The correct order of melting points of dichlorobenzenes is

Solution

Chemistry ma’am · 4 steps

Melting is about how well molecules pack in the crystal, and the symmetric para isomer packs best.

1. What melting needs. To melt a solid you must break up its crystal lattice. A molecule that fits neatly into the lattice holds on harder, so it melts at a higher temperature.
2. Para packs best. In p-dichlorobenzene the two Cl atoms sit opposite each other, so the molecule is highly symmetric. It stacks closely in the crystal and has by far the highest melting point.
3. Ortho beats meta. Neither ortho nor meta packs as well as para. Ortho is more polar than meta, because its two C–Cl dipoles point more nearly the same way. Its stronger dipole–dipole attractions give it a slightly higher melting point.

   Isomer	Symmetry and packing	Dipole moment	Melting point
   para (1,4)	highest, packs closely	zero	about 53 °C
   ortho (1,2)	lower	largest	about -17 °C
   meta (1,3)	lower	smaller than ortho	about -24 °C

4. So. The order is para > ortho > meta, which is option (a).

The idea to keep. For melting points of disubstituted benzenes, the para isomer wins because its symmetry lets it pack tightly in the crystal. Do not use dipole moment alone to rank melting points.

Answer from the NTA final answer key. Solution by PrepWiser.