Question

Give reasons for the following : (b) p-Dichlorobenzene has higher melting point than those of o- and m-isomers.

Hint:

In aromatic isomers, greater molecular symmetry generally results in better crystal packing and hence a higher melting point.

Answer 1

Step 1: Melting point depends on crystal packing efficiency.
A higher melting point reflects a more stable crystal lattice. Lattice stability depends on how well molecules pack in the solid state; better packing gives stronger intermolecular forces and a higher melting point.
Step 2: p-Dichlorobenzene is the most symmetric isomer.
In the para isomer the two Cl atoms sit at positions 1 and 4, directly opposite on the ring. This high molecular symmetry allows molecules to stack very uniformly and efficiently in the crystal lattice, maximising intermolecular contact.
Step 3: o- and m-isomers pack less efficiently.
In o-dichlorobenzene and m-dichlorobenzene the Cl atoms are on adjacent or alternate ring positions, making the molecules asymmetric. This asymmetry reduces packing efficiency, weakens the crystal lattice, and results in lower melting points than the para isomer.