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the ILs–acid systems have no such limitations; on the contrary
mesityl oxide can be dissolved at very low acid concentrations
or even under basic conditions, thereby increasing the scope of
mesityl oxide as an acidity probe past the point previously
1
3
reported. We can now see the full range of the utility of the
Dd of mesityl oxide acidity measurements that covers the range
of H
0
values from approximately À1 to À9.
Outside the range H
correspond to small changes in Dd, hence the precision of
the conversion is poor. This does raise the question of whether
0
= À1 to À9, large changes in H
0
Fig. 3 Dd vs. mol% H
2
SO
4
. Negative values of mol% acid indicates an it is better to use the Dd measurement directly as the acidity
excess of 1-butylimidazole as such À100 mol% acid equals [C
4
Him]
2
[SO
4
]. measurement rather than converting to H
. While it is true that
0
The spectra of some of the more viscous ILs (excess 1-butylimidazole)
using Dd directly avoids the problems associated with com-
pounding the errors of two different measurement methods,
were recorded at 80 1C, with only a marginal decrease in the Dd value
(for [C
4 4
Him][HSO ] Dd decreased by 0.98 ppm on heating from room
converting to H
0 0
does have the advantage that H is a well
temperature to 80 1C).
understood measurement.
A model derived from the Hammett equation was also fitted
to the data in Fig. 4 with some adjustments for separate
hydrogen bond donation and acceptance effects. It was
observed that there is a stronger mitigation of the effects of
protonation on the NMR spectrum of mesityl oxide in the more
acidic region below a H0 of À4, which could derive from a
hydrogen bond acceptance effect from the anion, compared to
the less acidic regions compositions where we would expect a
hydrogen bond donation effect towards mesityl oxide to
dominate. (For further details see ESI.†)
To summarise, Dd mesityl oxide offers an easy, quick and
robust single probe based system for the measurement of the
Fig. 4 Dd vs. H
0
for [C
4 4 2 4 4 1 4 2 4
Him][HSO ]–H SO , [C C im][HSO ]–H SO and
13
acidities of IL–acid solutions in the range of H
= À1 to À9. The
2 2 4
H O–H SO
(see ESI† for details of model).
0
use of NMR removes the need for a colourless ionic liquid,
which is often a problem for the Hammett UV-vis based
The form of the titration curve plotted in Fig. 3 is dependent methodology. The high concentration of mesityl oxide used
upon the probe’s sensitivity to changes in acidity in the in these measurements reduces the sensitivity of the measure-
different acidity ranges. The precision of any acidity measure- ment to small traces of impurities in the ionic liquid. That the
ment based upon measurements of Dd is determined by the technique relies on a single probe saves time in identifying
slope of the line at any given acid concentration. For which probe to use. However, the sheer number of Hammett
[C
4
Him][HSO
4 2 4
]–H SO this gives a range between 15–50 mol% probes available does mean that one of these can usually be
H
2
SO within which good precision can be achieved. Beyond identified to give very precise acidity values once the approxi-
4
this range to higher acidities, the measurement is possible, but mate acidity of the solution being measured is known. The
the lower slope of the curve leads to noticeably lower precision, advantage of the system described here is that it will allow
whilst at low acid concentrations the line becomes almost flat
Dd for 1-butylimidazole = 28.82 ppm, not included in the
graph) and no meaningful measurement can be made.
Having measured the acidity of various IL–H SO solutions
many more measurements of the acidity of ionic liquids and
their acid solutions than have been possible to date. We will
continue to do this in order to elucidate the behaviour of acids
in ionic liquids.
(
2
4
using both Dd of mesityl oxide and H
0
it is now possible to
calibrate the former against the latter. This was previously been
1
3
done by F ˘a rcas-iu for H
that system and also the data collected from both of our IL
systems. As can be seen the data from the two IL–H SO
systems show excellent agreement with the data for the H O–
H SO in the range that these overlap. This is expected since
O–H
SO
.
Fig. 4 shows the data for Notes and references
2
2
4
1
P. Wasserscheid and T. Welton, Ionic liquids in synthesis, Wiley-VCH,
Weinheim, 2003.
2
4
2 J. P. Hallett and T. Welton, Chem. Rev., 2011, 111, 3508–3576.
3 N. V Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37,
2
1
3
2
4
1
23–150.
the identities of the solvent and acid have no effect upon this
comparison, only the acidity of the solutions. The measure-
4
5
6
M. Maase, K. Massonne and K. Halbritter, US Pat., WO 2003 062171,
2008.
K. Weissermel and H.-J. Arpe, Industrial organic chemistry, VCH,
Weinheim, New York, 3rd comple., 1997.
C. Reichardt and T. Welton, Solvents and solvent effects in organic
chemistry, Wiley-VCH, Weinheim, Germany, 4th updat., 2011.
ments of the acidity of the H
2
O–H
SO
2
SO
4
system cover a smaller
acidity range, because at low H
2
4
concentrations the mesityl
oxide is insufficiently soluble to give a NMR signal. However,
7260 | Chem. Commun., 2014, 50, 7258--7261
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