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) \(\mathrm{p}\)-dichlorobenzene

Step-by-Step Solution

Answer

p-Dichlorobenzene has the highest melting point.

Step 1: Understand the Structures

Identify the structure of each compound: - Chlorobenzene has a benzene ring with one chlorine atom attached. - o-Dichlorobenzene has two chlorine atoms attached to adjacent carbons on the benzene ring. - m-Dichlorobenzene has two chlorine atoms with one carbon between them on the benzene ring. - p-Dichlorobenzene has two chlorine atoms directly opposite each other on the benzene ring.

Step 2: Analyze Molecular Symmetry

Consider the symmetry of each compound: - p-Dichlorobenzene is the most symmetrical, with substitutions at opposite ends of the benzene ring. - m-Dichlorobenzene and o-Dichlorobenzene have less symmetry, with chlorine atoms not directly opposing each other. - Greater symmetry generally leads to more efficient packing in the solid state.

Step 3: Determine Effect on Melting Point

Understand how symmetry affects melting points: - More symmetric molecules tend to pack more tightly in the crystalline solid state, leading to higher melting points. - p-Dichlorobenzene, being more symmetrical than the other options, will pack better in a solid state, leading to higher melting point than less symmetrical compounds.

Step 4: Conclusion

Based on symmetry and packing, among the given options, g delemparg)p-dichlorobenzene possesses the highest melting point due to its high symmetry and efficient packing in the solid state.

Key Concepts

Chlorobenzene

Chlorobenzene is a fascinating chemical compound often used in various industrial processes. It consists of a benzene ring, which is a six-carbon ring with alternating double bonds, forming a very stable structure. When one of these hydrogen atoms on benzene is replaced by a chlorine atom, chlorobenzene is created.

• Chlorobenzene retains the aromatic characteristics of benzene.
• The chlorine atom adds some interesting chemical properties due to its electronegativity.
• This results in distinct solubility and reactivity properties.

In terms of physical properties, chlorobenzene tends to be a colorless liquid at room temperature. Its melting point, which is the temperature at which it transitions from solid to liquid, is influenced by intermolecular forces and the structure of its molecules. Since it's less symmetrical compared to its dichlorinated relatives, it has a relatively lower melting point.

Molecular Symmetry

Molecular symmetry is a crucial concept in understanding the physical properties of substances, including their melting points. Symmetry in a molecule refers to the balanced distribution of its parts. When a molecule has a high degree of symmetry, its components can pack closely together in solid form, often resulting in a higher melting point.

• Symmetry helps molecules arrange themselves in more orderly, repeating patterns.
• This uniformity in structure allows for stronger intermolecular interactions.
• Greater interactions mean that more energy is needed to break the solid apart, implying a higher melting point.

In the context of the compounds in the exercise, p-dichlorobenzene exhibits a higher degree of symmetry compared to m- and o-dichlorobenzene, allowing it to effectively pack in a crystalline lattice and thus demonstrating a higher melting point.

Crystalline Structure

The crystalline structure of a compound is vital in determining its melting point and many other physical properties. Crystalline solids are characterized by a highly ordered and repeating arrangement of molecules.

• This order is due in part to the molecular symmetry of the substances involved.
• More symmetrical molecules contribute to forming more regular crystal lattices.
• These lattices are capable of withstanding high amounts of energy before the structure breaks apart during melting.

When looking at the compounds in the original exercise, p-dichlorobenzene's high symmetry ensures that it forms a more stable crystalline structure compared to its less symmetrical counterparts. Thus, it exhibits a higher melting point because the well-packed crystalline structure resists melting better than those formed by less symmetrical molecules.