Question

Amongst the following, the strongest base in the aqueous medium is :

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

Hint:

Basicity order of aliphatic amines in aqueous solution:
- Methyl group: \(2^\circ > 1^\circ > 3^\circ > \text{NH}_3\) (Code: 213).
- Ethyl group: \(2^\circ > 3^\circ > 1^\circ > \text{NH}_3\) (Code: 231).
In both cases, the secondary (\(2^\circ\)) amine is the strongest base!

Answer 1

The Correct Option is C

Concept:
The basicity of amines in aqueous solution is determined by the availability of the nitrogen unshared electron pair to accept a proton and the stability of the resulting substituted ammonium cation.
In water, three competing factors determine the stability of the conjugate acid:
1. Inductive effect (\(+I\) of alkyl groups),
2. Solvation effect (stabilization of cation through hydrogen bonding with water),
3. Steric hindrance to protonation and solvation.

Step 1: Comparing Aliphatic vs Aromatic and Substituted Amines:
- N-Methylaniline (\(\text{C}_6\text{H}_5\text{NHCH}_3\), compound D) is an aromatic amine where the nitrogen lone pair is delocalized into the benzene ring via resonance, substantially lowering its basicity compared to aliphatic amines.
- Aminoacetonitrile (\(\text{NCCH}_2\text{NH}_2\), compound B) contains a strong electron-withdrawing cyano group (\(-I\) effect) that withdraws electron density from the nitrogen atom, drastically reducing its basicity.

Step 2: Analysis of Aqueous Basicity in Methyl-Substituted Amines:
For methyl-substituted aliphatic amines in aqueous medium, the combined interplay of inductive, steric, and hydration effects gives the basicity order:
\[ (2^\circ) > (1^\circ) > (3^\circ) > \text{NH}_3 \] Specifically:
\[ (\text{CH}_3)_2\text{NH} > \text{CH}_3\text{NH}_2 > (\text{CH}_3)_3\text{N} > \text{NH}_3 \] In dimethylamine (\((\text{CH}_3)_2\text{NH}\)), two electron-releasing methyl groups supply electron density via \(+I\) induction while leaving room for effective hydration of the protonated cation through two hydrogen bonds.
This balance of induction and hydration makes dimethylamine more basic in water than methylamine (\(\text{CH}_3\text{NH}_2\)).

Step 3: Verification with \(pK_b\) Values:
The experimental \(pK_b\) values confirm this order:
- \((\text{CH}_3)_2\text{NH}\): \(pK_b = 3.27\) (strongest base)
- \(\text{CH}_3\text{NH}_2\): \(pK_b = 3.38\)
- \(\text{C}_6\text{H}_5\text{NHCH}_3\): \(pK_b = 9.30\)
Final Answer:
The strongest base in aqueous medium is \((\text{CH}_3)_2\text{NH}\), corresponding to option (C).