Why do para-isomers of dihalobenzenes have higher melting points compared to their ortho- and meta-isomers?

Para-isomers of dihalobenzenes generally have higher melting points compared to their ortho- and meta-isomers due to stronger intermolecular forces in the para-arrangement. In para-isomers, the halogen atoms are positioned opposite each other, allowing for more efficient packing of molecules in the solid state. This arrangement maximizes van der Waals forces, leading to a stronger crystal lattice and higher melting points. In contrast, the ortho- and meta-isomers have less favorable molecular packing, resulting in weaker intermolecular forces and lower melting points. The more symmetrical para-arrangement enhances the cohesive forces between molecules, contributing to the observed difference in melting points.