Problem 44 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following possesses highest melting point? (a) chlorobenzene (b) o-dichlorobenzene (c) \(\mathrm{m}\)-dichlorobenzene (d) p-dichlorobenzene

Step-by-Step Solution

Answer

p-Dichlorobenzene has the highest melting point due to its symmetry and efficient packing.

Step 1: Identify Melting Point Determinants

Melting point is primarily influenced by molecular structure, intermolecular forces, and molecular symmetry. Compounds with more symmetry and stronger intermolecular forces generally have higher melting points due to the efficient packing in solid form.

Step 2: Analyze Molecular Structures

Chlorobenzene has one chlorine atom attached to a benzene ring. The dichlorobenzene isomers have two chlorine atoms positioned differently: ortho (1,2-), meta (1,3-), and para (1,4-). The symmetry and ability of these molecules to pack efficiently will affect their melting points.

Step 3: Determine Symmetry and Efficient Packing

Among the options, p-dichlorobenzene ( p- positin) is the most symmetrical in terms of the placement of chlorine atoms. This symmetry allows for a more efficient solid-state packing, which usually correlates with a higher melting point compared to other isomers.

Step 4: Compare Melting Points Using Information

Known data supports that p-dichlorobenzene has the highest melting point due to its symmetrical structure, which facilitates solid packing. This property makes it solidify and resist melting at higher temperatures compared to its isomers and chlorobenzene.

Key Concepts

Intermolecular Forces

Intermolecular forces are critical in determining the physical properties of substances, such as melting and boiling points. These forces consist of three main types: dipole-dipole interactions, London dispersion forces, and hydrogen bonding. In the case of chlorobenzene and its isomers, the predominant intermolecular force is the dipole-dipole interaction. This is due to the polarity introduced by the presence of chlorine atoms, which have a higher electronegativity compared to carbon and hydrogen.

• Dipole-Dipole Interactions: These arise between molecules that possess a permanent dipole moment. For chlorobenzene and dichlorobenzene isomers, the electron-withdrawing chlorine atoms create a dipole, which strengthens the intermolecular forces.
• London Dispersion Forces: Although generally weaker than dipole-dipole interactions, London dispersion forces are present in all molecules. For larger molecules with more electrons, these forces become more significant.

Stronger intermolecular forces typically lead to higher melting points, as more energy is required to overcome these forces and change the state of the substance.

Molecular Symmetry

Molecular symmetry plays a significant role in how molecules pack in the solid state. Symmetrical molecules tend to pack more efficiently. This efficient packing allows the molecules to hold together more tightly, thus increasing the melting point. In the case of chlorobenzene and its isomers, symmetry is a key factor in determining melting point. Para-dichlorobenzene (p-dichlorobenzene) stands out because the chlorine atoms are positioned opposite each other on the benzene ring, giving it a high degree of symmetry. Due to this, it experiences less steric hindrance and can stack more neatly in a solid, crystalline form than ortho or meta isomers. This better packing in a crystalline lattice not only stabilizes the solid form but also requires more energy to disrupt during the transition to a liquid state.

Chlorobenzene Isomers

Chlorobenzene and its isomers, namely ortho-dichlorobenzene, meta-dichlorobenzene, and para-dichlorobenzene, are distinguished by the relative positions of chlorine atoms on the benzene ring. These positional differences contribute to variations in both symmetry and intermolecular forces, thus affecting physical properties like melting points.

• Ortho-Dichlorobenzene (-dichlorobenzene): With chlorine atoms positioned adjacently, there is a lack of symmetry, resulting in less efficient packing.
• Meta-Dichlorobenzene (m-dichlorobenzene): Although more symmetrical than ortho, the non-opposite positioning still prevents optimal packing.
• Para-Dichlorobenzene (p-dichlorobenzene): Exhibits the highest symmetry with chlorine atoms directly opposite each other, allowing snug and efficient crystalline packing.

This variation in molecular structure dictates the melting behavior of each compound. As a result, substances with more efficient packing, like the p-dichlorobenzene, boast higher melting points. Understanding these differences is crucial for predicting and explaining the melting point trends among these isomers.