Question

Consider the following silica-gel based thin-layer chromatogram of compounds A and B. Which one of the following statements is correct?

• A. B is more polar than A; A has R$_f$ = 0.75.
• B. B is more polar than A; A has R$_f$ = 0.25.
• C. A is more polar than B; B has R$_f$ = 0.33.
• D. B is less polar than A; B has R$_f$ = 0.75.

Hint:

TLC Polarity Rule:
Low spot (low $R_f$) = High polarity (clings tightly to the polar silica).
High spot (high $R_f$) = Low polarity (travels easily with the solvent).
$R_f$ is always a fraction between 0 and 1.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Thin-layer chromatography (TLC) on silica gel (a polar stationary phase) separates components based on polarity.
Polar compounds interact strongly with silica and move slower (lower spot), while non-polar compounds move faster (higher spot).
Key Formula or Approach:
The Retardation Factor (\( R_{f} \)) is calculated as:
\[ R_{f} = \frac{\text{Distance traveled by compound}}{\text{Distance traveled by solvent front}} \]

Step 2: Detailed Explanation:
From the chromatogram scale:
$\bullet$ Solvent front distance: 5.0 units.
$\bullet$ Compound A distance: 3.75 units.
\[ R_{f}(A) = \frac{3.75}{5.0} = 0.75 \]
$\bullet$ Compound B distance: 1.25 units.
\[ R_{f}(B) = \frac{1.25}{5.0} = 0.25 \]
$\bullet$ Polarity Analysis: Compound B has a much lower $R_{f}$ value than A. This means B adhered more strongly to the polar silica gel, indicating that B is more polar than A.

Step 3: Final Answer:
B is more polar and $R_{f}$ of A is 0.75.
This matches option (A).