Question

Arrange the following in increasing order of their pK\(_b\) values. 

• A. A less{} B less{} C
• B. C less{} B less{} A
• C. B less{} C less{} A
• D. A less{} C less{} B

Hint:

The higher the pK\(_b\) value, the weaker the base. Aliphatic amines (like ethylamine) are more basic than aromatic amines (like aniline) due to the inductive effect.

Answer 1

The Correct Option is B

The pK_b value of an amine is inversely related to its basic strength—lower pK_b means stronger base.

1. Ethylamine (C):
- Strongest base because the ethyl (-C₂H₅) group donates electrons, increasing the availability of the lone pair on nitrogen.
- Lowest pK_b value.

2. Benzylamine (B):
- Less basic than ethylamine because the benzyl (-CH₂C₆H₅) group has a weak +I effect but does not strongly delocalize electrons.

3. Aniline (A):
- Weakest base because the nitrogen lone pair is delocalized over the benzene ring, reducing its availability for protonation.
- Highest pK_b value.

Thus, the increasing order of pK_b values is:

C (Ethylamine) < B (Benzylamine) < A (Aniline)

This matches option (2).