Question

What is the correct increasing order of basic strength for the following amines when measured inside an aqueous medium?
\[ \text{(I) } \mathrm{CH_3NH_2} \] \[ \text{(II) } \mathrm{(CH_3)_2NH} \] \[ \text{(III) } \mathrm{(CH_3)_3N} \] \[ \text{(IV) } \mathrm{NH_3} \]

• A. IV < III  <  I  <  II
• B. IV & Lt; I & Lt; II & Lt; III
• C. III & Lt; I & Lt; II & Lt; IV
• D. I & Lt; II & Lt; III & Lt; IV

Hint:

Remember these structural trends for aqueous amines to save time: For Methyl groups, use sequence 213 (\(2^\circ > 1^\circ > 3^\circ\)). For Ethyl groups, use sequence 231 (\(2^\circ > 3^\circ > 1^\circ\)).

Answer 1

The Correct Option is A

Concept: 

In aqueous medium, the basicity of aliphatic amines depends on three important factors:

• Inductive electron donating effect \((+I)\)
• Steric hindrance
• Hydration stability of the conjugate acid through hydrogen bonding

Step 1: Compare the effect of alkyl groups on basicity.

Inductive effect trend: \[ 2^\circ > 3^\circ > 1^\circ \] More alkyl groups increase electron density on nitrogen, increasing basicity. 

Hydration trend in aqueous medium: \[ 1^\circ > 2^\circ > 3^\circ \] Primary amines are better hydrated, while tertiary amines suffer from steric hindrance and poor solvation. 

Combining all effects, the overall basicity order becomes: \[ 2^\circ > 1^\circ > 3^\circ > NH_3 \] 
Therefore: \[ (\mathrm{CH_3})_2\mathrm{NH} > \mathrm{CH_3NH_2} > (\mathrm{CH_3})_3\mathrm{N} > \mathrm{NH_3} \] 
Using the given labels: \[ \text{II} > \text{I} > \text{III} > \text{IV} \] Hence, arranging from lowest to highest basicity: \[ \boxed{\text{IV} < \text{III} < \text{I} < \text{II}} \]