Factors Affecting Boiling Points of Organic Compounds

The phase at which the attractive forces in liquids are completely broken for the liquid to
become gaseous is known as the boiling point. Boiling points differ based on the strength of
the attractive forces in the molecules. Four types of intermolecular forces hold most of the
organic substances and these are: Ionic forces, hydrogen bonding, dipole-dipole moments and
Van der Waals dispersion forces. According to Mumford and Phillips (1950 p.75-84), an
increase in polarization of the molecule consequently leads to an increase in boiling points
resulting from stronger intermolecular associations. The highest boiling points are in
compounds held by strong ionic bonds, followed by molecules held by relatively strong
hydrogen bonds, followed by those held by dipole-dipole interactions and lastly the
molecules head by weak Van der Waals dispersion forces have the least boiling points.
The loss or gain of electrons results in the formation of charged particles held together by
strong ionic bonds. The positively charged ions which are mostly metals like magnesium
(Mg+2) and aluminium (Al+3) are called cations whereas, the negatively charged ions which
are mostly non-metals like oxygen (O-2) and fluoride (F-) are called anions. Coulomb, in his
law describes the attraction between opposing forces which increases with an increase in the
charge density and decreases when the distance separating the ions is increased. According to
Mumford and Phillips (1950 p.75-84 ), ionic bonded molecules have very high boiling
points as a result of their very high polarization. In addition to this, these molecules are very
soluble in water.
According to Jim Clark (2013), the measure of the tendency of an atom to attract a bonding
pair of electrons is what is known as electronegativity. Various elements like fluorine,
oxygen, nitrogen and chlorine are very electronegative and hence end up drawing electrons

from the bonded atoms near them, creating temporary charges on these atoms. The most
electronegative atom acquires a temporary negative charge, while the lesser electronegative
atom acquires a temporary positive charge. The hydrogen proton which is positively charged
therefore gets attracted to these negatively charged atoms in solution, forming the hydrogen
bond. These bonds are transient and hence, are short-lived as a result of the rapid molecular
motion in the solution. Their strength is relatively high, about 9 kJ/mol, and hence, they need
huge amounts of energies to break. Hydrogen bonds become stronger with an increase in
electronegativity of the bonded atoms.
Besides the attraction of unlike charges, dipoles too can rearrange themselves in an attractive
manner creating the Van der Waal dipole-dipole interaction. These bonds are weaker than
hydrogen bonds as they are carbon-hydrogen bonds. The carbon has lower electronegativity
as compared to the halogens and nitrogen and hence, polarity of the molecule is reduced.
Since dipole-dipole interactions are weaker than hydrogen bonds, they will require less
energy to break therefore, have a lower boiling point.
According to Mumford and Phillips (1950 p.75-84), dispersion forces are adjacent dipole
attractions in a molecule that are very easy to break on application of energy. This is
attributed to the fact that dipoles are weak and transient and are dependent on the interaction
with molecules directly. Therefore, dispersion forces increase with surface area.
Consequently, an increase in molecular weight leads to an increase with the dispersion forces
and hence increase in boiling points in molecules in the same family.
The functionality of an organic compound influences boiling points. For instance,
compounds with hydroxyl groups like the alcohols that contain hydrogen bonds have greater
boiling points than the ketones that possess the carbonyl group that contains dipole-dipole
moments.

Molecular structural isotopes differ in their molecular surface area that sequentially
influences the strength of the Van der Waals dispersion forces holding them and thus, the
boiling points of the molecules are affected Burger et al (2011 p.121). Surface area of a
molecule is directly proportional to the molecular weight of the molecule and therefore, long
chain molecules with greater surface area have higher molecular weights, consequently, have
higher boiling points due to the increase in the strength of the bonds. In the process of
increasing surface area, the attraction ability of the molecules is also increased.
Table showing how the functional groups affect boiling points. Haynes, (2014 p.87).
The dispersion forces are weakened as a result of a decrease in the surface area of molecules
that are branched structurally. This explains why branched organic compounds have lower
boiling points.
Figure showing how branching of organic compounds affects the boiling points Haynes,
(2014 p.132)