Question

What is the order of pK$_b$ for the following molecules in an aqueous medium?

• A. P $<$ R $<$ Q $<$ S
• B. P $<$ Q $<$ R $<$ S
• C. S $<$ P $<$ R $<$ Q
• D. S $<$ P $<$ Q $<$ R

Hint:

Basicity: Aliphatic amines $>$ Aromatic amines.
For aromatic amines, adding electron-donating alkyl groups on the nitrogen increases basicity ($\text{p}K_b$ decreases).
Thus, the more methyl groups on aniline, the stronger the base (lower $\text{p}K_b$).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
$pK_{b}$ is inversely proportional to base strength. A lower $pK_{b$} means a stronger base.
We need to determine the basicity order of the four amines: Benzylamine (P), N,N-dimethylaniline (R), N-methylaniline (Q), and Aniline (S).

Step 2: Detailed Explanation:
$\bullet$ P (Benzylamine): The lone pair on Nitrogen is on an $sp^{3}$ carbon separated from the ring. It cannot participate in resonance. It is an aliphatic-like amine and is the strongest base (Lowest $pK_{b}$).
$\bullet$ S, Q, R (Aromatic Amines): The lone pair is in resonance with the benzene ring, making them significantly weaker bases than P.
$\bullet$ Among them, the methyl groups are electron-donating (+I effect). More methyl groups increase the electron density on N and stabilize the conjugate acid.
$\bullet$ R (N,N-dimethylaniline): Two methyl groups. Strongest of the aromatics.
$\bullet$ Q (N-methylaniline): One methyl group.
$\bullet$ S (Aniline): No methyl groups. Weakest base (Highest $pK_{b}$).
$\bullet$ Basicity Order: P $>$ R $>$ Q $>$ S.
$\bullet$ $pK_{b$ Order:} P $<$ R $<$ Q $<$ S.

Step 3: Final Answer:
The order is P $<$ R $<$ Q $<$ S.
This matches option (A).