Question

Identify compound from following having highest basic strength.

• A. $\text{CH}_3\text{CH}_2\text{NH}_2$
• B. $\text{NH}_3$
• C. $(\text{CH}_3)_2\text{NH}$
• D. $\text{C}_6\text{H}_5\text{NH}_2$

Hint:

For methyl-substituted aliphatic amines in water, remember the numerical shortcut sequence 213 (Secondary $\gt $ Primary $\gt $ Tertiary) to easily recall the absolute basicity order in exams!

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks us to identify the organic amine or nitrogenous base that possesses the highest basic strength from the provided options.

Step 2: Key Formula or Approach:
The basic strength of aliphatic amines in an aqueous solution is governed by a combination of three factors: 1.

Inductive effect ($+I$): Electron-donating alkyl groups increase electron density on the nitrogen atom. 2.

Steric hindrance: Bulkier alkyl groups obstruct the approach of a proton. 3.

Solvation effect: Smaller, less substituted ammonium cations form more stabilizing hydrogen bonds with water molecules.

Step 3: Detailed Explanation:
Let's analyze the basicity trends for each class of compound presented:

Aniline ($\text{C}_6\text{H}_5\text{NH}_2$): The lone pair of electrons on the nitrogen atom is delocalized into the aromatic benzene ring via resonance. This significantly lowers its availability for protonation, making aromatic amines exceptionally weak bases.

Ammonia ($\text{NH}_3$): Lacks any electron-donating alkyl groups to enhance nitrogen electron density.

Aliphatic Methyl Amines: When comparing methyl-substituted amines in an aqueous medium, the combined interplay of inductive, steric, and solvation factors establishes a strict basicity order: $$\text{Secondary (2}^\circ\text{)} \gt \text{Primary (1}^\circ\text{)} \gt \text{Tertiary (3}^\circ\text{)} \gt \text{NH}_3$$ $$(\text{CH}_3)_2\text{NH} \gt \text{CH}_3\text{NH}_2 \gt (\text{CH}_3)_3\text{N} \gt \text{NH}_3$$ Therefore, dimethylamine, $(\text{CH}_3)_2\text{NH}$, which is a secondary amine, experiences the optimal balance of $+I$ electron donation and minimal steric hindrance, giving it the highest basic strength.

Step 4: Final Answer:
The compound with the highest basic strength is $(\text{CH}_3)_2\text{NH}$, which corresponds to option (C).