Problem 21 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Among cellulose, poly (vinyl chloride), nylon and natural rubber, the polymer in which the intermolecular force of attraction is weakest is [2009S] (a) Nylon (b) Poly (vinyl chloride) (c) Cellulose (d) Natural rubber

Step-by-Step Solution

Answer

Natural rubber has the weakest intermolecular forces.

Step 1: Understanding Intermolecular Forces

Intermolecular forces are forces of attraction or repulsion between molecules. They determine many physical properties such as melting and boiling points.

Step 2: Analyzing the Polymers

Let's analyze each polymer: - **Nylon**: Known for strong hydrogen bonding due to amide groups; - **Poly (vinyl chloride)**: Has stronger dipole-dipole forces due to the polar C-Cl bond; - **Cellulose**: Has strong hydrogen bonding between the -OH groups; - **Natural rubber**: Mainly exhibits van der Waals forces, which are comparatively weaker than hydrogen bonding and dipole-dipole interactions.

Step 3: Comparing the Intermolecular Forces

Comparing the intermolecular forces: - **Nylon**, **Poly (vinyl chloride)**, and **Cellulose** all have stronger forces like hydrogen bonds or dipole-dipole interactions. - **Natural rubber** primarily has van der Waals forces, which are weaker compared to the other polymers.

Step 4: Conclusion

Based on the analysis, natural rubber has the weakest intermolecular forces as it mainly involves van der Waals forces.

Key Concepts

Van der Waals Forces

Van der Waals forces are a type of weak intermolecular force that arise due to temporary fluctuations in electron density around molecules. These forces occur in all molecules, whether polar or nonpolar, but they are the primary type of attraction in nonpolar molecules.
Van der Waals forces can be divided into three main types:

• Dispersion Forces: Also known as London forces, these are present in all molecules. They are the weakest of the van der Waals forces and result from temporary dipoles that form when electron clouds randomly shift around nuclei.
• Dipole-Induced Dipole Forces: These forces occur when a polar molecule induces a dipole in a nearby nonpolar molecule. The polar molecule's electric field distorts the electron cloud of the nonpolar molecule, creating a temporary dipole.
• Dipole-Dipole Forces: Occasionally categorized under van der Waals forces, although often treated separately due to their significance in polar compounds.

In polymers like natural rubber, van der Waals forces play a significant role. Since these forces are relatively weak, polymers relying mainly on van der Waals attractions usually have lower melting and boiling points compared to those with stronger intermolecular forces.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction. It occurs when a hydrogen atom bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine, experiences attraction to an electron-rich region of another molecule. This results in a relatively strong bond that can significantly influence the properties of molecules and polymers.

• In polymers like nylon and cellulose, hydrogen bonds are crucial in determining their structural integrity and physical properties.
• Nylon features amide groups capable of forming extensive hydrogen bonding networks along the polymer chain, offering strength and durability.
• Cellulose, composed of repeating glucose units, is laden with -OH groups, which engage in extensive hydrogen bonding. This gives cellulose its toughness and rigidity.

Hydrogen bonds confer higher melting and boiling points to polymers. They also contribute to increased tensile strength, meaning that materials like nylon can bear high loads and resist stretching.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that have permanent electric dipoles. In these molecules, one end is positively charged while the other is negatively charged, creating polarization that leads to attraction with other dipolar molecules.

• Polymers like poly (vinyl chloride) (PVC) demonstrate dipole-dipole interactions due to the presence of polar C-Cl bonds within their structure.
• The electronegativity difference between chlorine and carbon creates a dipole, which can attract similar dipoles in neighboring polymer chains.

Dipole-dipole interactions are stronger than van der Waals forces but generally weaker than hydrogen bonds. They provide polymers like PVC with enhanced stability and toughness compared to nonpolar polymers. Dipole-dipole forces are often responsible for considerable differences in mechanical and thermal properties among various polymeric materials.