Solvent is a substance, usually a liquid, that acts as a dissolving agent or
that is capable of dissolving another substance. In solutions of solids or gases
in a liquid, the liquid is the solvent. In all other homogeneous mixtures (i.e.,
liquids, solids, or gases dissolved in liquids; solids in solids; and gases in
gases), solvent is the component of the greatest amount. The minor proportion
substances are called solutes. The solvent offers several functions during a
chemical reaction. It solves not only the substance that reacts with another one
to produce a new set of substances (reactant) but also the compound that
supplies the molecule, ion, or free radical, which is considered as the
attacking species in a chemical reaction (reagent). The solvent is conductive to
collisions between the reactants and reagents to transform the reactants to new
products. The solvent also takes roll of temperature control, either to provide
the energy of the colliding particles for speedy reaction and to absorb heat in
exothermic reaction. The appropriate solvent should be selected based on the
inactivity in the reaction conditions, dissolving the reagents as well as
reactants, appropriate boiling point and easy removal at the end of the
reaction.
Polarity
The most common solvent is water. Other common
solvents which dissolve substances that are insoluble (or nearly insoluble) in
water are acetone, alcohol, formic acid, acetic acid, formamide. BTX, carbon
disulfide, diemthyl sulfoxide, carbon tetrachloride, chloroform, ether,
tetrahydrofuran, furfural, hexane and turpentine. They may be classified as
polar and non-polar. Polar solvents, like water, have molecules whose electric
charges are unequally distributed, leaving one end of each molecule more
positive than the other. Usually polar solvent has O-H bond of which water
(HOH), (CH3OH) and acetic acid (CH3COOH) are examples.
Propanol, butanol, formic acid, formamide are polar solvents. Dipolar solvents
which contain a C-O solid bond without O-H bond are acetone [(CH3)2C=O], ethyl
acetate (CH3COOCH2CH3), methyl ethyl ketone, acetonitrile, N,N-dimethylformamide
and diemthyl sulfoxide. Nonpolar solvents, like carbon tetrachloride (CCl4),
benzene (C6H6), and diethyl ether ( CH3CH2OCH2CH3), have molecules whose
electric charges are equally distributed and are not miscible with water. Hexane, tetrahydrofuran and methylene chloride are non-polar solvents. Polar
solvents are hydrophilic but non-polar solvents are lipophilic. Polar reactants
will dissolve in polar solvents. Non-polar solvents dissolve non-polar compounds
best. Oil and water don't mix but separate into two layers. There are three
measures of the polarity as "dipole moment", "dielectric constant" and
"miscibility with water". Though low dipole moments and small dielectric
constants indicates non-polar solvents, sharp boundaries between polar and
non-polar solvents are not available. The polarity reflects the balance between
a polar component (OH) and a non-polar hydrocarbon component, existing in the
same molecule. If hydrocarbon character increases relatively, the polarity
decreases. On an operational basis, solvents that are miscible with water are
polar.
Polar Protic and Dipolar Aprotic
Protic refers to a hydrogen atom
attached to an electronegative atom. Protic solvents can donate an H+ (proton)
since they contain dissociable H+, such as hydrogen attached to oxygen as in a
hydroxyl group, nitrogen as in a amine group. Examples are water, methanol,
ethanol, formic acid, hydrogen fluoride and ammonia. Aprotic solvents don't has
O-H bond but a C=O bond typically. Examples are acetone [(CH3)2C=O] and ethyl
acetate (CH3COOCH2CH3). Polar protic solvents are useful in SN1 reaction, while
polar aprotic solvents are SN2 reaction.
Solvents |
Boiling
point C |
Dipole Moment |
Dielectric Constant
|
Density
(g/ml)
|
Polar Protic |
Water |
100 |
1.85 |
80 |
0.998
|
Methanol |
68 |
1.70 |
33 |
0.791
|
Ethanol |
78 |
1.69 |
24.3 |
0.789
|
n-Propanol |
97 |
1.68 |
20.1 |
0.803
|
n-Butanol |
118 |
1.66 |
17.8 |
0.810
|
Formic acid |
100 |
1.41 |
58 |
1.21
|
Acetic acid |
118 |
1.74 |
6.15 |
1.049
|
Formamide |
210 |
3.73 |
109 |
1.134 |
Polar Aprotic |
Acetone |
56 |
2.88 |
20.7 |
0.786
|
Tetrahydrofuran |
66 |
1.63 |
7.52 |
0.886
|
Methyl ethyl ketone |
80 |
2.78 |
18.5 |
0.805
|
Ethyl acetate |
78 |
1.78 |
6.02 |
0.894
|
Acetonitrile |
81 |
3.92 |
36.6 |
0.786
|
N,N-Dimethylformamide |
153 |
3.82 |
38.3 |
0.944
|
Diemthyl sulfoxide |
189 |
3.96 |
47.2 |
1.092
|
Non-Polar |
Hexane |
69 |
- |
2.02 |
0.655
|
Benzene |
80 |
0 |
2.28 |
0.879
|
Diethyl ether |
35 |
1.15 |
4.34 |
0.713
|
Methylene chloride |
40 |
1.60 |
9.08 |
1.326
|
Carbon tetrachloride |
76 |
0 |
2.24 |
1.594 |
|