Q.Arrange the following compounds in increasing order of their boiling points.

Concept: Boiling Point Trends — For isomeric alkyl halides, boiling point depends on the surface area and branching. More branching → more compact shape → weaker London forces → lower boiling point.

Reasoning:

1. All three are isomers of $\mathrm{C_4H_9Br}$, so molecular weight is identical.
2. Boiling point is governed by van der Waals forces, which increase with molecular surface area.
3. Branching reduces surface area:
   • (b) 1-bromobutane is straight-chain → largest surface area → highest boiling point. …

Boiling point trends in organic compounds are governed by two main factors: molecular mass and intermolecular forces. For isomeric alkyl halides like these three C₄H₉Br compounds, the molecular weight is identical (137 g/mol), so the deciding factor is the strength of van der Waals forces — specifically, how well the molecules can pack together.

The key insight: more branching means a more compact, spherical molecule. A compact molecule has less surface area available for intermolecular contact, so the van der Waals forces are weaker, and the boiling point is lower. A straight-chain molecule, by contrast, is long and flexible, allowing many points of contact between neighbouring molecules.

Let’s examine each compound:

1. Compound (c): 2-bromo-2-methylpropane — This is the most branched. The central carbon is bonded to three methyl groups and one bromine. The molecule is nearly spherical. Very little surface area for neighbouring molecules to “grip” each other. This will have the lowest boiling point.

2. Compound (a): 1-bromo-2-methylpropane — Here the longest chain is only three carbons (a propane backbone) with a methyl branch on the middle carbon and the bromine on C-1: $\mathrm{(CH_3)_2CHCH_2Br}$. The single branch makes the molecule less elongated than the straight chain, yet it is still not as compact as (c). Its surface area is intermediate, so its boiling point falls in the middle. …

Concept: Boiling Point Trends in Alkyl Halides

The boiling point of alkyl halides depends on molecular mass and branching. For isomers (same molecular formula), more branching → lower boiling point because branching reduces the surface area for intermolecular forces (van der Waals forces).

Method: Branching Effect on Boiling Points

Steps:

1. Identify molecular formula

   All three compounds are isomers of $\mathrm{C_4H_9Br}$ — same molecular mass.

2. Analyze branching

   • (b) 1-bromobutane — straight chain (no branching)
   • (a) 1-bromo-2-methylpropane — one methyl branch …

Let’s break this down step-by-step — first the concept, then the common mistakes, and finally how to avoid each.

Core Concept: Boiling Points of Alkyl Halides

Boiling points of organic compounds depend on:

• Molecular mass (larger mass → higher boiling point, generally)
• Surface area (more linear → more surface contact → stronger London dispersion forces → higher boiling point)
• Branching (more branching → more compact shape → less surface area → lower boiling point)

All three compounds here are isomers (same molecular formula $\mathrm{C_4H_9Br}$), so molecular mass is identical.

Thus, branching decides the order.

Correct Order

• Least branched = 1-bromobutane (b) → highest boiling point
• Moderately branched = 1-bromo-2-methylpropane (a)
• Most branched = 2-bromo-2-methylpropane (c) → lowest boiling point

Increasing order:

So the correct option is (iii).

Common Mistakes & How to Avoid Them

✗ Mistake 1: Assuming higher branching increases boiling point

Why it happens: Students confuse branching with stronger intermolecular forces.

Reality: Branching decreases surface area → weaker London forces → lower boiling point.

✓ How to avoid:

Visualise the molecules — draw them out. A straight chain has more contact points with neighbours than a compact, branched one.

✗ Mistake 2: Ignoring that all are isomers

Why it happens: Students see different names and assume different molecular masses.

Reality: All three have formula $\mathrm{C_4H_9Br}$ — same mass.

✓ How to avoid:

Always check molecular formula first. If they are isomers, mass is not the deciding factor — shape is.

✗ Mistake 3: Confusing boiling point with melting point trends

Why it happens: Melting point trends for branched alkanes can be different (symmetry helps packing). …