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

Factors Influencing Boiling Point

1. Molecular Weight (Size):
Increased molecular size correlates with higher boiling points due to stronger van der Waals forces.

2. Branching:
Molecular branching reduces surface area available for intermolecular interactions, thus lowering boiling points.

3. Polarity:
Polar molecules exhibit stronger dipole-dipole interactions, leading to higher boiling points compared to nonpolar molecules of comparable size.

Compound Analysis

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

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

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

Ordering by Boiling Point

1. Molecular Weight:
All three compounds possess similar molecular weights due to the presence of bromine and a four-carbon alkyl chain. Compound (ii) has a marginally higher molecular weight as it is unbranched.

2. Branching:
Branching significantly affects boiling points. Compound (iii) exhibits the most branching, resulting in the lowest boiling point, followed by compound (i). Compound (ii) is unbranched.

Consequently, the order of increasing boiling point is: (iii) < (i) < (ii).

Answer:
The correct option is (B).