Question

Identify the compound from following having lowest boiling point.

• A. $\mathrm{n-C_4H_9NH_2}$
• B. $\mathrm{C_2H_5CH(CH_3)_2}$
• C. $\mathrm{C_2H_5N(CH_3)_2}$
• D. $\mathrm{(C_2H_5)_2NH}$

Hint:

Alkanes are essentially non-polar hydrocarbons and can only form weak London dispersion forces. When compared against polar functional groups like amines or alcohols of similar molecular weight, alkanes will always have the lowest boiling point.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
We need to compare the physical properties of four different organic compounds (a primary amine, a tertiary amine, a secondary amine, and a branched alkane) of comparable molecular mass and determine which one possesses the lowest boiling point.

Step 2: Key Formula or Approach:
Boiling point values are directly governed by the relative strengths of the present intermolecular forces. The generalized hierarchy of these forces in order of decreasing strength is: $$\text{Hydrogen Bonding } (\text{Primary} > \text{Secondary}) > \text{Dipole-Dipole Interaction} > \text{London Dispersion Forces}$$

Step 3: Detailed Explanation:
Let's analyze the structural binding properties of each individual option: Option (A) $\mathrm{n-C_4H_9NH_2}$ (butylamine) is a primary amine containing two highly polarized $\mathrm{N-H}$ bonds, allowing it to establish strong intermolecular hydrogen-bonding networks. Option (D) $\mathrm{(C_2H_5)_2NH}$ (diethylamine) is a secondary amine with one $\mathrm{N-H}$ bond, enabling moderate intermolecular hydrogen bonding. Option (C) $\mathrm{C_2H_5N(CH_3)_2}$ (ethyldimethylamine) is a tertiary amine. It has no hydrogen atoms directly bonded to nitrogen ($\mathrm{N-H}$ bonds are absent), preventing it from forming hydrogen bonds with itself, though it still exhibits weak dipole-dipole attractions. Option (B) $\mathrm{C_2H_5CH(CH_3)_2}$ (2-methylbutane) is a branched hydrocarbon alkane. It is entirely non-polar and relies exclusively on weak London dispersion forces. Because dispersion forces are weaker than both dipole-dipole interactions and hydrogen bonds, it requires minimal thermal energy to break apart into the gas phase.

Step 4: Final Answer:
The compound with the lowest boiling point is the alkane, 2-methylbutane, which corresponds to option (B).