Question

Which of the following compounds will have the highest melting point?
A.Chlorobenzene
B.o-dichlorobenzene
C.m-dichlorobenzene
D.p-dichlorobenzene

Answer 1

Hint: The melting point is usually defined as the point at which the material changes from solid to a liquid. Moreover, melting points may be defined in various ways, each corresponding to a different residual amount of solid or liquid. The main criteria for melting point of such compounds is based on molecular packing.

Complete step by step answer:
Generally, the temperature at which the solid changes its state to liquid at atmospheric pressure is called the melting point of that liquid. Now, in case of these compounds like o-dichlorobenzene, m-dichlorobenzene and p-dichlorobenzene, the main criteria for determining the melting point is molecular packing. The melting point is directly proportional to the symmetry of the compound. We can say that the symmetry of the compound goes in the same manner as melting point.
So, p-dichlorobenzene is more symmetrical than the o- and m- isomers. Therefore, it fits more closely in the crystal lattice and hence, more energy is required to break the crystal lattice of p-dichlorobenzene. So, it is clear that p-dichlorobenzene will have the highest melting point than the o- and m- isomers. It has a melting point of ${53.5^ \circ }C$ . The three types of isomers are as shown:

Hence, option D is correct.

Note: p-dichlorobenzene is used as a fumigant insecticide to control clothes moths. It is also found in deodorant blocks made for trash cans and toilets. Most of the people recognize the odor of this compound as the smell of mothballs. Further, acute inhalation or oral exposure to high concentrations may result in liver damage and may cause severe respiratory tract issues.