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
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
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