Highest Melting Point: p-Dichlorobenzene

Explanation:
- Melting point is the temperature at which a solid substance changes into a liquid state.
- It is influenced by several factors including intermolecular forces, molecular weight, and molecular structure.
- In the given options, the highest melting point is possessed by p-Dichlorobenzene.
- Let's analyze the molecular structure and intermolecular forces for each option to understand why p-Dichlorobenzene has the highest melting point.

a) Chlorobenzene:
- Chlorobenzene has a molecular formula of C6H5Cl.
- It is a benzene ring with a chlorine atom attached.
- The chlorine atom in chlorobenzene induces a dipole moment in the molecule.
- However, the dipole-dipole interactions between chlorobenzene molecules are relatively weak, resulting in a lower melting point compared to the other options.

b) o-Dichlorobenzene:
- o-Dichlorobenzene has a molecular formula of C6H4Cl2.
- It has two chlorine atoms attached to adjacent carbon atoms on the benzene ring.
- The presence of the two chlorine atoms induces stronger dipole moments in the molecule compared to chlorobenzene.
- The dipole-dipole interactions between o-Dichlorobenzene molecules are stronger than in chlorobenzene, leading to a higher melting point than chlorobenzene.

c) m-Dichlorobenzene:
- m-Dichlorobenzene also has a molecular formula of C6H4Cl2.
- It has two chlorine atoms attached to carbon atoms in the meta position on the benzene ring.
- The dipole moments induced by the chlorine atoms in m-Dichlorobenzene are similar to o-Dichlorobenzene.
- However, the arrangement of the chlorine atoms in the meta position results in weaker intermolecular forces compared to o-Dichlorobenzene.
- Consequently, m-Dichlorobenzene has a lower melting point than o-Dichlorobenzene.

d) p-Dichlorobenzene:
- p-Dichlorobenzene also has a molecular formula of C6H4Cl2.
- It has two chlorine atoms attached to carbon atoms in the para position on the benzene ring.
- The arrangement of the chlorine atoms in the para position leads to the strongest dipole-dipole interactions among all the options.
- These stronger intermolecular forces make p-Dichlorobenzene have the highest melting point among the given options.

In summary, p-Dichlorobenzene possesses the highest melting point due to the stronger dipole-dipole interactions resulting from the arrangement of two chlorine atoms in the para position of the benzene ring.

D