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 Question:
This question asks us to analyze a thin-layer chromatography (TLC) plate with two compounds, A and B, to calculate their retardation factors ($R_f$) and determine their relative polarities based on their elution behavior on silica gel.

Step 2: Key Formula or Approach:
The retardation factor ($R_f$) is defined as:
\[ R_f = \frac{\text{Distance traveled by the solute (compound)}}{\text{Distance traveled by the solvent front}} \] On a silica-gel based TLC plate (polar stationary phase):
- Silica gel contains highly polar silanol ($\text{Si}-\text{OH}$) groups.
- Polar compounds interact strongly with the silica gel via hydrogen bonding and dipole-dipole forces, causing them to move slowly (resulting in a lower $R_f$ value).
- Less polar compounds interact weakly with the stationary phase and travel further with the mobile phase, resulting in a higher $R_f$ value.

Step 3: Detailed Explanation:
Let's perform the calculations based on the given chromatogram:

• 1. Total distance traveled by the solvent front:
- Distance from baseline ($0\text{ cm}$) to solvent front $= 5.0\text{ cm}$.

• 2. Calculate $R_f$ for Compound A:
- Distance traveled by Compound A $= 3.75\text{ cm}$.
\[ R_f(\text{A}) = \frac{3.75\text{ cm}}{5.0\text{ cm}} = 0.75 \]
• 3. Calculate $R_f$ for Compound B:
- Distance traveled by Compound B $= 1.25\text{ cm}$.
\[ R_f(\text{B}) = \frac{1.25\text{ cm}}{5.0\text{ cm}} = 0.25 \]
• 4. Relative Polarity Analysis:
- Since Compound B has a much lower $R_f$ value ($0.25$) than Compound A ($0.75$), Compound B has migrated a much shorter distance.
- This indicates that B is more strongly adsorbed to the polar silica gel, meaning B is more polar than A.

Step 4: Final Answer:
Compound B is more polar than A, and the $R_f$ of A is $0.75$. This corresponds exactly to option (A).