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: What stabilises the ion.
In water these ammonium type ions are made stable by hydrogen bonding with water molecules. This stabilising by water is called solvation.
Step 2: The key factor.
The more N-H bonds an ion has, the more hydrogen bonds it can make with water, so the more it is stabilised by solvation.
Step 3: Count the hydrogens.
$\text{R}_3\text{NH}^+$ has $1$ N-H, $\text{R}_2\text{NH}_2^+$ has $2$, $\text{RNH}_3^+$ has $3$, and $\text{NH}_4^+$ has $4$.
Step 4: The extra effect of R groups.
The bulky R groups also crowd the ion and block water from getting close, which lowers solvation further for the alkyl ions.
Step 5: Order the stability.
So solvation stabilises in the order $\text{NH}_4^+ > \text{RNH}_3^+ > \text{R}_2\text{NH}_2^+ > \text{R}_3\text{NH}^+$.
Step 6: Final choice.
The most stabilised by solvation is $\text{NH}_4^+$, which is option 3.
\[ \boxed{\text{NH}_4^+} \]