A chromatography column, packed with silica gel as stationary phase, was used to separate a mixture

Q: Why doesn't aniline elute first since it can form hydrogen bonds?

While aniline forms strong hydrogen bonds, it forms them primarily with the stationary phase (silica gel), which slows it down. The highly polar mobile phase prefers compounds with higher overall dipole moments like acetophenone.

Q: What is the role of hexane in the mobile phase?

Hexane is a non-polar solvent. Mixing it with ethyl acetate allows chemists to fine-tune the overall polarity of the mobile phase to achieve the perfect separation.

Q: How do we know acetophenone has a higher dipole moment than aniline?

The carbonyl group (C=O) in acetophenone has a very strong dipole due to the high electronegativity difference between carbon and oxygen, without the opposing vectors found in some other functional groups.

Q: What does the $R_f$ value indicate?

The Retention factor ($R_f$) indicates how far a compound travels relative to the solvent front. A higher $R_f$ means the compound moves faster and elutes earlier.

Q: Would the order change if we used a purely non-polar mobile phase?

Yes! With a non-polar mobile phase, the polar compounds would stick tightly to the silica gel, and the least polar compound would likely elute first.

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 : ethyl acetate (20 : 80), the sequence of obtained compounds is

Options:

(A) (C), (A) and (B)

(B) (A), (B) and (C)

(C) (B), (C) and (A)

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

Sigma Percentile · Accepted Answer

Imagine a race track, but instead of cars, we have molecules, and instead of asphalt, the track is made of silica gel. This is the fascinating world of **Column Chromatography**! In this problem, we are tasked with separating a mixture of three organic compounds: Benzanilide, Aniline, and Acetophenone.

### The Setup: The Track and the Wind
In any chromatography experiment, there are two main players: the **stationary phase** (the track) and the **mobile phase** (the wind pushing the racers).

Here, our stationary phase is **silica gel**, which is known for being highly polar due to its surface hydroxyl ($-OH$) groups.

Our mobile phase is a solvent mixture of hexane and ethyl acetate in a $20:80$ ratio. Because ethyl acetate is quite polar and makes up $80\%$ of the mixture, our mobile phase is acting like a strong, highly polar wind.

### The Principle of Elution
So, who wins the race? The rule of thumb in chromatography is simple: **like dissolves like**.

Molecules that have a stronger affinity for the mobile phase will be carried down the column faster, resulting in a higher Retention factor ($R_f$) and an earlier elution time. Conversely, molecules that stick strongly to the stationary phase will lag behind.

Since our mobile phase is highly polar, it will preferentially grab onto the most polar molecules in our mixture. Therefore, the compound with the **greatest dipole moment** will be swept away fastest and elute first!

### Analyzing the Competitors
Let's evaluate the dipole moments of our three contenders:

**1. Acetophenone (C):** 
This molecule features a carbonyl group ($-C=O$). Oxygen is highly electronegative, pulling electron density away from the carbon atom and creating a very strong, localized dipole. Acetophenone boasts the highest dipole moment among the three ($\approx 3.0 	ext{ D}$).

**2. Benzanilide (A):** 
Benzanilide contains an amide linkage ($-NH-CO-$). While amides are generally polar, the presence of two large, bulky benzene rings on either side helps to distribute the electron cloud, slightly dampening the overall dipole moment ($\approx 2.7 	ext{ D}$). It is polar, but not quite as polar as acetophenone.

**3. Aniline (B):** 
Aniline has an amine group ($-NH_2$). While it is capable of hydrogen bonding, its overall dipole moment is the lowest of the group ($\approx 1.5 	ext{ D}$). Furthermore, its ability to act as a strong hydrogen bond donor means it interacts very strongly with the polar silica gel, causing it to drag its feet during the race.

### The Final Sequence
Putting it all together, the polar mobile phase will carry the molecules in decreasing order of their dipole moments.

Acetophenone **(C)**, being the most polar, crosses the finish line first. Benzanilide **(A)** follows in second place. Finally, Aniline **(B)**, with the lowest dipole moment and strongest attachment to the silica gel, emerges last.

Therefore, the sequence of obtained compounds is **(C) > (A) > (B)**. The great chromatography race is complete!

Select Answer:

Visualized Solution

$Chromatography Setup$

$Principle of Elution$

$Polarity of Acetophenone (C)$

$Polarity of Benzanilide (A)$

$Polarity of Aniline (B)$

$Elution Sequence$

The Sigma Insight: Nomenclature and Characterisation

The Setup

The Principle of Elution

Analyzing the Competitors

The Final Sequence

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