Question

Which of the following conjugate bases is stabilized to greater extent due to solvation of ammonia and amines?

• A. $\text{R}_3\text{NH}^+$
• B. $\text{RNH}_3^+$
• C. $\text{NH}_4^+$
• D. $\text{R}_2\text{NH}_2^+$

Hint:

When ranking basicity of amines in aqueous solutions, remember that the trend is a battle between +I (inductive) effect of alkyl groups and solvation energy! More hydrogens = better solvation!

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks us to identify which of the given cationic species (conjugate acids of ammonia and amines, though referred to as conjugate bases in the raw text) experiences the maximum stabilization through solvation (specifically hydration in water).

Step 2: Key Formula or Approach:
In an aqueous solution, the stability of a substituted ammonium cation depends heavily on its ability to form hydrogen bonds with water molecules.
The greater the number of hydrogen atoms directly attached to the positively charged nitrogen atom, the more extensive the hydrogen bonding with water, and the higher the hydration (solvation) energy.

Step 3: Detailed Explanation:
Let's count the number of hydrogen atoms available for hydrogen bonding in each ion:
(A) $\text{R}_3\text{NH}^+$ (Tertiary ammonium ion): Has only 1 hydrogen atom. Minimum hydrogen bonding.
(D) $\text{R}_2\text{NH}_2^+$ (Secondary ammonium ion): Has 2 hydrogen atoms.
(B) $\text{RNH}_3^+$ (Primary ammonium ion): Has 3 hydrogen atoms.
(C) $\text{NH}_4^+$ (Ammonium ion): Has 4 hydrogen atoms. Maximum hydrogen bonding.
Because $\text{NH}_4^+$ has the most N-H bonds, it is stabilized to the greatest extent by solvation. The bulky alkyl (R) groups in the other options also cause steric hindrance, further decreasing solvation.
The order of stabilization by solvation is: $\text{NH}_4^+ > \text{RNH}_3^+ > \text{R}_2\text{NH}_2^+ > \text{R}_3\text{NH}^+$.

Step 4: Final Answer:
The $\text{NH}_4^+$ ion is stabilized to the greatest extent, which corresponds to option (C).