Question

Which among the following has least \( \text{p}K_b \) value?

• A. Methanamine
• B. Ethanamine
• C. N-ethylethanamine
• D. N,N-diethyl ethanamine

Hint:

Always assume an aqueous medium unless specified otherwise. Memorize the basicity orders:
Ethylamines: $2^\circ>3^\circ>1^\circ>\text{NH}_3$
Methylamines: $2^\circ>1^\circ>3^\circ>\text{NH}_3$

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The $\text{pK}_b$ value is a quantitative measure of the basic strength of a compound. A lower $\text{pK}_b$ value indicates a stronger base (since $\text{pK}_b = -\log \text{K}_b$). The basicity of aliphatic amines in an aqueous solution depends on a delicate balance of inductive effect, solvation (hydration) effect, and steric hindrance.

Step 2: Key Formula or Approach:
Identify the class of each amine ($1^\circ, 2^\circ, 3^\circ$) and apply the established trend for their basicity in an aqueous medium. For ethyl-substituted amines, the order of basicity is $2^\circ>3^\circ>1^\circ>\text{NH}_3$.

Step 3: Detailed Explanation:
Let's classify the given options:
- (A) Methanamine: Primary ($1^\circ$) methylamine
- (B) Ethanamine: Primary ($1^\circ$) ethylamine
- (C) N-ethylethanamine: Secondary ($2^\circ$) ethylamine (Diethylamine)
- (D) N,N-diethylethanamine: Tertiary ($3^\circ$) ethylamine (Triethylamine)
In an aqueous environment, secondary amines are generally the strongest bases. They possess a good balance: two alkyl groups provide a strong electron-donating inductive (+I) effect to increase electron density on the nitrogen, while still retaining a hydrogen atom to allow for effective stabilization of the conjugate acid via hydrogen bonding with water.
Tertiary amines have a stronger +I effect but suffer from significant steric hindrance, which prevents effective hydration of their conjugate acid, making them weaker bases than secondary amines in water. Primary amines have less steric hindrance but a weaker +I effect.
Therefore, N-ethylethanamine (a $2^\circ$ amine) is the strongest base among the choices and will have the highest $\text{K}_b$ and the least $\text{pK}_b$ value.

Step 4: Final Answer:
N-ethylethanamine has the least $\text{pK}_b$ value.