In normal-phase TLC, which compounds move slowest?

In normal-phase TLC, which compounds move slowest?

• A. Nonpolar compounds
• B. Polar compounds
• C. Aromatic compounds
• D. Ionic compounds

Answer: (B)

In normal-phase TLC, the stationary phase is polar, so how far a compound moves depends on how strongly it sticks to that polar surface. Polar molecules have strong interactions with the polar stationary phase—hydrogen bonding and dipole-dipole attractions with silanol groups—so they are retained longer on the plate and migrate more slowly. The mobile phase in normal-phase TLC is relatively nonpolar, so polar solutes aren’t carried far by it and stay adsorbed to the surface longer. Nonpolar compounds, by contrast, interact weakly with the polar stationary phase and are carried along more by the mobile phase, moving farther up the plate. Aromaticity alone doesn’t set the retention, since an aromatic compound can be polar or nonpolar depending on its substituents, and ionic species would also tend to strong retention, but the key factor here is overall polarity.

In normal-phase TLC, the stationary phase is polar, so how far a compound moves depends on how strongly it sticks to that polar surface. Polar molecules have strong interactions with the polar stationary phase—hydrogen bonding and dipole-dipole attractions with silanol groups—so they are retained longer on the plate and migrate more slowly. The mobile phase in normal-phase TLC is relatively nonpolar, so polar solutes aren’t carried far by it and stay adsorbed to the surface longer. Nonpolar compounds, by contrast, interact weakly with the polar stationary phase and are carried along more by the mobile phase, moving farther up the plate. Aromaticity alone doesn’t set the retention, since an aromatic compound can be polar or nonpolar depending on its substituents, and ionic species would also tend to strong retention, but the key factor here is overall polarity.