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Recrystallization and Melting Point

Recrystallization and Melting Point. January 19. Purification and Characterization. Purification: separation of target compound from impurities Characterization Identity: Do I have what I think I have? Purity: How pure is the compound?. Recrystallization. Purification for solids

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Recrystallization and Melting Point

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  1. Recrystallization and Melting Point January 19

  2. Purification and Characterization • Purification: separation of target compound from impurities • Characterization • Identity: Do I have what I think I have? • Purity: How pure is the compound?

  3. Recrystallization • Purification for solids • Useful for • Large samples • Final purification step • Based on differential solubility

  4. Practical Aspects Impure Solid  Dissolve Recrystallize Filter www.whfreeman.com/mohrig3e then Movies and Spectra  Williamson Movies  5.3 Recrystallization (macroscale) (Also look at others, such as Drying Organic Liquids and Melting Points)

  5. Recrystallization: Theory • IMF in solids • Temperature dependence • Equilibrium process in slow crystal formation

  6. Important Points • Choice of solvent • Mixed solvents • Maintaining hot solvent • Safety when boiling • Hot filtration • Inducing crystallization • Washing crystals • Too much/too little solvent • Oiling out

  7. Melting Point • Physical property to characterize substance • Identity • purity capillary Melt-temp

  8. Theory and Application • Theory • Pure crystals and impure crystals • Intermolecular Forces • Lattice energy • Application • Depressed MP • Broad MP • Mixed MP Eutectic mixture

  9. Extraction Lab Question: How does the identity of an impurity affect the choice of extraction solvent? Techniques: Extraction, drying, evaporation, melting point

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

  11. Separation AND Characterization

  12. 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 ________)

  13. 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?

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

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

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

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

  18. 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?

  19. Common Problems • Overspotting • Underspotting • Wrong solvent

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