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Chapter I: Classification of Solvents
Solvent molecules can be inorganic or organic in nature, and consequently they vary widely both
in physical and chemical properties. It is thus an extremely difficult task to categorize solvents.
Many attempts have been made to classify solvent molecules based on i) chemical constitution,
ii) physical properties, chemical reactivity, polarity and so on.
The solvents based on chemical constitution, i.e., characteristic chemical bonds, can be classified
as follows:
a. molecular liquids
b. ionic liquids
c. atomic liquids
Molecular liquids
All organic solvents: aliphatic and aromatic hydrocarbons and their halo and nitro derivatives,
alcohols, carboxylic acids, carboxylic esters, ethers, ketones, aldehydes, nitriles, amines,
sulphoxides, sulphones belong to this group. The study of their properties reveals that they
always prefer to solvate molecules of related functional groups leading to the general rule that:
like dissolves like. Besides, proper knowledge of their properties derived from functional groups
present in them help us to choose proper solvent for a particular reaction. For example, if we
were to carry out a condensation reaction, one would not choose a solvent such as acetone which
itself has the potential to participate in such reactions.
Use of water as a molecular liquid currently has found renewed interests due to its
environmentally benign attributes and pronounced potential to solvate apolar solutes through
hydrophobic hydration. Perfluorohydrocarbons that are nonpolar, hydrophobic, chemically inert
and non-toxic are mostly used for spectroscopic measurements and various kinds of organic
reactions. They are known to form biphasic systems. With differential solubilities for educts,
reagents, products, and catalysts, biphasic solvent systems with a “fluorous phase” can facilitate
the separation of products from the reaction mixtures.
Liquid crystals which are normally long, rigid, flat molecules and have the following general
structure:
R2
R1
Bridge
Common central bridges
CH2
CH 2
CH
C
CH
C
R- , RO- , HO- , R-CO-O-,
R-O-CO-O-, H2N-,
R-O-(CH2)n-O-, O2N-,
Cl-, Br-, I-
COO
N
Common terminal substituents
N
R
The degree of ordering present in a liquid crystal is in general in between that of isotropic liquids
and crystals, and is dependent on the nature of bridge and terminal substituents. Liquid crystals
are further classified into lyotropic and thermotropic categories depending on the nature of
origin. If the liquid crystal is generated by controlling the concentration of the amphiphilic solute
in a polar solvent, then it is called lyotropic liquid crystal. On the other hand, if the variation of
temperature leads to generation of a liquid crystal, it is called thermotropic liquid crystal.
Thermotropic liquid crystals are further classified as nematic, smectic, and cholesteric liquid
crystals depending upon how the molecules are arranged.
Ionic liquids
In recent times, ionic liquids have become increasingly important as solvents for applications in
organic and inorganic reactions. High thermal stability, excellent electrical conductivity, large
liquid range (providing a possibility of using high reaction temperature), low vapor pressure
along with their excellent capability to dissolve wide range of organic compounds, metals and
salts make them extremely useful reaction media. Dues to their very low vapor pressure, they are
environmentally benign, and can be recycled easily. Since they are viscous in nature, they have
excellent ability to stabilize transition states in organic reactions. Examples of ionic liquids are:
R3
N
1
R
N
R
BF4
R4NBr
,
,
N
PF6
,
etc.
R2
Atomic liquids
Liquid atoms with potential of being used as a solvent falls under this category. Liquid mercury
and sodium have been used as atomic solvents in the early stages of evolution of chemistry. It is
not difficult to believe that they were used with the intention of studying something new in those
media. Unless situation demands, they are not used as solvents simply because of the hazards
associated. Consequently, atomic liquids have not received much attention from the chemists.
Solvent effects on acid-base equilibria
Solvents play an important role in determining the acid-base equilibria not only due to their own
acidic/basic nature, but also due to the effects they can exert by way of solvation. We say that
benzoic acid is a stronger acid than acetic acid. There is an inherent element of incompleteness in
the statement, unless we specify the solvent. The true statement should be “benzoic acid is
stronger acid than acetic acid in water”. This is because they ionize to different magnitudes to
give varying amounts of the active acid species, namely the hydronium ion. However, the
ionization may become equal when the solvent is changed from water to say, ammonia.
Ammonia is sufficiently basic in nature so that both the acids are completely ionized and they
appear equal in their acidities in ammonia. This simple phenomenon illustrates how important
the role of solvents could be in controlling the position of acid-base equilibria.
Gas phase acidities and basicities of organic compounds should be different. This is because the
gas phase acidity or basicity reflects the intrinsic properties of molecules, whereas in the solution
phase acidity or basicity reflects the combined effect of molecule and solvent, their nature of
interaction and influence of solvent on the molecule.
