Question

$p$-dichlorobenzene has higher melting point than those of $o$- and $m$-isomers. Discuss.

Answer 1

$\text{Melting point comparison}$

Melting point depends on the crystal lattice structure.


Here we can see that $p$-dichlorobenzene is more symmetrical than $o$- and $m$-isomers and due to this it fits more closely than $o$- and $m$- isomers in the crystal lattice.


Therefore, more energy is required to break the crystal lattice of $p$-dichlorobenzene.

As a result, $p$-dichlorobenzene has a higher melting point than $o$-dichlorobenzene and $m$-dichlorobenzene.