How do boiling points relate to polar bonds?
Boiling point increases with increasing strength of polar bonds, because more energy is required to break the stronger bonds.
There are 4 main types of polar intermolecular bonds:
The strongest polar bonds, these form between a hydrogen atom, and an electronegative acceptor atom.
The most common example of hydrogen bonds are those found in water, between hydrogen and the electronegative oxygen atom:
Permanent dipole - Permanent dipole bonds
These bonds form between two molecules possessing permanent dipoles a.k.a polar molecules. Usually the bond forms between an electronegative (𝛿-) atom, and a more electropositive (𝛿+) atom on a neighbouring molecule.
For example, in hydrochloric acid there is a permanent dipole-permanent dipole bond between the 𝛿- chlorine, and 𝛿+ hydrogen on a neighbouring molecule:
Permanent dipole - Induced dipole bonds
These bonds form between a polar molecule, and non-polar molecule.
When a polar molecule moves close to a non-polar molecule, the high electron density present at the 𝛿- end repels electrons in the neighbouring molecule. This causes an uneven distribution of electrons in the neighbouring molecule, leading to a 𝛿- and 𝛿+ end.
The, now polar, molecule, can now form an intermolecular bond with the original polar molecule.
Instantaneous dipole - Induced dipole bonds
Also known as London Dispersion forces, these bonds are formed in every species, regardless of polarity. They are the weakest polar bond.
They occur when there is a random uneven distribution of electrons in a molecule, leading to 𝛿+ and 𝛿- ends. When this polar molecule moves close to another molecule, it induces a dipole, and the polar bond is formed.
How are these different bonds related to boiling point?
When a liquid boils, the intermolecular bonds are broken, and the molecules are dispersed in the gas phase. Breaking these bonds requires energy, provided by heat.
A stronger intermolecular bond requires more energy to break, and a higher temperature is required to provide this energy. Thus, molecules with stronger intermolecular bonds have higher boiling points.
Boiling point decreases in the order:
Hydrogen bonds > Permanent dipole - Permanent dipole > Permanent dipole - Induced dipole > Instantaneous Dipole - Induced Dipole