In gas phase the basicities of substituted amines run as: R3N > R2NH > RNH2 > NH3. This is
expected considering the inductive effect (+I) of the methyl group. However, the effect of
solvent becomes vivid when we consider the basicity order of the same species in aqueous
medium. It runs as: NH3 < RNH2 < R2NH > R3N. If the inductive effect was the sole factor, then
on the basis of same token the ordering should have been exactly similar. However that it has
been reversed between R2NH and R3N indicates that solvent molecule i.e. water is playing its
role. Actually there is a competition between inductive effect and salvation. When methyl group
is introduced the inductive effect is, no doubt, increased; however there is a loss of energy in
terms of loss of one hydrogen bonding as one hydrogen atom is replaced by alkyl group.
R
R2NH + H3O
R
N
H
H
O
H
H
O
H
H
R
R 3N
+ H3 O
R
N
R
H
H
O
H
As the above scheme shows when there is lesser number of alkyl group there is more
stabilization of the reactive acid viz. ammonium ion due to greater solvation by hydrogen
bonding with solvent water molecules.
Similar example is the following:
In gas phase:
NH2
N
NH3
In solution phase:
NH2
N
NH3
A similar explanation is applicable to over here also. However the gas phase basicity needs little
clarification. The incipient ammonium ion would be more stable if there are alkyl groups which
sort of shield the nitrogen center and provide electron density by +I effects.
The solution and gas-phase acidities of C-H bonds are interesting. Let us have a look at those.
In gas phase the following is the order of C-H acidities:
CH4 < H2O < CH3OH < C6H5CH3 < HCCH < CH3SOCH3 < CH3CN < CH3COCH3 < CH3CHO
< C6H5COCH3 < CH3NO2 < Cyclopentadiene < CH3COOH.
This is worth noting that toluene is more acidic than water in gas phase. Thus if add OH- to
toluene we expect acid base reaction to get water and C6H5CH2-. This is rather surprising at a
first glance.
The gas phase acidity order of the haloacetic acid X-CH2COOH is H < F < Cl < Br < I, i.e. the
order is reversed compared to that in aqueous solution. Thus it can be concluded that the wellknown aqueous acidity order of the above acids is not caused by the increasing inductive effect
of the substituent, as is generally assumed, but rather by solvation effects.
α-Amino acids are known to exist in zwitterionic form. Actually there is equilibrium between
neutral and zwitterionic form. However, the gas-phase acidity measurement data show that these
species exist in neutral form. This means that it is not the intramolecular acid-base reaction
between amine and carboxylic acid group but the solvation that causes the amino acids to exist
zwitterionic form. Also the amount of zwitterions present in DMSO is significantly lower than
that present in aqueous solution which can be explained if we only invoke the concept of better
solvation and hence stabilization carboxylate group in aqueous medium.
Solvent effect on tautomerism
Solvents have pronounced effect on tautomeric equilibrium. This is because the different
tautomers have differential solvents in the solvent concerned and thus the shift of equilibrium in
different solvents. Here is an example:
H
O
O
O
O
OEt
A
OEt
H
O
O
O
O
B
Medium
%A
%B
Gas-phase
42.5
92
Cyclohexane
62
98
EtOH
12
85
H2O
6.5
19
DMSO
5
67
1) For both the molecules the keto form is more polar than the enol form. Thus on going
from gas phase (where auto-solvation is possible) to Cyclohexane the molecules are
separated from each other and hence auto-solvation is not possible. Thus the amount of
more polar form that is keto is decreased in a non-polar environment. Thus amount of
enol content is greater in Cyclohexane than in gas-phase.
2) The amount of enol form for B in all medium is greater than that of A because of greater
acidity of enolic proton due to presence of keto group in B instead of ester group that is
present in A.
3) In the enol form there is intramolecular hydrogen bonding that stabilizes this form in
nonpolar solvents. However if the solvent is such that it can provide the enol form the
opportunity to form intramolecular hydrogen bonding with it, then there is no reason why
the enol form would be predominant. That is why with the increase in polarity and
hydrogen bonding ability of the solvents the amount of enol form decreases for both the
compounds.
Another example is the following:
O
O
HO
O
Solvent
Enol/Keto
Diethyl ether
6.81
EtOH
1.67
Acetone
0.85
The above table also shows that with increase in solvent polarity and hydrogen bonding
capability of the solvent the amount of enol form decreases.
Hydrogen bonding is crucial in some cases to determine the relative amounts of the tautomers.
