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

OSuccinimide

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