Chlorobenzene Isomers and Melting Points

The question asks which isomer of chlorobenzene has a high melting point. Chlorobenzene has three structural isomers: ortho (o-), meta (m-), and para (p-), depending on the relative position of the chlorine atom on the benzene ring.

The melting point of a substance is related to how well its molecules pack together in the solid crystal lattice and the strength of the intermolecular forces between these molecules. Stronger intermolecular forces and more efficient packing generally lead to higher melting points.

Factors Affecting Melting Point of Chlorobenzene Isomers

Several factors influence the melting point of these isomers, including polarity and molecular symmetry.

• Polarity: The C-Cl bond is polar. The overall polarity of the molecule depends on the arrangement of this bond relative to the molecular symmetry.
  • o-Chlorobenzene and m-Chlorobenzene have dipole moments and are polar molecules.
  • p-Chlorobenzene is symmetrical, and the two C-Cl bond dipoles cancel each other out, resulting in a zero net dipole moment. It is a nonpolar molecule overall despite having polar bonds.
• Molecular Symmetry: The symmetry of a molecule significantly affects how well it can pack into a crystal lattice. Highly symmetrical molecules can pack more efficiently, leading to stronger van der Waals forces and a more stable crystal structure.

Comparing the Isomers

Let's consider the symmetry of each chlorobenzene isomer:

• o-Chlorobenzene: The two chlorine atoms are adjacent (1,2 positions). It is less symmetrical.
• m-Chlorobenzene: The two chlorine atoms are at the 1,3 positions. It is also less symmetrical than the para isomer.
• p-Chlorobenzene: The two chlorine atoms are opposite to each other (1,4 positions). This arrangement gives the molecule high symmetry.

Because of its high symmetry, p-chlorobenzene molecules can pack very efficiently and tightly in the crystal lattice. This close packing allows for stronger van der Waals forces (specifically London dispersion forces, which are significant due to the presence of chlorine atoms and the larger electron cloud) between adjacent molecules compared to the ortho and meta isomers.

Although o- and m-chlorobenzene are polar and experience dipole-dipole forces in addition to dispersion forces, the effect of efficient crystal packing in p-chlorobenzene often outweighs the contribution of dipole-dipole forces in the solid state for melting point.

Melting Point Comparison

The experimental melting points typically show the following trend:

As seen in the table, p-chlorobenzene has a significantly higher melting point compared to both o- and m-chlorobenzene.

Conclusion on Melting Point

The high melting point of p-chlorobenzene is primarily attributed to its symmetrical structure, which allows for very efficient packing in the crystal lattice. This efficient packing leads to stronger intermolecular interactions in the solid state, requiring more energy (and thus a higher temperature) to overcome these forces and melt the solid.

Therefore, p-chlorobenzene is the isomer with the high melting point among the three chlorobenzene isomers.