Why does p-dichlorobenzene have a higher melting point than its o- and m-isomers?

p-Dichlorobenzene has a higher melting point compared to its o- and m-isomers primarily due to differences in molecular symmetry and packing efficiency.

In p-dichlorobenzene, the two chlorine atoms are positioned opposite each other (para position) on the benzene ring. This arrangement allows for the formation of more efficient packing in the solid state. The molecules can pack together more closely in an organized manner, leading to stronger intermolecular forces such as van der Waals forces and dipole-dipole interactions between adjacent molecules. As a result, more energy is required to overcome these forces, resulting in a higher melting point.

On the other hand, in o-dichlorobenzene and m-dichlorobenzene, the chlorine atoms are positioned at the ortho and meta positions, respectively. These isomers have less efficient packing arrangements due to steric hindrance caused by the proximity of the chlorine atoms. As a result, the intermolecular forces are weaker, and less energy is required to overcome them, resulting in lower melting points compared to p-dichlorobenzene. Additionally, the spatial orientation of the chlorine atoms in o- and m-isomers results in less favorable interactions between molecules, further contributing to their lower melting points.