Question

The correct order of melting points of dichlorobenzenes is

• A.
• B.
• C.
• D.

Hint:

Symmetry plays a crucial role in determining the packing efficiency of molecules, which in turn affects their melting and boiling points.

Answer 1

The Correct Option is D

The melting points of organic compounds such as dichlorobenzenes depend on their molecular structure, which dictates how the molecules pack in the solid state. In the case of dichlorobenzenes, the position of chlorine atoms on the benzene ring significantly influences their melting points. There are three isomers of dichlorobenzene: 

1. 1,2-Dichlorobenzene (ortho-dichlorobenzene)
2. 1,3-Dichlorobenzene (meta-dichlorobenzene)
3. 1,4-Dichlorobenzene (para-dichlorobenzene)

When considering the melting points of these isomers, the packing efficiency in the solid-state plays a crucial role, which is determined by the symmetry of the molecule:

• 1,4-Dichlorobenzene (para): This isomer is the most symmetrical, allowing for tight packing of the molecules in a crystalline lattice. Hence, it typically has the highest melting point.
• 1,3-Dichlorobenzene (meta): This isomer is less symmetrical than the para isomer, resulting in a lower melting point.
• 1,2-Dichlorobenzene (ortho): This isomer is the least symmetrical and has the lowest melting point due to inefficient packing.

Therefore, based on the symmetry and packing consideration, the order of melting points is:

• 1,4-Dichlorobenzene (para) > 1,3-Dichlorobenzene (meta) > 1,2-Dichlorobenzene (ortho)

The correct answer is the option that reflects this order.

Thus, the correct order of melting points is represented by the image option provided.