Which of the following possesses the highest melting point?

p-dichlorobenzene

The melting point of a compound is determined by the strength of the intermolecular forces between the molecules. The stronger the intermolecular forces, the higher the melting point.

In the case of dichlorobenzenes, the para isomer (p-dichlorobenzene) has the highest melting point. This is because the p-isomer is more symmetrical than the ortho and meta isomers. Symmetry leads to more compact molecules, which can pack more tightly together in the crystal lattice. This results in stronger intermolecular forces and a higher melting point.

The melting points of the dichlorobenzenes are as follows:

• p-dichlorobenzene: 53.5 °C
• o-dichlorobenzene: 45.8 °C
• m-dichlorobenzene: 49.2 °C

Therefore, the correct answer is (4) p-dichlorobenzene.