Which of the following possess the highest melting point?

p-Dichlorobenzene

The answer is (4) p-Dichlorobenzene.

The melting point of a compound is affected by the symmetry of the molecule. The more symmetrical the molecule, the higher the melting point.

In the case of the dichlorobenzenes, the p-isomer is the most symmetrical. This is because the chlorine atoms are on opposite sides of the benzene ring, which makes the molecule more symmetrical.

The other isomers, the o- and m-isomers, are less symmetrical because the chlorine atoms are on adjacent carbon atoms. This makes the molecules less symmetrical and lowers their melting points.

The melting points of the dichlorobenzenes are as follows:

• p-Dichlorobenzene: 53.5°C
• o-Dichlorobenzene: 45.8°C
• m-Dichlorobenzene: 44.1°C
• Chlorobenzene: -45.58^oC

Therefore, p-dichlorobenzene has the highest melting point.