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Difficulty: MediumPaper and Column Chromatography

A mixture containing four organic compounds of varying polarities—Compound P (strongly polar), Compound Q (moderately polar), Compound S (weakly polar), and Compound R (non-polar)—is separated using paper chromatography with a polar stationary phase (water bound to cellulose) and a non-polar mobile phase solvent. What is the correct sequence of these compounds when arranged in order of increasing RfR_f value (from smallest RfR_f to largest RfR_f)?

  1. 1Compound P (strongly polar)
  2. 2Compound Q (moderately polar)
  3. 3Compound S (weakly polar)
  4. 4Compound R (non-polar)

Answer

Compound P (strongly polar) → Compound Q (moderately polar) → Compound S (weakly polar) → Compound R (non-polar)
In paper chromatography using a polar stationary phase and non-polar mobile phase, retention factor (RfR_f) is inversely related to solute polarity. The strongly polar compound (Compound P) binds most tightly to the polar water molecules on the paper, retarding its movement and giving it the smallest RfR_f value. As compound polarity decreases (Q → S → R), solubility in the non-polar mobile phase increases, allowing the solute to travel further toward the solvent front, resulting in progressively larger RfR_f values.

Step-by-Step Solution

1
Analyze the stationary and mobile phase characteristics.
Stationary phase is polar (water on paper), and mobile phase is non-polar solvent.
Chromatographic separation depends on partitioning between polar and non-polar phases.
2
Determine phase affinities for each compound based on polarity.
Strongly polar compounds adsorb strongly onto the polar stationary phase, while non-polar compounds dissolve preferentially in the non-polar mobile phase.
'Like dissolves like' governs partitioning between stationary and mobile phases.
3
Relate distance traveled to Retention Factor (RfR_f).
Rf=distance traveled by solutedistance traveled by solvent frontR_f = \frac{\text{distance traveled by solute}}{\text{distance traveled by solvent front}}. Solutes that travel further have larger RfR_f values.
Retention factor directly measures relative mobility.
4
Order compounds by increasing RfR_f value.
Compound P (lowest RfR_f) → Compound Q → Compound S → Compound R (highest RfR_f).
Increasing polarity corresponds to decreasing distance traveled in normal-phase paper chromatography.

Key Concept

Partitioning and Retention Factor (RfR_f) in Paper Chromatography
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Topics

Paper and Column Chromatography

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