1. Laying the Groundwork for Investing the Cage Effects of Ionic Liquids
Andrew Diorio,* Lora Walsh* and Amy E. Keirstead**
Department of Chemistry and Physics, University of New England, Biddeford, Maine, USA 04005
*UNE undergraduate student **e-mail: akeirstead@une.edu
Introduction
Ionic liquids (ILs) are salts that are liquids at or below room temperature and are typically composed
of an organic cation and an inorganic anion; Figure 1 displays some common ILs.1-2 Over the past
decade, ILs have generated much attention as environmentally friendly alternatives to conventional
industrial solvents. The properties of individual ILs can be tailored to fit specific needs via changing the
anion/cation combination, warranting them the title “designer solvents.” Their robustness under
extreme conditions and variability make them ideal for a wide array of industrial applications. Examples
of ionic liquids’ use are found in chemical synthesis, as both solvents and catalysts. Research has also
begun on the use of ILs as electrolytes in batteries and dye-sensitized solar cells (DSSCs). Despite the
potential use of ionic liquids in a wide variety of applications, little is known about their chemical
properties, such as polarity, viscosity, and the cage effects of ILs.3 The cage effect is the constraint
placed on solute molecules by the solvent cage and describes how effectively the solute molecules
can escape the solvent cage. Considering the recent interest in the use of ILs as “green” solvents,
cage effects are an important property worth characterizing. A strong cage effect could make a DSSC
or battery less efficient or could dramatically influence the outcome of a chemical reaction.
Figure 1. Examples of commercial available ionic liquids (a) butylmethylimidizolium
hexafluorophosphate [BMIM][PF6],
(b) 1-butyl-4-methylpyridinium tetrafluoroborate [BMPy][BF4], (c) trihexyltetradecylphosphonium
chloride [P-6,6,6,14][Cl].
Project Goals
References
1. Rogers, R. D.; Seddon, K. R., Ionic Liquids. Industrial Applications for Green Chemistry. American Chemical Society: Washington, DC,
2002; Vol. 818.
2. Rogers, R. D.; Seddon, K. R., Ionic Liquids as Green Solvents. Progress and Prospects. American Chemical Society: Washington, D.C.,
2003; Vol. 856.
3. Pagni, R. M.; Gordon, C. M., Photochemistry in Ionic Liquids. In CRC Handbook of Organic Photochemistry and Photobiology, Second
Edition, Horspool, W. M.; Lenci, F., Eds. CRC Press: Boca Raton, FL, 2004.
4. Furniss, B.S.; Hannaford, A.J.; Smith, P.W.G.; Tatchell A.R., The synthesis and purification of 2-naphthyl benzoate. InVogels Textbook of
Practcal Organic Chemistry, Fifth Edition, 1989
Purification of Products
The esters that were synthesized were purified in order to remove residual naphthol and/or acid chloride
by one of two techniques. 2-naphthyl benzoate was purified using a technique called recrystallization
using rectified spirit (absolute ethanol) as a recrystallization solvent. The product was recrystallized twice
in order to obtain white crystals that are pure in appearance. The rest of the esters were purified by a
technique called column chromatography using silica as the stationary phase and 5% ethyl acetate in
hexanes as the mobile phase for 2-naphthyl pivalate and 2% ethyl acetate in hexanes as the mobile
phase for 1-naphthyl pivalate and 1-naphthyl benzoate. The mobile phase was determined by TLC trials
where the ratio of ethyl acetate to hexanes was changed in order to optimize separation of the product
from impurities.
Analysis of Ester Purity
After purification, the esters that were synthesized were analysed for purity using a variety of techniques.
The most useful technique for determining purity would be nuclear magnetic resonance spectroscopy
(NMR), however; the NMR Spectrometer in the lab was non-functioning all summer. Instead, techniques
like melting point determination, thin layer chromatography (TLC) and gas chromatography coupled with
mass spectrometry (GC/MS) were used to help determine purity of compounds. Thin layer
chromatography was used to determine how many components made up each product. By spotting
material on a TLC plate and observing it under UV light after elution it can be determined if the material
is made of one or more components. For a compound to be pure it has to have one component.
Impurities will cause the melting point to deviate from the literature values so average melting point
ranges were collected for all compounds and compared to the literature. The data for this can be seen in
Table 1 and suggest that the esters are relatively pure. The technique GC/MS was used to more
accurately determine how many components were in the material with the mass spectrometer being able
to predict what the components are. The data for this can be seen in Table 2 and suggest that the
starting materials have been successfully synthesized and purified with the exception of an unidentified
peak for 2-naphthyl benzoate. Data for 1-naphthyl benzoate was not collected because the ester was
synthesized during the last few days of the project.
