Correct Option: (D) Pentanamine
Step 1 (Governing Principle): [1/3 Mark]
The boiling point of amines is primarily governed by the extent of intermolecular hydrogen bonding and the surface area of the molecule. For isomeric primary amines, the boiling point decreases with an increase in branching because branching reduces the surface area, thereby decreasing the magnitude of van der Waals forces of attraction.
Step 2 (Step-by-Step Analysis): [1/3 Mark]
- (A) 2,2-Dimethylpropanamine: Highly branched (neopentyl structure). Lowest surface area.
- (B) 3-Methylbutanamine: Branched chain.
- (C) 2-Methylbutanamine: Branched chain.
- (D) Pentanamine: Straight-chain primary amine ($CH_3CH_2CH_2CH_2CH_2NH_2$).
Comparing the structures, Pentanamine is a straight-chain isomer. The van der Waals forces are proportional to the surface area of the molecule:
$$ \text{Boiling Point} \propto \text{Surface Area} $$Since Pentanamine has the maximum surface area among the given isomers, it experiences the strongest intermolecular van der Waals forces.
Step 3 (Final Concluded Answer): [1/3 Mark]
Because Pentanamine has the least branching, it possesses the largest surface area, leading to stronger intermolecular forces compared to the branched isomers. Therefore, Pentanamine has the highest boiling point.
💡 EL10 Concept Insight: Think of molecules like pieces of Velcro; the straighter and longer the molecule, the more "Velcro" surface it has to stick to its neighbors, making it harder to pull them apart (higher boiling point). Branching makes the molecule more like a ball, giving it less surface area to stick, so it boils much more easily!