Problem 9 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

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: (a) (B), (C) and (A) (b) (B), (A) and (C) (c) (C), (A) and (B) (d) (A), (B) and (C)

Step-by-Step Solution

Answer

The answer is option (c): (C), (A), (B).

Step 1: Understand the Separation Mechanism

Chromatography separates compounds based on their interactions with the stationary phase (silica gel) and the mobile phase (hexane:ethyl acetate). In normal phase chromatography, the stationary phase is polar, so polar compounds will have stronger interactions with it and elute slower than non-polar compounds.

Step 2: Analyze Compound Polarity

1. Aniline (B) - a primary amine, is fairly polar due to the presence of an amino group. 2. Acetophenone (C) - a ketone, has moderate polarity due to the carbonyl group. 3. Benzanilide (A) - contains an amide group; is polar but less so compared to aniline.

Step 3: Predict Elution Order

Since the stationary phase is polar, less polar compounds will elute first when using a polar stationary phase: 1. Acetophenone (C) - less polar due to the presence of a ketone, will elute before the others. 2. Benzanilide (A) - is moderately polar due to the amide group. 3. Aniline (B) - being the most polar, will elute last.

Step 4: Match to Given Options

Order established is (C), (A), (B). Check which of the given options matches this order: - (a) (B), (C), (A) - (b) (B), (A), (C) - (c) (C), (A), (B) - (d) (A), (B), (C) The correct sequence is option (c) (C), (A), and (B).

Key Concepts

Stationary Phase

In chromatography, the stationary phase is a vital component that helps in separating different compounds within a mixture. In our scenario, silica gel serves as the stationary phase. The role of this phase is to provide a surface for the compounds in the mixture to interact with as they pass through. Silica gel is known for its polar characteristics. This property can significantly influence how different compounds interact with it.

A polar stationary phase like silica gel will generally interact more strongly with polar compounds. This stronger interaction means that polar substances will adhere more to the stationary phase and thus move more slowly through the chromatography column. In contrast, non-polar compounds will not adhere as strongly and will travel more quickly. This is why understanding the nature of the stationary phase is crucial in predicting elution order. It determines how long a compound will remain within the column before being eluted.

Elution Order

Elution order is all about the sequence in which compounds exit the chromatography column. This sequence is heavily influenced by the interactions between the compounds and both the stationary phase and the mobile phase.

In the given exercise, we used a mixture of solvents (hexane and ethyl acetate) as the mobile phase and silica gel as the stationary phase. The polar nature of the silica gel means polar compounds will have stronger interactions and thus elute slower than their less polar counterparts. This is a principle of normal phase chromatography, where less polar substances will move through faster.

For instance, in our exercise, acetophenone (C) elutes first because it has a lower polarity compared to benzanilide (A) and aniline (B). Benzanilide comes next as it has moderate polarity. Lastly, aniline, being the most polar, binds the strongest to the stationary phase and therefore elutes last.

Compound Polarity

Understanding compound polarity is key to predicting how substances behave in chromatography. Polarity is about how the electrons are distributed across a compound, affecting its interaction with polar and non-polar environments.

In this exercise:

• **Aniline (B)** - This is a primary amine, which is quite polar due to the presence of a nitrogen atom. Polar substances have a tendency to form hydrogen bonds, resulting in strong interactions with polar stationary phases like silica gel.
• **Acetophenone (C)** - This compound has moderate polarity due to its carbonyl group (C=O). The oxygen atom creates a region of negativity that interacts less strongly compared to aniline, making acetophenone elute before more polar compounds.
• **Benzanilide (A)** - It contains a polar amide group, but it is less polar than aniline. This is significant enough to anchor it somewhat to the stationary phase, delaying its elution compared to acetophenone.

By understanding these polar characteristics, students can predict the interaction strengths and thus, the order of elution in chromatography experiments.