Question

Choose the correct decreasing order of basic strength of amines in aqueous solution:

• A. $NH_3 > CH_3NH_2 > (CH_3)_2NH > (CH_3)_3N$
• B. $(CH_3)_2NH > (CH_3)_3N > NH_3 > CH_3NH_2$
• C. $(CH_3)_3N > NH_3 > CH_3NH_2 > (CH_3)_2NH$
• D. $(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3$
• E. $CH_3NH_2 > (CH_3)_2NH > (CH_3)_3N > NH_3$

Hint:

Memorize the "213" rule for methyl amines ($2^\circ, 1^\circ, 3^\circ$) and the "231" rule for ethyl amines ($2^\circ, 3^\circ, 1^\circ$). It saves precious time in MCQ exams.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Basic strength of amines in aqueous solution depends on three factors:
1. Inductive effect (+I effect of alkyl groups).
2. Solvation effect (hydrogen bonding with water).
3. Steric hindrance.
Step 2: Key Formula or Approach:
For Methyl-substituted amines in aqueous solution, the experimental order is 2{o} \(>\) 1{o} \(>\) 3{o} \(>\) NH\textsubscript{3}.
Step 3: Detailed Explanation:
- Inductive effect favors 3{o} \(>\) 2{o} \(>\) 1{o}.
- Solvation/Steric factors favor 1{o} \(>\) 2{o} \(>\) 3{o}.
The net result for methyl amines is that the Secondary amine is most basic, followed by the Primary amine, then the Tertiary amine (due to steric crowding preventing effective solvation), and lastly Ammonia.
Order: \( (CH_3)_2NH>CH_3NH_2>(CH_3)_3N>NH_3 \).
Step 4: Final Answer:
The correct order is Option D.