Problem 65 - A Complete Resource Book in Chemistry for JEE Main

Question

The molecule which the highest boiling point is (a) \(\mathrm{CH}_{3}-\mathrm{CHCl}-\mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{3}-\mathrm{CHOH}-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}\) (d) \(\mathrm{CH}_{3}-\mathrm{CHOH}-\mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

(b) has the highest boiling point.

Step 1: Identify Boiling Point Factors

The boiling point of a molecule is influenced by intermolecular forces: hydrogen bonding, dipole-dipole interactions, and London dispersion forces. More hydrogen bonds typically indicate a higher boiling point.

Step 2: Analyze Each Molecule

(a) \( \mathrm{CH}_{3}-\mathrm{CHCl}-\mathrm{CH}_{3} \) has dipole-dipole interactions due to Cl but no hydrogen bonds.(b) \( \mathrm{CH}_{3}-\mathrm{CHOH}-\mathrm{CH}_{2} \mathrm{CH}_{2}\mathrm{OH} \) has two hydroxyl groups, allowing strong hydrogen bonding.(c) \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}\mathrm{Cl} \) also experiences dipole-dipole interactions, but no hydrogen bonds.(d) \( \mathrm{CH}_{3}-\mathrm{CHOH}-\mathrm{CH}_{3} \) contains one hydroxyl group, allowing hydrogen bonding, but less than (b).

Step 3: Compare Molecules Based on Hydrogen Bonding

Compound (b) has two OH groups, providing it with the strongest hydrogen bonding capability, leading to a higher boiling point compared to compounds with fewer or no OH groups.

Step 4: Final Decision: Identify the Highest Boiling Point

Compound (b) \( \mathrm{CH}_{3}-\mathrm{CHOH}-\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{OH} \) will have the highest boiling point due to its ability to form multiple hydrogen bonds, which heavily influence boiling points.

Key Concepts

Intermolecular Forces

Intermolecular forces are the attractions that occur between molecules, and they play a crucial role in determining the physical properties of compounds, such as boiling points. The strength of these forces dictates how much energy is required to separate molecules from one another. The three main types of intermolecular forces are hydrogen bonds, dipole-dipole interactions, and London dispersion forces. Each type of force varies in strength and plays a different role depending on the molecular structure involved.

When comparing substances, molecules exhibiting stronger intermolecular forces typically possess higher boiling points. This is due to the additional energy needed to break these interactions during phase transitions. Understanding these forces provides insight into why certain substances are liquids while others remain as gases or solids at room temperature.

• Hydrogen Bonds: Typically the strongest among the intermolecular forces.
• Dipole-Dipole Interactions: Moderate strength and occur in polar molecules.
• London Dispersion Forces: Present in all molecules but weaker compared to other forces.

Recognizing which forces predominate in a given compound helps in predicting and explaining its boiling point.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction and occurs only when hydrogen is covalently bonded to highly electronegative elements such as nitrogen, oxygen, or fluorine. In the context of boiling points, hydrogen bonds significantly increase the energy required to separate molecules, thus leading to higher boiling points in substances exhibiting this type of bonding.

A molecule like \( \mathrm{CH}_{3}-\mathrm{CHOH}-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} \) features two hydroxyl groups, which facilitates the formation of multiple hydrogen bonds. This extensive hydrogen bonding network explains why this compound has an elevated boiling point compared to others that might have fewer hydrogen bonding opportunities.

Generally, the more hydrogen bonds present in a molecule, the greater the energy required to transition from liquid to gas, leading to increased boiling points. This concept is invaluable in predicting the thermal properties of various compounds and is essential when considering potential solvents and biochemical interactions.

• Key Feature: Occurs with N-H, O-H, or F-H bonds.
• Impact: Substantial increase in boiling point.
• Examples: Water's high boiling point is largely due to hydrogen bonding.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, where positive ends of one molecule are attracted to the negative ends of another. These forces are generally stronger than London dispersion forces but weaker than hydrogen bonds. They significantly impact the boiling points of polar molecules, making them higher than those experiencing only London dispersion forces.

For instance, in the molecule \( \mathrm{CH}_{3}-\mathrm{CHCl}-\mathrm{CH}_{3} \), chlorine's electronegativity induces a dipole, allowing for dipole-dipole interactions. Although these interactions contribute to a higher boiling point than nonpolar counterparts, compounds with hydrogen bonding, like those with hydroxyl groups, will typically have higher boiling points.

While dipole-dipole interactions alone may not give rise to the highest boiling points among all types of intermolecular forces, they are crucial for understanding the physical properties of polar compounds. Detecting these interactions helps classify substances and predict their behavior in different chemical contexts.

• Formation: Arises in molecules with a permanent dipole moment.
• Consequences: Moderate increase in boiling point in polar molecules.
• Comparative Strength: Weaker than hydrogen bonds but stronger than London dispersion forces.

These interactions are essential in determining the physical properties of vast numbers of organic and inorganic compounds.