Rf values

Meaning: In TLC, a spot is placed on a plate coated with a stationary phase, usually polar silica or alumina. A solvent (mobile phase) moves up the plate by capillary action and carries compounds along. Each compound partitions between the two phases. A compound that sticks strongly to the stationary phase moves little. A compound that dissolves well in the mobile phase moves far.

Equation: Rf = dcompound / dsolvent, where dcompound is the distance from the origin line to the center of the spot (cm or mm) and dsolvent is the distance from the origin line to the solvent front (same unit). Rf has no units.

On the MCAT: Expect a drawn TLC plate, with spots for a mixture and for standards. You may be asked to rank compounds by polarity, calculate Rf from distances, or predict what happens when a more polar solvent is used. On silica, polar compounds (alcohols, acids, amides) have lower Rf than nonpolar ones (alkanes, ethers, alkyl halides). A more polar solvent moves all spots farther, so Rf values rise and the spots crowd together.

Common confusion: Students reverse the trend and assume polar compounds travel farthest. On a polar stationary phase, polar compounds bind more strongly and travel less. The trend flips in reverse-phase chromatography, where the stationary phase is nonpolar. Also, measure to the solvent front, not to the top of the plate. Rf cannot exceed 1, because a compound cannot outrun the solvent.