Chemical processes cannot exist without
intimate involvement of solvent in synthesis and separation steps. Because of
this reason, choosing the right solvent is important in most organic
reactions; thus, the search for new solvent with wide-ranging suitability is an
on-going research topic in Organic Chemistry. With growing concern over
green chemistry, carbon dioxide has stepped into the limelight because of
several characteristics. For example, carbon dioxide is inexpensive, nontoxic,
harmless to living things, easily disposable, nonflammable, and chemically
inert under many conditions. Furthermore, it is liquefiable at reasonable
pressure so that these properties make carbon dioxide suitable for use as a
solvent in organic chemistry (2). Researchers continued to study the liquid
form of carbon dioxide as a solvent. Many labs pressurize carbon dioxide,
converting it into a near-liquified form called supercritical fluid carbon
dioxide. They then determine whether supercritical carbon dioxide is appropriate
or not as an organic solvent by measuring solubility of several compounds in
supercritical carbon dioxide solvent.
While carbon dioxide, especially supercritical
carbon dioxide, turned out to be a suitable organic solvent for some non-polar
and polar molecules with low molecular weight, it is still a poor solvent for
most high molar mass molecules under mild conditions (6). It corresponds to its
relatively inert characteristic as a chemical reactant due to its non-dipolar
property. Consequently, researchers often use Lewis basic substrates, catalysts,
or metal-based reagents to increate the rate of carbon dioxide incorporation.
Supercritical
Carbon Dioxide
A supercritical
fluid is a substance in which both temperature and pressure are above the
critical values. The critical point of carbon dioxide is at 304K and 73 atm.
Above this point, carbon dioxide exists only in the supercritical fluid form
and behaves as a gas with higher density than liquid carbon dioxide (ca. 0.47 g
cm-1). Supercritical carbon dioxide can also behave as a good
solvent; this is due to its special combination of gas-like properties which
have important implications for reaction kinetics such as higher diffusivity,
lower viscosity, and lower surface tension, and liquid-like density that allow
solvation of many compounds. As mentioned before, its conditions (304K and
73atm) can easily be attained, and can be simply removed by depressurization.
Supercritical carbon dioxide keeps all the advantages of carbon dioxide; furthermore,
as it shows enhanced diffusivity, viscosity, and surface tension over
gas/liquid carbon dioxide, supercritical carbon dioxide seems to be a future as
a solvent.
Figure 1. phase diagram
Carbon
Dioxide Behaves like Hydrocarbon Solvent
Some of the solvent molecules with a zero dipole
moment as a result of molecular symmetry and correspondingly low dielectric
force have significant bond dipoles with resulting multipole moments due to
their charges on individual atoms. This system leads to a solvation of
compounds with multipole-dipole interactions and multipole-induced dipole
interactions and works as a non-dipolar solvent (7).
Figure 2. (7). Bond dipoles and the atomic charges
Carbon dioxide is one of the great example of these
molecules. Even though carbon dioxide has a zero dipole moment, it is
considered a “charged-separated molecule with significant nonzero bond dipole
moments.” This charge separation leads to a quadrupole moment. As a result,
carbon dioxide can be regarded as a quadrupolar solvent (7). Consequently, carbon
dioxide often acts as a non-polar solvent that can occasionally dissolve polar
molecules as well.
Therefore, John A. Hyatt extensively studied the
behavior of supercritical carbon dioxide as a non-polar solvent. In his paper,
he presented all previous results from preceding studies with his own research
to show where carbon dioxide should be assigned among conventional organic
solvents (2). His paper generalized the results into several ideas:
(1) Supercritical carbon dioxide behaves like a hydrocarbon solvent.
(2) Supercritical carbon dioxide does not interact strongly with bases
such as anilines, pyrroles, pyridines as a solvent.
(3) Supercritical carbon dioxide is a good solvent for aliphatic
hydrocarbons up to twenty carbons. Most small aromatic hydrocarbons and some of
the polycyclic hydrocarbons show substantial solubility.
(4) Halocarbons, aldehydes esters, ketones, and low alcohols are freely
soluble in supercritical carbon dioxide.
(5) Polar compounds such as amides show poor solubility in supercritical
carbon dioxide.
(6) Molecules with more than 500 molecular weights are mostly not soluble
in supercritical carbon dioxide.
Based on these observations, supercritical carbon
dioxide was proven to behave as a hydrocarbon solvent.
