Question

Which of the following amine has lowest $pK_b$ value in aqueous phase?

• A. Ethanamine
• B. Methanamine
• C. N-Methylmethanamine
• D. N-Ethylethanamine
• E. N,N-Diethylmethanamine

Hint:

In aqueous solution, secondary amines are generally more basic than primary and tertiary amines.

Answer 1

The Correct Option is D

Concept:
Lower $pK_b$ means stronger base. In aqueous medium, basic strength of aliphatic amines depends on +I effect and solvation.
Step 1: Compare types of amines.
- Primary amines: methanamine, ethanamine - Secondary amine: N-ethylethanamine - Other substituted amines
Step 2: Aqueous phase trend.
In water, secondary amines are usually strongest because: - Two alkyl groups increase electron density on nitrogen. - Solvation is still effective. General trend: \[ 2^\circ>1^\circ>3^\circ>NH_3 \]
Step 3: Choose strongest base.
N-ethylethanamine is a secondary amine. Hence it has lowest $pK_b$.
Step 4: Final answer.
\[ \boxed{\text{N-Ethylethanamine}} \]
Hence, correct option is (D).