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TLC and Dihydroxylation

TLC and Dihydroxylation. Studying the Outcome of Reactions. Big question in organic chemistry lab: What did we make?. Stereochemistry. Possible outcomes? Predicted outcomes?. Table of Reagents. For SYNTHETIC RXNS, fill out stoichiometry table

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TLC and Dihydroxylation

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  1. TLC and Dihydroxylation

  2. Studying the Outcome of Reactions Big question in organic chemistry lab: What did we make?

  3. Stereochemistry Possible outcomes? Predicted outcomes?

  4. Table of Reagents • For SYNTHETIC RXNS, fill out stoichiometry table • Don’t include solvents unless part of reaction • Reference: www.sigmaaldrich.com/chemistry

  5. Characterization • Could attempt a melting point, but, separation would be difficult. (Why?) • Is there another characterization technique?

  6. TLC • Thin layer chromatography • Stationary phase • Mobile phase

  7. Separation AND Characterization

  8. 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 ________)

  9. 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?

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

  11. Solvent effect on Rf • Polar solvents outcompete compounds, drive them up plate

  12. Choosing a Developing Solvent • Adjust solvent to give Rf values around 0.4 • Common mixtures • Ether/Hex • EtOAc/Hex • CH2Cl2/methanol • Determined experimentally

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

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

  15. Common Problems • Overspotting • Underspotting • Wrong solvent

  16. Stereochemistry of TLC • TLC plate is achiral • How many TLC spots would be observed for • Mix of A and B • Mix of B and C • Mix of A, B, and C?

  17. Your Lab • In this lab, you will: • Conduct two reactions • Run TLC of your products vs authentic samples • Draw conclusions based on TLC • Suggest consistent mechanisms • Write a formal experimental section • Common problems • Difficult to spot aqueous samples (also, lots of solid impurities) • Overspotting or underspotting • Impurities in developing jar (WATER!!!)

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