The vast majority of chemical reactions are performed in solution.
The solvent fulfills several functions during a chemical reaction. It
solvates the reactans and reagents so that they dissolve. This
facilitates collisions between the reactant(s) and reagents that must
occur in order to transform the reactant(s) to product(s). The
solvent also provides a means of temperature control, either to
increase the energy of the colliding particles so that they will
react more quickly, or to absorb heat that is generated during an
exothermic reaction. The selection of an appropriate solvent is
guided by theory and experience. Generally a good solvent should meet
the following criteria.
The second criterion invokes the adage "Like
dissolves like". Non-polar reactants will dissolve in
non-polar solvents. Polar reactants will dissolve in polar solvents.
For our purposes there are three measures of the polarity of a
Molecules with large dipole moments and high dielectric constants
are considered polar. Those with low dipole moments and small
dielectric constants are classified as non-polar. On an operational
basis, solvents that are miscible with water are polar, while those
that are not are non-polar; remember the saying "Oil and water don't mix".
Chemists have classified solvents into three categories according
to their polarity.
Let's start with the meaning of the adjective protic. In the
context used here, protic refers to a hydrogen atom attached to an
electronegative atom. For our purposes that electronegative atom is
almost exclusively oxygen. In other words, polar protic solvents are
compounds that can be represented by the general formula ROH. The
polarity of the polar protic solvents stems from the bond dipole of
the O-H bond. The large difference in electronegativities of the
oxygen and the hydrogen atom, combined with the small size of the
hydrogen atom, warrant separating molecules that contain an OH group
from those polar compounds that do not. Examples of polar protic
solvents are water (HOH), methanol (CH3OH), and acetic
Here the key word is aprotic. In the context used here, aprotic
describes a molecule that does not contain an O-H bond. Solvents in
this class all contain a bond that has a large bond dipole. Typically
this bond is a multiple bond between carbon and either either oxygen
or nitrogen. Most dipolar aprotic solvents contain a C-O double bond.
Examples are acetone [(CH3)2C=O] and ethyl
Non-polar solvents are compounds that have low dielecrtic
constants and are not miscible with water. Examples include benzene
(C6H6), carbon tetrachloride (CCl4),
and diethyl ether (
Table 1 presents a list of solvents that are commonly used in
chemical reactions. The boiling point, dipole moment, and dielectric
constant of each solvent is included. All of these solvents are
clear, colorless liquids. The hydrogen atoms of the protic solvents
are highlighted in red.
It should be apparent from the table that there are no sharp
boundaries between polar and non-polar solvents, at least if you use
dielectric constants or dipole moments as a measure. There is,
however, a correlation between chemical structure and dielectric
constant that provides a useful way to think about polarity. Look at
the series of polar protic compounds water, methanol, ethanol,
1-propanol, and 1-butanol. As Figure 1 demonstrates, each compound in
the series differs from the ones before and after it by one
As the number of CH2 groups in ROH increases, the
dielectric constant decreases. If you think of these molecules as
containing a polar component (OH) and a non-polar component (R), then
the polarity of a compound reflects the balance between these two
components. As the relative amount of hydrocarbon character
increases, the polarity decreases. Note that hexane, which is 100%
hydrocarbon, is the least polar solvent in the table.
Now that we've looked at the various types of solvents that you
can expect to see, let's examine how those
solvents interact with solutes.