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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
• Theory– 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
O
OH
O
OCH2CH3
O
H2N
Fluorene Fluorenone
Benzoic Acid Ethyl 4-Aminobenzoate
NH
O
O
Succinimide
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
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)?
Common Problems
• Overspotting• Underspotting• Wrong solvent