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

The molecule with the highest boiling point is (b) \(\mathrm{CH}_{3}-\mathrm{CHOH}-\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{OH}\).

Step 1: Identify the Types of Intermolecular Forces

To determine which molecule has the highest boiling point, identify the types of intermolecular forces present in each. Stronger forces lead to higher boiling points. Hydrogen bonding, present in alcohols, is particularly strong; it occurs when a hydrogen atom is bonded to highly electronegative atoms like oxygen or nitrogen.

Step 2: Analyze the Structure of Each Molecule

(a) \(\mathrm{CH}_{3}-\mathrm{CHCl}-\mathrm{CH}_{3}\) can exhibit dipole-dipole interactions due to the C-Cl bond, but lacks hydrogen bonds. (b) \(\mathrm{CH}_{3}-\mathrm{CHOH}-\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{OH}\) has two alcohol groups capable of hydrogen bonding. (c) \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{Cl}\) exhibits dipole-dipole interactions. (d) \(\mathrm{CH}_{3}-\mathrm{CHOH}-\mathrm{CH}_{3}\) has one alcohol group capable of hydrogen bonding.

Step 3: Compare the Strengths of Intermolecular Forces

Molecule (b) with two hydroxyl groups will have the strongest hydrogen bonding among the options, resulting in the strongest intermolecular forces and consequently, the highest boiling point. Hydrogen bonding is stronger than dipole-dipole interactions found in (a) and (c). While (d) also exhibits hydrogen bonding, it is less than in (b) due to having only one hydroxyl group.

Step 4: Determine the Molecule with the Highest Boiling Point

Based on the analysis, molecule (b) \(\mathrm{CH}_{3}-\mathrm{CHOH}-\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{OH}\) has the highest boiling point because it can form the most hydrogen bonds compared to the others.

Key Concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles, such as atoms, molecules, or ions. These forces are key to understanding many physical properties of substances, such as boiling and melting points. There are various types of intermolecular forces, including:

• London dispersion forces: These are weak forces present in all molecules, caused by the movement of electrons which may create a temporary dipole.
• Dipole-dipole interactions: These occur between molecules with permanent dipole moments, where positive and negative ends attract.
• Hydrogen bonding: A strong type of dipole-dipole interaction occurring when hydrogen is bonded to highly electronegative atoms like oxygen or nitrogen.

In general, the stronger the intermolecular forces, the higher the boiling point of the substance. This is because more energy is needed to overcome these forces during the phase change from liquid to gas.

Hydrogen Bonding

Hydrogen bonding is a special type of strong dipole-dipole interaction that significantly affects the physical properties of molecules. It occurs when a hydrogen atom is directly bonded to a small, highly electronegative atom, such as nitrogen, oxygen, or fluorine. In molecules containing O-H or N-H bonds, the hydrogen atom acquires a partial positive charge (hem]*) while the electronegative atom obtains a partial negative charge. This results in an attraction that is much stronger than typical dipole-dipole interactions. For example:

• Water (hem]*) is a classic example where hydrogen bonding explains its high boiling point relative to other molecules of similar size.
• Ethanol, which has an alcohol group, also displays hydrogen bonding, making it polar and giving it a higher boiling point than nonpolar molecules of similar molecular weight.

Hydrogen bonds significantly influence the boiling points of substances. Molecules such as hem]*(OH) will have higher boiling points due to the presence of hydrogen bonds, making them require more energy to separate into gaseous states.

Dipole-Dipole Interactions

Dipole-dipole interactions represent attractions between polar molecules, characterized by permanent dipoles. These interactions occur when the positive end of one molecule attracts the negative end of another. The strength of dipole-dipole interactions depends on the magnitude of the dipoles and the distance between molecules. Here’s how they influence boiling points:

• For molecules like hem]*(Cl), dipole-dipole interactions are significant but weaker than hydrogen bonds, leading to moderate boiling points.
• Although weaker than hydrogen bonds, dipole-dipole interactions are still stronger than London dispersion forces.

Molecules with stronger dipole-dipole interactions tend to have higher boiling points compared to those that rely solely on London forces. However, substances that can form hydrogen bonds will always exhibit higher boiling points than those limited to dipole-dipole interactions, due to the greater strength of hydrogen bonds.