Question

Among the following amines, the strongest base in aqueous solution is: \[ \text{NH}_3,\; \text{CH}_3\text{NH}_2,\; (\text{CH}_3)_2\text{NH},\; (\text{CH}_3)_3\text{N} \]

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

Hint:

For amines in aqueous solution: \[ 2^\circ \text{ amine } > 1^\circ \text{ amine } > 3^\circ \text{ amine} \] because solvation of protonated amines is very important.

Answer 1

The Correct Option is C

Concept: Basic strength of amines depends upon:

• Availability of lone pair on nitrogen atom.
• Electron donating \(+I\) effect of alkyl groups.
• Stability of the protonated amine in aqueous solution due to solvation.

In gaseous state: \[ 3^\circ > 2^\circ > 1^\circ > \text{NH}_3 \] But in aqueous solution, solvation plays an important role. Secondary amines become most basic because they have:

• Strong \(+I\) effect
• Effective solvation

Hence: \[ 2^\circ > 1^\circ > 3^\circ > \text{NH}_3 \]

Step 1: Understand the effect of alkyl groups. Methyl groups donate electrons toward nitrogen through \(+I\) effect, increasing electron density on nitrogen and making the amine more basic. \[ \text{NH}_3 < \text{CH}_3\text{NH}_2 < (\text{CH}_3)_2\text{NH} \]

Step 2: Compare tertiary amine in aqueous medium. Although tertiary amine has maximum \(+I\) effect, its protonated form is less stabilized due to poor solvation because of steric hindrance. Therefore: \[ (\text{CH}_3)_3\text{N} < (\text{CH}_3)_2\text{NH} \]

Step 3: Final order of basic strength in aqueous solution. \[ \boxed{ (\text{CH}_3)_2\text{NH} > \text{CH}_3\text{NH}_2 > (\text{CH}_3)_3\text{N} > \text{NH}_3 } \] Hence, the strongest base is: \[ \boxed{(\text{CH}_3)_2\text{NH}} \]