Let as consider the following two examples:
a)
HO
O
H H
Solvent
Keto form
Enol form
Chloroform
Present(exclusive)
absent
Water
present(exclusive)
absent
Methanol
present
Present
1,4-dioxane
present
present
DMSO
present
present(exclusive)
In chloroform and water no enol form is detectable but in solvents such as DMSO and
acetonitrile the enol form has been found to be exclusive. This immediately calls for hydrogen
bonding which is the most important factor here to control these relative amounts. The enol form
is hydrogen donor and thus solvents which are hydrogen acceptors would stabilize maximum and
thus the enol form is exclusive in acetonitrile and DMSO. But in solvents like water, chloroform,
dichloromethane which are hydrogen bond donors would preferentially stabilize the keto form, a
hydrogen bond acceptor.
b)
O
Solvent
Keto
Gas phase
exclusive
Iso-octane/benzene
exclusive
OH
Enol
DMF
exclusive
Pyridine
exclusive
DMSO
exclusive
Similar arguments hold here also. The keto form which is polar as well as hydrogen bond
acceptor will be predominant in nonpolar and in hydrogen donor solvents. Conversely, the enol
form will be favored if the solvent is hydrogen acceptor. Thus we see in DMF, Pyridine, DMSO
the enol form is exclusive. Also the addition of triethylamine to benzene solution of 9-anthrone
leads to a gradual shift of equilibrium towards the enol form supporting our point of view.
There are numerous such examples and it is worth mentioning that the effects of solvents are
not limited to keto-enol tautomerism described above but can be extended to several other
systems. Here is a concise listing of few of them:
i)
N
OH
N
H
O
N
H
O
Here it has been found that increase in solvent polarity shifts the equilibrium towards right i.e.
the pyridine-form. This is because this form is more dipolar than the hydroxyl-form due to the
contribution from the charge separated mesomeric form. Also hydrogen bonding plays a crucial
role in a manner as described earlier.
ii)
N
N
N
H
N
The left one is the colorless form and the one in the right is colored. In presence of hydrogen
bond acceptor solvents the color intensity increases. This means that the equilibrium shifts
towards right. Similarly, in hydrogen bond donor solvents the solution becomes colorless; the
equilibrium stays in favor of the species in the left.
Solvents effect on conformational equilibria
Solvents can also influence the conformational existence of a molecule. We know that a
molecule can exist in several conformations that lie on plateaus of an energy profile diagram. If
the solvation energies with different conformations are largely different and are of the order of
the separation of energy barrier of different conformations then solvents show huge impact on
the relative amounts of the different conformers. This is why it has been often observed that one
conformer is present predominantly in one solvent but not in another.
Example 1:
H
Cl
H
O
O
Cl
H
H
H
(µ≈0)
H
(µ=+ve)
Thus we expect that in a more polar solvent the conformer with higher dipole moment will be
more stable. The following table containing the data supports the expectation.
Solvent
µ=+ve conformer (%)
Cyclohexane
44
Benzene
58
Acetone
72
DMF
78
DMSO
84
Example 2:
N
N
(A)
(B)
For the conformational equilibrium in the phencyclidine system as shown above we have the
following information:
Solvent
Major Conformer
[D2]dichloromethane
A
CD2Cl2/CD3OD (1:2)
B
Ratio (B:A)
99:1
CD3COCD3
1:1
CD3CN
1:1
From the steric point of view both A and B is of similar energy. But a simple qualitative
treatment gives the feeling of (A) being more stable from stereoelectronic considerations. The CH bond being antiperiplanar to the polar C-N bond and it’s sort of extended anomeric effect.
Thus in dichloromethane and other non-polar solvents (A) is the major conformer. However, if
there is possibility of hydrogen bonding the scenario can change a lot. When the equilibrium is
studied in a HBD (hydrogen bond donor) solvent the equilibrium is far towards (B). This is
because hydrogen bonding increases the crowding around N center. Thus the conformer with
axial piperidine ring will become more destabilized as compared to other one and thus (B)
becomes almost exclusive in CD2Cl2/CD3OD (1:2). Again in CD3COCD3 and CD3CN which are
polar but non-HBD solvents we have almost equal population of two conformers.
Solvent Effects on Electron‐Transfer Equilibria CH 2 OOEt
COOEt
2
COOEt
+
N
N
N
Et
Et
Et
For the above equilibrium we have the following information:
Solvent
Ion pair/Radical
DMF
4 x 10 -13
Acetonitrile
10 -13
Methanol
10 -8
Water
5 x10 -5
As expected, the equilibrium shifts towards right with the increase in solvent polarity. More
polar solvents result in better solvation of the ion pair as compared to the neutral radical species.
Solvent Effects on Isomerization Equilibria
Valence isomers are isomers that are related by simple reorganization of bonding electrons
without migration of any atoms. They have different properties since they have different
structures and thus we can expect that solvents will have an impact in their equilibrium. Here is
one example:
N3
S
N
S
N
N N
N
The isomer in the left has dipole moment smaller (µ = 5.2 x 10 -30 Cm) than the one in the right
(µ = 13.7 x 10 -30 Cm). In gas phase and in other non-polar solvents such as carbon tetrachloride
and benzene the left isomer is the major and stable isomer. However in polar solvents such as
DMSO and HMPA the bicyclic valence isomer is the dominant species.