silica gel
Summary
Silica gel is porous silicon dioxide (SiO₂) with polar Si–OH groups on its surface. It is the usual polar stationary phase in TLC and column chromatography, so polar compounds stick more strongly and move slowly (low Rf), while nonpolar compounds elute first.
Detail
Meaning: Silica gel is a porous, granular form of silicon dioxide (SiO₂) whose surface is covered with polar Si–OH groups. In thin-layer and column chromatography it is the usual polar stationary phase, and a less polar solvent (the eluent) is the mobile phase. Compounds separate by how strongly they adsorb to the silica, mainly through hydrogen bonding and dipole interactions. Polar molecules such as alcohols, carboxylic acids and amines bind tightly and move slowly. Nonpolar molecules such as alkanes bind weakly and travel with the solvent.
Equation: Rf = d(spot) / d(front), where each distance is measured in m from the origin line (to the spot center or to the solvent front); Rf is dimensionless and between 0 and 1.
On the MCAT: A passage shows a TLC plate or a column separating a mixture. Rank the compounds by polarity: the highest Rf, or the first fraction off the column, is the least polar. Making the eluent more polar (more ethyl acetate in hexane) pushes all spots farther and raises every Rf.
Common confusion: Silica separates by polarity, not size, unlike size-exclusion chromatography. Reverse-phase chromatography uses a nonpolar stationary phase, which flips the order: there, polar compounds elute first.
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.