Question

The elution sequence of a mixture of compounds containing chlorobenzene, anthracene and p-cresol developed on an alumina column using a solvent system of progressively increasing polarity is

• A. anthracene \(\rightarrow\) chlorobenzene \(\rightarrow\) p-cresol
• B. anthracene \(\rightarrow\) p-cresol \(\rightarrow\) chlorobenzene
• C. chlorobenzene \(\rightarrow\) p-cresol \(\rightarrow\) anthracene
• D. chlorobenzene \(\rightarrow\) anthracene \(\rightarrow\) p-cresol
• E. p-cresol \(\rightarrow\) anthracene \(\rightarrow\) chlorobenzene

Hint:

"Like dissolves like, but opposites attract in chromatography." The polar stationary phase holds onto polar molecules tightly, letting non-polar ones slip by first.

Answer 1

The Correct Option is A

Concept: In column chromatography with a polar adsorbent like alumina (\(Al_2O_3\)), compounds are separated based on their polarity.
• Least polar compounds have weak interactions with the adsorbent and elute (come out) first.
• Most polar compounds have strong interactions and elute last.

Step 1: Rank the compounds by polarity.

• Anthracene: A pure hydrocarbon (polycyclic aromatic). It is non-polar.
• Chlorobenzene: Contains a polar \(C-Cl\) bond, making it slightly polar.
• p-Cresol: Contains a hydroxyl (\(-OH\)) group, which can form hydrogen bonds with alumina. It is the most polar of the three.

Step 2: Determine elution order.
Order of polarity: Anthracene < Chlorobenzene < p-Cresol. Therefore, the elution sequence is: Anthracene \(\rightarrow\) Chlorobenzene \(\rightarrow\) p-Cresol.