Table 1. Melting point determination data for naphthyl esters for assessing purity
Table 2. GC/MS data collected for assessing purity of naphthyl esters
Acknowledgements
This work was funded by an American Chemical Society Petroleum Research Fund grant #51292UNI4
(AEK PI). Summer stipends for AD and TJR were funded through the UNE College of Arts and Sciences
Summer Undergraduate Research Experience (SURE) program and the Green Family Foundation. The
instrumentation used in this project was purchased through funds from the Green Family Foundation
and the National Science Foundation (MRI #1229519). We also thank the Department of Chemistry and
Physics for supplying gases, solvents, and consumables.
Conclusion and Future Directions
There is no way to tell the esters made this summer are pure without using a more sensitive technique
like NMR spectroscopy. The data collected suggests that they are pure but there is no way to know for
certain the degree of purity the staring materials have. The ester that were synthesized and purified
over the summer will be analysed again by NMR spectroscopy to ensure purity. Once the starting
materials are pure they will be used with an internal standard in a photochemical probe reaction called
the Photo-Fries reaction, depicted in figure 3, will be used and coupled with GC/MS in order to quantify
the cage effects of ionic liquids and hexanes. It is essential to do the experiment in hexanes first in an
attempt to replicate data previously obtained in order to determine that the procedure and technique is
correct.
Synthesis of Naphthyl Esters
The major goal for this summer was to complete the preliminary work for the photolysis of naphthyl
esters in ionic liquids in order to quantify their cage effects. Their cage effects can be quantified using
a photo chemical probe reaction called the photo-Fries reaction. The reaction scheme for this reaction
can be found in Figure 4, and involves the irradiation of a sample in order to create geminate radical
pairs that can recombine is different ways based on the strength of the solvent cage. This reaction will
be done with an internal standard and analyzed by gas chromatography/mass spectrometry (GC/MS)
in order to quantify the product ratios and there for the media’s cage effect. This work included
synthesizing the starting materials needed for the photolysis reaction, the photo-Fries reaction. The
starting materials are a series of naphthyl based esters as shown in Figure 2. These Esters then
needed to be purified using different purification techniques, but mostly a technique called column
chromatography was used. Once believed to be pure the products had to be analyzed for purity using
several techniques in order to accurately determine the quality of the starting materials. The starting
materials must be highly pure because the reaction they will be used in is a photochemical reaction
meaning there cannot be any optical interferences. They must also must be highly pure because any
impurities will change the chemical environment and can lead to deviations in data.
Figure 2. Naphthyl Esters that will be used during the photolysis reaction and the later part of the
experiment.
The starting materials were synthesized following a common procedure and the procedure is the same
for all starting materials. The reactions procedure involves reacting equal amounts of naphthol with its
corresponding acid chloride in basic, dark and cold conditions for varying amounts of time. The basic
solution mentioned is a 5% sodium hydroxide solution for all reactions. 1-naphthyl benzoate was
synthesized from the reaction between 1-naphthol and benzoyl chloride. The reaction needed to take
place over forty-eight hours in order to obtain tangible product. 2-naphthyl benzoate was made by
reacting 2-naphthol with benzoyl chloride and this reaction took place over a few hours. 1-naphthyl
pivalate was synthesized by reacting 1-naphthol with pivaloyl chloride. This reaction was set stirring for
twenty-four hours to obtain product. The 2-naphthyl pivalate was made by reacting 2-naphthol with
pivaloyl chloride and this reaction only needed a few hours to go to completion. In all cases naphthol
was first fully dissolved in the solvent and then the acid chloride was added dropwise.4
ICIC
IC
IC
IC
OCIf R = CH3,
Methylnaphthalene
R
R = CH3 (acetate)
C(CH3)3 (pivalate)
C6H5 (benzoate)
(CH2)11CH3 (myristate)
R
R
R
R
R
R-R
OC
R
R
R
R
Figure 4. Reaction scheme for a typical photo-Fries reaction showing the in-cage (IC) and out-of-cage
(OC) products formed upon photolysis of a generic 1-naphthyl ester.
Compound Melting Point (oC) Lit. Melting Point (oC)
1-naphthyl pivalate 36.5-37.7 35.5-36
2-naphthyl benzoate 106.6-106.9 106-108
2-naphthyl pivalate 62.3-64.0 65-66
Compound Retention Time (min.) Identity
1-naphthyl pivalate 19.331 1-naphthyl pivalate
2-naphthyl pivalate 19.45 2-naphthyl pivalate
2-naphthyl benzoate 10.169 2-naphthyl benzoate
19.336 ?