In following study, to observe solvent effects on
aldehyde and ketone C double bond O (C=O) stretching frequencies, Hyatt
obtained IR spectra of acetone and cyclohexanone in liquid and supercritical carbon
dioxide. Both of these tables showed similar results. C double bond O (C=O)
stretching frequencies of acetone and cyclohexanone in all solvents showed similar
values. This data suggested that supercritical carbon dioxide exhibited similar
behavior as both non-polar hydrocarbon and halocarbon solvents (2).
Based on the results, supercritical carbon dioxide
exhibits behavior comparable to hydrocarbon solvents. While it is a good
solvent for many non-polar low molecular weight compounds and fluoropolymers, carbon
dioxide is not as good of a solvent for high molecular weight polymers.
Example
of a Supercritical Carbon Dioxide as an Organic Solvent
For further characterization, Jefferson and his
research team conducted an experiment measuring the rate of a Diels-Alder
reaction between N-ethylmaleimide and 9-hydroxymethylanthracene in supercritical
carbon dioxide to compare its behavior to the reaction conducted in
hydrocarbon solvents. As expected, the reaction showed dramatic rate
acceleration in supercritical carbon dioxide solvent. It was nearly twenty-five
times faster than that measured in acetonitrile. This dramatic acceleration
demonstrated that using supercritical carbon dioxide as a solvent could have
significant advantages along with its well-known environmentally-friendly
features (8).
Figure 3. (8) Reaction of Diels-Alder cycloaddition
of 9-hydroxymethyl-anthracene and N-ethylmaleimide showed dramatic rate
accelerations in supercritical carbon dioxide
Reactivity
of Carbon Dioxide: Inert Characteristic as a Reactant
While carbon dioxide can be regarded as a good
solvent for most non-polar and some polar molecules of lower molar mass, it is
a poor solvent for most high molar mass molecules under mild conditions (6).
This means that we cannot ignore any of its underlying features that contribute
to its low toxicity. Ease of handling is based on its relatively inert
characteristic as a chemical reactant. As a result, employing carbon dioxide as
a reagent is typically limited to Lewis basic substrates that display
sufficient nucleophilicity to react with the poorly-electrophilic carbon
dioxide. Also, metal-based reagents are often used to increase the rate of carbon
dioxide incorporation (9). For example, carbon dioxide with its two Lewis basic
oxygen atoms can be activated with bidentate catalyst, because the strong
bidentate interaction fixates the carbon dioxide molecule to enhance the
electrophilicity of the carbon atom. This activated carbon can be used for the
conversion of carbon dioxide to methane or methanol (10).
Figure 4. (10) Activation of carbon dioxide by a
bidentate catalyst for the conversion to methane or methanol.
Also, by using a metal-based system, five-membered
cyclic carbonates can be generated with an insertion of carbon dioxide into
activated epoxides (9).
Figure 5. (9) Metal-based system including carbon
dioxide to increase the rate of carbon dioxide incorporation.
Capture
of Carbon Dioxide with Amine
One of the most important reactions concerning
carbon dioxide is carbon dioxide capture from the air by using amines. As the
consumption of fossil fuels has increased, accumulated carbon dioxide in the
atmosphere is one of the more pressing concerns of mankind due to its effect on
climate change and ocean acidification. Thus, carbon dioxide management is a
challenging issue of the present age. According to Loker Hydrocarbon Research
Institute and Department of Chemistry of University of Southern California, a
possible pathway of direct carbon dioxide capture from air through use of amine
was discovered. Aqueous solutions of primary and secondary amines are proved to
be used as large-scale capture carbon dioxide from industrial streams with
their ability to chemisorb acidic gases like carbon dioxide. It is great way to
recycle carbon dioxide captured from the air, which in turn can be converted
into useful materials and fuels.
Figure 6. (11) Reaction of carbon dioxide with
amines
Summary
Carbon dioxide exhibited similar solvent properties
of hydrocarbon solvents. It seemed to be a good solvent for many non-polar low
molecular weight compounds and fluoropolymers, but not as good for higher
molecular weight polymers. Supercritical carbon dioxide, which did not show
significant difference in polarity compared to that of liquified carbon dioxide,
could be regarded as a good organic solvent due to its advantages over the carbon
dioxide with its gas and liquid-like properties. In other words, supercritical
carbon dioxide can be act as a great hydrocarbon solvent with unique properties
such as its compressibility, which can cause low surface tension and viscosity,
and low polarizabilty. However, despite its advantages, carbon dioxide is a
relatively inert molecule as a reactant, and possesses a high energy barrier,
meaning that catalysts or metal-based reagents that can increase the rate of
incorporation are required. One of the most important reactions with carbon
dioxide is the capture of carbon dioxide with amines, where carbon dioxide is
“captured” from the atmosphere and recycled into useful materials and fuels.
