Question

Which among the following is most basic in aqueous medium

• A. Benzenamine
• B. N-ethylethanamine
• C. N,N-diethylethanamine
• D. Methylamine

Hint:

Always memorize the basicity order of alkylamines in an aqueous phase:
For Ethyl group: \(2^\circ>3^\circ>1^\circ>\text{NH}_3\)
For Methyl group: \(2^\circ>1^\circ>3^\circ>\text{NH}_3\)

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The basicity of amines in an aqueous solution depends on a delicate balance of three primary factors:
1. The +I (inductive) effect of the alkyl groups, which increases electron density on the nitrogen.
2. Solvation (hydration) of the conjugate acid, where more N-H bonds allow for better stabilization via hydrogen bonding with water.
3. Steric hindrance, which can prevent water molecules from solvating the ion effectively.
Step 2: Key Formula or Approach:
The approach here is to evaluate the competing effects of inductive electron donation (favoring \(3^\circ>2^\circ>1^\circ\)) and hydration/steric effects (favoring \(1^\circ>2^\circ>3^\circ\)) to determine the overall experimental trend in an aqueous medium.
Step 3: Detailed Explanation:
Let's analyze each option:
- i) Benzenamine (Aniline): The lone pair on nitrogen is delocalized over the benzene ring via resonance. This makes it the least available for accepting a proton. It is a very weak base.
- ii) N-ethylethanamine (Diethylamine): This is a secondary (\(2^\circ\)) amine. In aqueous solutions, secondary amines generally represent the optimal balance between the +I effect of two alkyl groups and sufficient hydration of the conjugate acid (which still has two hydrogen bonds to form with water).
- iii) N,N-diethylethanamine (Triethylamine): This is a tertiary (\(3^\circ\)) amine. Although it has the maximum +I effect, its conjugate acid is highly sterically hindered and can only form one hydrogen bond with water. Thus, its solvation energy is low, reducing its basicity compared to the \(2^\circ\) amine.
- iv) Methylamine: This is a primary (\(1^\circ\)) amine. It has less +I effect than secondary or tertiary amines.
For ethyl-substituted amines, the experimentally determined order of basic strength in an aqueous medium is:
\(2^\circ \text{ (Diethylamine)}>3^\circ \text{ (Triethylamine)}>1^\circ \text{ (Ethylamine)}>\text{Ammonia}\)
Therefore, N-ethylethanamine is the most basic among the given choices.
Step 4: Final Answer:
The most basic compound is ii) N-ethylethanamine.