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

Concept: The melting point of an organic compound depends largely upon the efficiency of packing of its molecules in the crystal lattice. More symmetrical molecules generally pack more efficiently and possess stronger intermolecular attractions in the solid state. As a result, such compounds usually have higher melting points.

Step 1: Examine the structures. The three isomers are: \[ o\text{-Dichlorobenzene} \] \[ m\text{-Dichlorobenzene} \] \[ p\text{-Dichlorobenzene} \] Among these, para-dichlorobenzene possesses the most symmetrical structure.

Step 2: Crystal packing. Because of its symmetrical arrangement, p-dichlorobenzene molecules fit together more efficiently in the crystal lattice. This leads to:

• Better packing.

• Greater lattice stability.

• Stronger intermolecular attractions.

Step 3: Compare with ortho and meta isomers. The ortho and meta isomers are comparatively less symmetrical. Their molecules cannot pack as efficiently in the crystal lattice. Consequently, the crystal structure is less stable.

Step 4: Conclusion. Since p-dichlorobenzene exhibits maximum symmetry and efficient crystal packing, it has a higher melting point than the ortho and meta isomers. \[ \boxed{\text{p-Dichlorobenzene has the highest melting point due to its symmetrical structure and efficient crystal packing.}} \]