Question

Arrange the following compounds in increasing order of their boiling points:
 

• A. (ii) < (i) < (iii)
• B. (iii) < (i) < (ii)
• C. (i) < (ii) < (iii)
• D. (iii) < (ii) < (i)

Hint:

Boiling point increases with chain length and molecular size due to stronger London dispersion forces.

Answer 1

The Correct Option is B

Understanding Factors Affecting Boiling Point

1. Molecular Weight (Size):
Larger molecules generally have higher boiling points due to increased van der Waals forces.

2. Branching:
Branching decreases the surface area for intermolecular interactions, leading to lower boiling points.

3. Polarity:
Polar molecules have stronger dipole-dipole interactions, resulting in higher boiling points than nonpolar molecules of similar size.

Analyzing the Compounds

(i) 2-bromo-3-methylbutane:
This molecule has branching.

(ii) 1-bromobutane:
This is a straight-chain alkyl halide.

(iii) 2-bromo-2-methylpropane:
This molecule has significant branching around the bromine atom.

Determining the Order

1. Molecular Weight:
All three compounds have a similar molecular weight due to the presence of bromine and a 4-carbon alkyl chain. However, compound (ii) has a slightly higher molecular weight as it is a straight-chain.

2. Branching:
Branching significantly impacts boiling points. Compound (iii) has the most branching, causing it to have the lowest boiling point, followed by compound (i). Compound (ii) is straight-chained.

Therefore, the order of increasing boiling point should be: (iii) < (i) < (ii).

Answer:
The correct answer is (B).