In normal-phase TLC, how does polarity relate to Rf?

In normal-phase TLC, how does polarity relate to Rf?

• A. More polar compounds always have higher Rf
• B. Polarity has no effect on Rf
• C. More polar compounds typically have lower Rf
• D. Rf is independent of polarity

Answer: (C)

In normal-phase TLC, the stationary phase is polar, so polarity governs how strongly a compound sticks to the plate. More polar molecules form stronger interactions with the polar stationary phase (like hydrogen bonding and dipole interactions), so they migrate more slowly and cover a shorter distance. Since Rf is the distance moved by the compound divided by the distance moved by the solvent front, these strongly retained, more polar compounds end up with a smaller Rf. Less polar compounds interact weakly with the stationary phase, travel farther with the mobile phase, and thus show higher Rf values. In short, polarity and Rf move in opposite directions in normal-phase TLC: higher polarity tends to give lower Rf.

In normal-phase TLC, the stationary phase is polar, so polarity governs how strongly a compound sticks to the plate. More polar molecules form stronger interactions with the polar stationary phase (like hydrogen bonding and dipole interactions), so they migrate more slowly and cover a shorter distance. Since Rf is the distance moved by the compound divided by the distance moved by the solvent front, these strongly retained, more polar compounds end up with a smaller Rf. Less polar compounds interact weakly with the stationary phase, travel farther with the mobile phase, and thus show higher Rf values. In short, polarity and Rf move in opposite directions in normal-phase TLC: higher polarity tends to give lower Rf.