Why is the melting point of p-dichlorobenzene higher than those of o-dichlorobenzene and m-dichlorobenzene?

Question

I was going through alkyl and aryl halides and noted the following statement in my textbook: > _p_-dichlorobenzene has higher melting point and solubility than those of _o_-dichlorobenzene and _m_-dichlorobenzene. I thought over it but could not come to a conclusion. Why is the above assertion true?

Answer 1

This is due to the inter-molecular forces within the different isomers. In the para-dichlorobenzene, the molecule is symmetric and fits perfectly in a crystal lattice, allowing the molecule to have stronger molecular forces.

With the other isomers, they are asymmetric and don't fit perfectly within a crystal lattice. As a result, they have comparable weaker inter-molecular forces.

As the boiling point is determined by how much heat energy is needed to overcome the inter-molecular forces, p-dichlorobenzene with the stronger forces requires more heat energy to break its bonds. Therefore, it has a higher melting point than the two isomers.