Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of increasing boiling points is (a) isobutane \(<\mathrm{n}\)-butane \(<\mathrm{n}\)-butyl chloride \(<\mathrm{n}\)-butanol (b) isobutane \(<\mathrm{n}\)-butyl chloride \(<\mathrm{n}\)-butane \(<\mathrm{n}\)-butanol (c) \(\mathrm{n}\)-butane \(<\mathrm{n}\)-butanol \(<\mathrm{n}\)-butyl chloride \(<\) isobutane (d) \(\mathrm{n}\)-butane \(<\) isobutane \(<\mathrm{n}\)-butyl chloride \(<\mathrm{n}\)-butanol

Step-by-Step Solution

Answer

(a) isobutane < n-butane < n-butyl chloride < n-butanol is the correct order.

Step 1: Understand the properties affecting boiling points

To determine the boiling points, consider these factors: molecular weight, structure (branching vs linear), and intermolecular forces (e.g., hydrogen bonding). More branching decreases boiling points, while hydrogen bonding increases them.

Step 2: Identify molecular structures and expected intermolecular forces

Isobutane is branched and has van der Waals forces; n-butane is linear with van der Waals forces; n-butyl chloride has van der Waals forces and dipole-dipole interactions; n-butanol has hydrogen bonding, which is the strongest intermolecular force.

Step 3: Rank the compounds by intermolecular forces

Hydrogen bonds in n-butanol make it the highest boiling point. n-butyl chloride has dipole-dipole interactions, higher than the pure van der Waals forces in n-butane and isobutane. n-butane has a higher boiling point than isobutane because of its linear structure.

Step 4: Arrange the boiling points from lowest to highest

Order by boiling points based on intermolecular forces and structure: Isobutane (lowest due to branching) < n-butane (next, as it's linear) < n-butyl chloride (dipole-dipole forces) < n-butanol (highest, due to hydrogen bonding).

Key Concepts

Intermolecular Forces

Intermolecular forces are the attractions between molecules. These forces determine many physical properties such as boiling points. Understanding them is crucial to predicting how compounds will behave when heated.

These forces come in several varieties:

• Van der Waals forces: Weak attractions that occur between all molecules. They arise from temporary dipoles formed when electrons move about a nucleus.
• Dipole-dipole interactions: Occur between polar molecules with permanent dipoles. These interactions are stronger than van der Waals forces but weaker than hydrogen bonds.
• Hydrogen bonds: Special kind of dipole-dipole interaction involving hydrogen atoms bound to electronegative elements like oxygen or nitrogen. They are the strongest of the typical intermolecular forces.

The strength of these intermolecular forces directly influences boiling points. Generally, the stronger the force, the higher the boiling point.

Hydrogen Bonding

Hydrogen bonding occurs when hydrogen is covalently bonded to electronegative elements such as oxygen, nitrogen, or fluorine. In these bonds, the hydrogen atom carries a partial positive charge, while the electronegative atom carries a partial negative charge. This creates a strong dipole interaction with other nearby electronegative atoms.

Hydrogen bonds significantly elevate the boiling point of a substance due to the increased energy required to break them:

• For example, water has an unusually high boiling point because each molecule can form hydrogen bonds with others.
• n-Butanol exhibits hydrogen bonding because of the -OH (hydroxyl) group, leading to a higher boiling point in comparison to other compounds with similar molecular weights.

The presence of hydrogen bonds in a molecule makes it behave differently in terms of boiling point compared to those with only van der Waals or dipole-dipole forces.

Molecular Structure Effects on Boiling Point

The molecular structure of a compound, including its shape and branching, plays a vital role in determining boiling points.

Linear chains typically result in higher boiling points compared to branched structures. Here's why:

• Linear molecules pack closely together, increasing van der Waals forces. This leads to higher boiling points.
• Branched compounds, like isobutane, are more compact, reducing contact area and van der Waals forces, resulting in lower boiling points.

Thus, in molecules with similar types of intermolecular forces, those with less branching will usually have higher boiling points. n-Butane is an example that illustrates this principle as it has a slightly higher boiling point than its branched isomer, isobutane.