Question

A chromatography column, packed with silica gel as stationary phase, was used to separate a mixture of compounds consisting of (A) benzanilide (B) aniline and (C) acetophenone. When the column is eluted with a mixture of solvents, hexane : ethylacelate (20:80), the sequence of obtained compounds is :

• A. (A), (B) and (C)
• B. (B), (A) and (C)
• C. (C), (A) and (B)
• D. (B), (C) and (A)

Hint:

In column chromatography on polar silica gel, the less polar (less strongly adsorbed) compound elutes first and the most polar elutes last.

Answer 1

The Correct Option is C

Step 1: Silica gel is a polar stationary phase, so compounds that bind more strongly (more polar, better hydrogen bonding) move slowly and elute later.
Step 2: Acetophenone (C) is a ketone with only a C=O group (hydrogen-bond acceptor, no N-H/O-H), so it is the least strongly adsorbed.
Step 3: Benzanilide (A) is a secondary amide: a polar C=O plus an N-H, but the N lone pair is delocalised and the N-H is hindered by two aryl groups, so it adsorbs more than the ketone but less than the free amine.
Step 4: Aniline (B) has a free primary NH₂ group which hydrogen-bonds strongly to silanol groups, so it is retained longest.
Step 5: Order of elution: acetophenone (C), benzanilide (A), aniline (B).
Answer: (C) (C), (A) and (B)