Hydrogen bonding in water.

A preoccupation in organic chemistry is how the electronegativity difference of bonded atoms leads to polarities which in turn determine the degree of intermolecular force. The carbonyl group of aldehydes and ketones is a prototypical organic functional group with dipole-dipole intermolecular forces leading to respectably strong intermolecular attractions.

Even stronger intermolecular forces are produced, however, with hydroxyl group or amine group, where the positive pole is hydrogen. With large electronegativity differences between bonded atoms, the resulting dipoles lead to strong intermolecular forces, especially if the positive pole is hydrogen, in which case the resulting intermolecular force is called a hydrogen bond.

Because the intermolecular forces corresponding to hydrogen bonding are stronger, a primary or secondary alcohol will be higher boiling than a similar aldehyde or ketone, respectively, which will in term be higher boiling than a similarly sized alkane, which will have only weak Van der Waals intermolecular forces (caused by temporary induced dipoles).

The stronger the intermolecular forces, the more energy is required to overcome the deep potential energy well of mutual attraction and lead to vaporizaton. In summary, electronegativity difference leads to bond polarity, which, in turn, leads to intermolecular force. Intermolecular force, in turn, plays a role in determining physical properties such as boiling point.