Problem 11 - A Complete Resource Book in Chemistry for JEE Main

Question

Correct order of boiling point for the alkyl halide is (a) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}>\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br}>\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{I}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{I}>\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br}>\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{I}>\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}>\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br}>\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{I}>\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\)

Step-by-Step Solution

Answer

The correct order is (b) C_{2}H_{5}I > C_{2}H_{5}Br > C_{2}H_{5}Cl .

Step 1: Understanding the Concept

Boiling points of alkyl halides depend on molecular weight and intermolecular forces, particularly Van der Waals forces. The larger and heavier the molecule, the stronger these forces, leading to a higher boiling point.

Step 2: Identify Alkyl Halides

The alkyl halides given are ethyl chloride ( C_{2}H_{5}Cl ), ethyl bromide ( C_{2}H_{5}Br ), and ethyl iodide ( C_{2}H_{5}I ). Their molecular weights increase from chlorine (Cl) to bromine (Br) to iodine (I).

Step 3: Analyze Molecular Weight and Forces

As molecular weight increases (from Cl to I), the van der Waals forces also increase, meaning the boiling point should increase from ethyl chloride to ethyl iodide.

Step 4: Order the Boiling Points

Based on molecular weight and intermolecular forces, the correct order of boiling points is C_{2}H_{5}I > C_{2}H_{5}Br > C_{2}H_{5}Cl .

Key Concepts

Alkyl Halides

Alkyl halides are organic compounds where a halogen atom is bonded to an alkyl group. This specific group of compounds includes molecules like ethyl chloride, ethyl bromide, and ethyl iodide. Each of these compounds consists of ethyl groups

• Ethyl Chloride: C₂H₅Cl
• Ethyl Bromide: C₂H₅Br
• Ethyl Iodide: C₂H₅I

The halogen part (Cl, Br, I) attaches to the ethyl group. The differences in halogens lead to variations in their physical properties, such as boiling points. This variability is due primarily to the increasing size and molecular weight of the halogens, in addition to variations in intermolecular forces.

Molecular Weight

Molecular weight is a crucial factor that influences the boiling points of alkyl halides. Molecular weight refers to the sum of the atomic weights of all the atoms in a molecule. For the series of alkyl halides under discussion, the molecular weights vary as follows:

• Ethyl Chloride: Low
• Ethyl Bromide: Moderate
• Ethyl Iodide: High

As we move from ethyl chloride to ethyl iodide, the molecular weight increases. The heavier a molecule, the stronger its intermolecular forces because it has more electrons that contribute to these forces. Therefore, a higher molecular weight correlates with a higher boiling point, as seen in this series of alkyl halides.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between different molecules. These forces play a crucial role in determining physical properties such as boiling points. In alkyl halides:

• Dipole-Dipole Interactions: These occur because of electronegativity differences between the alkyl group and the halogen atom.
• Van der Waals Forces: These are generally the most significant forces in determining boiling points in non-polar sections of the molecule.

While dipole-dipole interactions are present, it is the Van der Waals forces that primarily contribute to the boiling points of alkyl halides. The increased surface area and more substantial molecular weight usually enhance these forces.

Van der Waals Forces

Van der Waals forces are weak electric forces that generally contribute to the boiling points of molecules, including alkyl halides. These forces include attractions between molecules that have temporary dipoles:

• They grow stronger as the molecular weight increases.
• They depend largely on the surface area of the molecule.
• For alkyl halides, larger halogens like iodine create stronger Van der Waals forces than smaller ones like chlorine.

As such, the intensity of Van der Waals forces in ethyl iodide is greater than in ethyl bromide or ethyl chloride, contributing significantly to its higher boiling point. These forces are crucial when analyzing why ethyl iodide has a higher boiling point than its lighter counterparts.