The melting points of the two constitutional isomers are as follows: ortho-hydroxy acetophenone: 4-6 °C; para-hydroxy acetophenone: 109-111 °C. Explain the large difference in melting points exhibited by these two compounds.

Hydrogen bonding is a special type of the dipole-dipole interaction and it occurs between hydrogen atom that is bonded to highly electronegative atom which is either fluorine, oxygen or nitrogen atom.
Partially positive end of the hydrogen atom is attracted to partially negative end of these atoms which is present in another molecule. It is strong force of attraction between the molecules.

The bonding can be intramolecular and intermolecular. ortho-hydroxy acetophenone can form intramolecular hydrogen bonding which means that hydrogen bonding exist within the same molecule. On the other hand, para-hydroxy acetophenone forms intermolecular hydrogen bonding in which the bonding is in between the different molecules. Hence, more amount of energy is required to melt these molecules in case of para-hydroxy acetophenone.