The melting point of p-isomers of dichloro benzene is higher than that of o- and m-isomers. This is because, p-isomer has symmetrical structure and therefore, its molecule can easily pack closely in crystal lattice. Hence, it has stronger intermolecular forces of attraction than o- and m- isomers. Therefore, greater energy is required to break the intermolecular forces to melt or dissolve the p-isomer than the corresponding o- and m- isomers. In other words, the melting point of p-isomer is...

Nucleophilic substitution in haloarene is more difficult than haloalkane because the C-X bond acquires partial double bond character due to resonance and carbon atom holding halo group in haloarene is sp² hybridized which is relatively more electronegative than sp³ hybridized carbon of haloalkane and makes C-X bond less polar.