Arrange the following amines in decreasing order of $pK_b$ values in aqueous solution:

(A) Phenylmethanamine
(B) N-Ethylethanamine
(C) N-methylaniline
(D) Benzenamine

Choose the correct answer from the options given below:

(D), (C), (A), (B)

The correct answer is Option (1) → (D), (C), (A), (B)

To determine the correct order, it is important to remember that $pK_b$ values are inversely proportional to basic strength. A higher $pK_b$ value indicates a weaker base, while a lower $pK_b$ value indicates a stronger base.

Thus the basicity order is: Benzenamine < N-methylaniline < Phenylmethanamine < N-ethylethanamine

Since pKb is inversely related to basicity, the decreasing order of pKb becomes:

Benzenamine > N-methylaniline > Phenylmethanamine > N-ethylethanamine

Analysis of Basicity (Increasing Basic Strength)

1. (D) Benzenamine (Aniline) – Highest $pK_b$ (Weakest Base)

In aniline, the lone pair of electrons on the nitrogen atom is delocalized into the benzene ring through resonance. This makes the lone pair significantly less available for protonation. It is the weakest base in this group.

2. (C) N-Methylaniline

Like aniline, the lone pair is delocalized into the ring. However, the presence of one methyl group ($-CH_3$) provides a slight electron-donating inductive effect ($+I$), making it slightly more basic than aniline.

3. (A) Phenylmethanamine (Benzylamine)

In benzylamine ($C_6H_5CH_2NH_2$), the nitrogen atom is attached to an $sp^3$ hybridized carbon, not directly to the benzene ring. Therefore, the lone pair is not involved in resonance with the ring and is much more available for donation than in aromatic amines.

4. (B) N-Ethylethanamine (Diethylamine) – Lowest $pK_b$ (Strongest Base)

This is a secondary aliphatic amine. It benefits from the $+I$ effect of two ethyl groups which increases electron density on the nitrogen. In aqueous solution, secondary amines like diethylamine typically show the highest basicity due to the combined effects of inductive electron donation and cation stabilization via solvation.