Question

Which of the following amines acts as strongest base?

• A. $\text{CH}_3\text{NH}_2$
• B. $(\text{C}_2\text{H}_5)\text{N}(\text{CH}_3)_2$
• C. $(\text{CH}_3)_2\text{NH}$
• D. $(\text{CH}_3)_3\text{N}$

Hint:

Memorize these two specific basicity orders for aqueous solutions:
Methyl series: $2^\circ > 1^\circ > 3^\circ > \text{NH}_3$ (Order: 213)
Ethyl series: $2^\circ > 3^\circ > 1^\circ > \text{NH}_3$ (Order: 231)

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We need to determine which of the provided aliphatic amines acts as the strongest base (most readily accepts a proton) in an aqueous solution.

Step 2: Key Formula or Approach:
The basic strength of amines in water is determined by a delicate balance of three competing factors:
1.

+I (Inductive) effect of alkyl groups (increases basicity).
2.

Solvation (hydration) effect of the conjugate acid (more hydrogen bonding increases basicity).
3.

Steric hindrance (bulky groups block the approach of the proton, decreasing basicity).

Step 3: Detailed Explanation:
For methyl-substituted amines specifically, the interplay of these three factors creates a unique trend in aqueous solutions.
Primary ($1^\circ$) amines have high solvation but low +I effect.
Tertiary ($3^\circ$) amines have high +I effect but suffer from severe steric hindrance and very poor solvation.
Secondary ($2^\circ$) amines hit the "sweet spot" with a strong +I effect from two alkyl groups and moderate, effective solvation.
Therefore, the standard basicity order for methylamines in aqueous solution is:
Secondary ($2^\circ$) $>$ Primary ($1^\circ$) $>$ Tertiary ($3^\circ$) $>$ Ammonia ($\text{NH}_3$).
Applying this to the options, the secondary amine $(\text{CH}_3)_2\text{NH}$ (dimethylamine) is the strongest base.

Step 4: Final Answer:
The strongest base is $(\text{CH}_3)_2\text{NH}$, corresponding to option (C).