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 Useful for Large samples Final purification step Based on differential solubility

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)

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

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

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

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

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

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 Rf Polar solvents outcompete compounds, drive them up plate

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

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

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?

Common Problems Overspotting Underspotting Wrong solvent