Question

Which among following amines has lowest \( pK_b \) values?

• A. \( \text{CH}_3\text{CH}_2\text{NH}_2 \)
• B. \( (\text{CH}_3\text{CH}_2)_2\text{NH} \)
• C. \( (\text{CH}_3\text{CH}_2)_3\text{N} \)
• D. \( \text{C}_6\text{H}_5\text{NH}_2 \)

Hint:

In aliphatic amines, the basicity order in water: secondary > primary > tertiary. For aniline, the lone pair is involved in resonance with the ring, making it much less basic.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
\( pK_b \) is a measure of base strength: lower \( pK_b \) means stronger base. We need the amine with the lowest \( pK_b \) (strongest base) among the given.

Step 2: Key Formula or Approach:
For aliphatic amines in aqueous solution, the basicity order is generally: secondary > primary > tertiary (due to steric hindrance to solvation of the conjugate acid). Aromatic amines (aniline) are much weaker due to resonance delocalization of the lone pair into the benzene ring.

Step 3: Detailed Explanation:
- (A) Ethylamine (primary): \( pK_b \approx 3.36 \)
- (B) Diethylamine (secondary): \( pK_b \approx 3.00 \) (stronger base, lower \( pK_b \))
- (C) Triethylamine (tertiary): \( pK_b \approx 3.25 \) (less basic than secondary due to steric hindrance)
- (D) Aniline (aromatic): \( pK_b \approx 9.38 \) (very weak base)
Thus, diethylamine has the lowest \( pK_b \).

Step 4: Final Answer:
Option (B) is correct.