(3.3)
Experimental
Standard deviations in parentheses.
Materials
Methyl p-nitrobenzenesulfonate was purchased from Fisher
and used as received; 1-methylimidazole was purchased from
Avocado and distilled from potassium hydroxide; iodoethane,
1-chlorobutane and 1-bromobutane were purchased from
Lancaster and distilled from phosphorus pentoxide. Tetra-
fluoroboric acid was purchased from Lancaster and used as
received.
relative reactivity is of the order chloride (1.06) : bromide (1) :
iodide (1.41). It would appear that chloride and bromide are of
approximately equal nucleophilicity in [bmim][BF ], with iodide
4
being the most nucleophilic of the halides in this system.
The normal order of nucleophilicities in protic solvents (e.g.
Ϫ
Ϫ
Ϫ
water or methanol) is I > Br > Cl . The observed relative
nucleophilicity of the halides is dependent upon a number of
factors including the polarisability of the anion, and solvation
Spectroscopy
9
of the halide (including such factors as ion–dipole, dipole–
Proton NMR spectra were recorded on a JEOL 270 MHz
spectrometer. FAB Mass spectra were recorded on a VG
AutoSpec-Q mass spectrometer. UV/vis spectra were recorded
using a Perkin Elmer Lambda 2 spectrophotometer with a
thermostatted sample holder.
dipole and H-bonding interactions). Which of these factors is
most important in the observation made in this study is not
clear from study of just one ionic liquid.
The only literature comparison available to halide nucleo-
philicity in ionic liquids shows that the relative nucleophilicities
of the chloride, bromide and iodide anions is that found by
3
Synthesis
Ford et al. (2.1 : 1.2 : 1 respectively) for reactions in a trimeth-
ylhexylammonium trimethylhexylboride ionic liquid. This is
not the same as the trend that was determined in [bmim][BF4].
The salts [bmim]Cl, [bmim]Br and [emim]I were prepared by
reaction of 1-methylimidazole with the appropriate halo-
alkane and recrystallised from acetonitrile.
Clearly, different ionic liquids do not show the same behaviours
10
and cannot be treated as if the same. Although it is not possible
at this stage of the investigation to be certain of the reason for
this difference, it is notable that 1,3-dialkylimidazolium cations
1
-Butyl-3-methylimidazolium tetrafluoroborate. In a Schlenck
3
flask [bmim]Cl (158 g, 0.905 mol) and water (50 cm ) were com-
10
form strong hydrogen bonds to halide ions, whereas this inter-
action is not possible for simple tetraalkylammonium salts.
bined and stirred at room temperature until dissolved. Then
3
tetrafluoroboric acid (113 cm of 50% w/w aqueous solution, 1
equivalent) was added slowly with stirring. The mixture was
Arrhenius activation energies
stirred for 24 hours, after which the resulting [bmim][BF ] was
4
3
extracted with dichloromethane (4 × 50 cm ). The combined
dichloromethane fraction was washed with water (3 × 10 cm )
The reactions of [bmim]Cl and [bmim]Br with 1 were studied in
3
[
bmim][BF ] over a range of temperatures (the reaction of
4
until the aqueous fraction was pH neutral, and observed to be
[emim]I was not studied because of difficulties in obtaining
free of chloride (AgNO ). The dichloromethane was removed
3
reproducible data). The data obtained were analysed using
Arrhenius plots, allowing calculation of activation energies and
the results are shown (Table 2).
The Arrhenius activation energy for the reaction of chloride
ion with 1 was measured to be 58.2 kJ mol . This value was
somewhat lower than the value of activation energy determined
for reaction of the ion pair (∆E = 83.3 kJ mol ) and similar to
by rotary evaporation. After isolating the crude ionic liquid,
it was further purified by mixing with activated charcoal then
filtered through a plug of acidic alumina into a Schlenck tube,
dried by heating in vacuo and stored under nitrogen. The ionic
liquid thus obtained (140 g, 0.620 mol, 69%) was a colourless,
viscous liquid.
Ϫ1
Ϫ1
a
δH (ppm) (DMSO): 9.04 (1H, s, CH-2), 7.69 (2H, 2 s,
CH-4,5), 4.14 (2H, t, N-CH ), 3.83 (3H, s, N-CH ), 1.75 (2H, t,
the activation energy determined for reaction of the naked
Ϫ1
2
3
anion (∆E = 57.4 kJ mol ) with the same substrate in dichloro-
a
6
N-CH -CH -), 1.25 (2H, q, N-CH -CH -CH ), 0.88 (3H, m,
2 2 2 2 2
methane. The activation energy for the reaction of [bmim]Br in
Ϫ1
N-CH -CH -CH -CH ).
2 2 2 3
FABϩ MS: 365 ([(bmim) BF ] , 1%), 139 ([bmim] , 100%).
[
bmim][BF ] was almost identical at 60.0 kJ mol . Although
4
ϩ
ϩ
2
4
one would expect the solvation energies of the chloride and
bromide ions in [bmim][BF ] to be different to each other,
the Arrhenius activation energy arises from the difference
between the initial and transition states. The values of the pre-
exponential A term found in this work are significantly lower
than those found for the same reaction by the ion (ln A = 23.4)
Ϫ
ϩ
FABϪ MS: 313 ([(bmim)(BF ) ] , 8%), 87 ([BF ] , 92%), 19
4
2
4
4
Ϫ
(
F , 100%). No peaks due to chloride containing species were
observed in the mass spectra. The UV cut off point of this
solvent was found to be approximately 240 nm.
6
General method for studying reactions by UV spectroscopy
or ion pair (ln A = 31.9) in dichloromethane.
3
A solution of [bmim]Cl (0.2 mmol) in [bmim][BF ] (1.5 cm ,
4
freshly outgassed in vacuo) was weighed into a 0.5 cm path
length UV/vis quartz cuvette under anaerobic conditions. At
known time, an aliquot of methyl p-nitrobenzenesulfonate (5 ×
10 mol) in dichloromethane (0.1 cm ) was added and spectra
were recorded at regular time intervals.
Conclusions
This work has demonstrated that it is possible to carry out
kinetic measurements by using in situ techniques in ionic
liquids. The relative reactivities of chloride, bromide and iodide
Ϫ7
3
J. Chem. Soc., Perkin Trans. 2, 2001, 2267–2270
2269