chromatography
DESCRIPTION
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) - PowerPoint PPT PresentationTRANSCRIPT
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Chromatography
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TLC
• Thin layer chromatography
• Stationary phase• Mobile phase
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Separation AND Characterization
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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 ________)
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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?
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Quantitative Characterization
• Retention factor• Distance traveled/
solvent front distance
• Unitless• For silica gel TLC,
based on polarity of the compound(s)
Must report solvent!
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Solvent effect on Rf
• Polar solvents outcompete compounds, drive them up plate
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Choosing a Developing Solvent
• Adjust solvent to give Rf values around 0.4
• Common mixtures– Ether/Hex– EtOAc/Hex– CH2Cl2/methanol
• Determined experimentally
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Visualization
• Most compounds are invisible on TLC
• UV lamp• Stains• Iodine chamber
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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)?
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Common Problems
• Overspotting• Underspotting• Wrong solvent
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Column Chromatography
• Similar to TLC in separation• Preparative process– 0.1g to 5 g scale
• Purify small quantities of liquids/solids (contrast recrystallization)
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• 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 ____
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Practical Considerations
• Preparing the column• Loading the sample• Choosing the solvent• Separation capacity
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Practical Considerations
• Preparing the column• Loading the sample• Choosing the solvent• Separation capacity
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Practical Considerations
• Preparing the column• Loading the sample• Choosing the solvent• Separation capacity
Most common: Hexane/ethyl acetate CH2Cl2/methanol
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Practical Considerations
• Preparing the column• Loading the sample• Choosing the solvent• Separation capacity– Effect of diameter– Effect of length– Effect of Silica gel grade
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Flash Chromatography
• Faster separation• Tighter
separations
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HPLC