Chromatography

TLC Thin layer chromatography Stationary phase Mobile phase

Separation AND Characterization

Chromatography Basics Based on different affinities for stationary and mobile phases Silica gel: polar, water-covered surface – Compound(s) Polar: _______ affinity for plate, travels _______ Nonpolar: _______ affinity for plate, travels ______ – Developing solvent Polar: higher affinity for plate, travels slower, displaces compound more (compound travels __________) Nonpolar: lower affinity for plate, travels faster, displaces compound less (compound travels ________)

Test your Understanding Which spot represents a more polar compound? What would happen to each spot if a less polar solvent were used? Why should you ALWAYS report your developing solvent with any TLC data?

Quantitative Characterization Retention factor Distance traveled/ solvent front distance Unitless For silica gel TLC, based on polarity of the compound(s) Must report solvent!

Solvent effect on R f Polar solvents outcompete compounds, drive them up plate

Choosing a Developing Solvent Adjust solvent to give R f values around 0.4 Common mixtures – Ether/Hex – EtOAc/Hex – CH 2 Cl 2 /methanol Determined experimentally

Visualization Most compounds are invisible on TLC UV lamp Stains Iodine chamber

Application of TLC Purity Identity Reaction Progress What can we determine about the identity of the unknown? Column 1 is your target compound; column 2 is an expected impurity. What can you determine about your reaction (column 3)?

Common Problems Overspotting Underspotting Wrong solvent

Column Chromatography Similar to TLC in separation Preparative process – 0.1g to 5 g scale Purify small quantities of liquids/solids (contrast recrystallization)

Mobile phase: eluent similar to TLC Stationary phase: silica gel similar to TLC Column is upside down from TLC, so a larger Rf for a compound means it comes out ____

Practical Considerations Preparing the column Loading the sample Choosing the solvent Separation capacity

Practical Considerations Preparing the column Loading the sample Choosing the solvent Separation capacity

Practical Considerations Preparing the column Loading the sample Choosing the solvent Separation capacity Most common: Hexane/ethyl acetate CH 2 Cl 2 /methanol

Practical Considerations Preparing the column Loading the sample Choosing the solvent Separation capacity – Effect of diameter – Effect of length – Effect of Silica gel grade

Flash Chromatography Faster separation Tighter separations

HPLC