The p-isomer of dichlorobenzene has higher melting point than O-and M-isomer. Why ?

Explanation

The structures of three isomeric dichlorobenzenes are as follows:

The p-isomer being more symmetrical fits closely in the crystal lattice and thus has stronger intra-molecular forces of attraction than o and n-isomers. Since during melting or dissolution, the crystal lattice breaks, therefore, a larger amount of energy is needed to melt or dissolve the p-isomer than the corresponding o-and m-isomers. In other words, the melting point of the p-isomer is higher and its solubility lower than the corresponding o and m-isomers.