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

(b) C_2H_5I > C_2H_5Br > C_2H_5Cl.

Step 1: Understanding Boiling Point

The boiling point of a substance is influenced by the type and strength of the intermolecular forces present. For alkyl halides, boiling point tends to increase with the size and polarizability of the halogen atom.

Step 2: Analyzing Molecular Interactions

In alkyl halides, as we move down the halogen group in the periodic table (i.e., Cl, Br, I), the atoms get larger, have more electrons, and are more polarizable. This increases the London dispersion forces, leading to higher boiling points.

Step 3: Considering Haloalkane Series

The order of increasing atomic size and mass for halogens in alkyl halides is: Cl < Br < I. Therefore, the boiling point typically increases from chloroethane (C_2H_5Cl) to iodoethane (C_2H_5I) due to increased dispersion forces with larger atoms.

Step 4: Determining the Correct Order

Given the trend in molecular size and polarizability, the correct order of boiling points from highest to lowest for ethyl halides is C_2H_5I > C_2H_5Br > C_2H_5Cl.

Key Concepts

Intermolecular Forces

Intermolecular forces are the interactions that occur between molecules, and they play a crucial role in determining the physical properties of substances, such as boiling points. In the case of alkyl halides, the types of intermolecular forces include dipole-dipole interactions and London dispersion forces. Dipole-dipole interactions happen because alkyl halides are polar molecules, meaning they have regions of partial positive and negative charges that attract each other.
\( \ \)The boiling point of a compound is significantly influenced by the strength of these intermolecular forces. Stronger intermolecular forces result in higher boiling points as more energy is required to disrupt the interactions between molecules and transition the substance from liquid to gas. Among alkyl halides, while dipole-dipole interactions exist, it is largely the dispersion forces that dictate the trend in boiling points as we move down the group in the periodic table.

Polarizability

Polarizability refers to the ease with which an electron cloud of a molecule can be distorted by an external electric field. This concept is important in understanding alkyl halides' boiling points since it affects how strong the London dispersion forces can be. Larger atoms, such as iodine compared to chlorine, have more diffuse electron clouds that are more susceptible to distortion.
\( \ \)With greater polarizability, the temporary induced dipoles become stronger, which results in enhanced London dispersion forces. This is why as we go from chloroethane (\( \text{C}_2\text{H}_5\text{Cl} \)) to iodoethane (\( \text{C}_2\text{H}_5\text{I} \)), we observe an increase in boiling points. As halogen atoms get larger down the group, their polarizability increases, making their contribution to intermolecular forces more significant.

London Dispersion Forces

London dispersion forces are a type of weak intermolecular force that arises due to temporary fluctuations in electron density within a molecule, creating momentary dipoles. These forces are universally present in all molecules, whether polar or nonpolar, but they are particularly important in larger, nonpolar molecules or atoms.
\( \ \)In alkyl halides, London dispersion forces are the key to understanding differences in boiling points. As we move from lighter to heavier halogens like chlorine, bromine, and iodine, the boiling point increases due to stronger dispersion forces. Larger atoms exhibit stronger dispersion forces because they have more electrons that can create larger temporary dipoles.

• Increased atomic size leads to stronger dispersion forces.
• More electrons increase polarizability, leading to more significant temporary dipoles.
• Ultimately, this raises the boiling point as seen in the order \( \text{C}_2\text{H}_5\text{I} > \text{C}_2\text{H}_5\text{Br} > \text{C}_2\text{H}_5\text{Cl} \).

By understanding London dispersion forces, we appreciate why larger halogens exhibit higher boiling points.