Solvent Systems for Flash Column Chromatography
Flash column chromatography is usually carried out with a mixture of two solvents, with a polar and a nonpolar component.
Occasionally, just one solvent can be used. The only appropriate one-component solvent systems (listed from the least polar
to the most polar):
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Hydrocarbons: pentane, petroleum ether, hexanes
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Ether and dichloromethane: (very similar polarity)
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Ethyl acetate
The most common two-component solvent systems (listed from the least polar to the most polar):
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Ether/Petroleum Ether, Ether/Hexane, Ether/Pentane: Choice of hydrocarbon component depends upon availability
and requirements for boiling range. Pentane is expensive and low-boiling, petroleum ether can be low-boiling, hexane is
readily available.
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Ethyl Acetate/Hexane: The standard, good for ordinary compounds and best for difficult separations.
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Methanol/Dichloromethane: For polar compounds.
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10 percent Ammonia in Methanol Solution/Dichloromethane: Sometimes moves stubborn amines off the baseline.
Good Starting Points
- Polar compounds: 100% EtOAc or 5% MeOH/dichloromethane
- Normal compounds: 10-50% EtOAc/Hexane
- Nonpolar compounds: 5% EtOAc/hexane, 5% ether/hexane, 100% hexane
See Also:
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A compound with an Rf of 0.5 in 10 percent ethyl acetate/hexane will have an Rf of 0.5 in 20 percent ether/hexane. This conversion
factor is general.
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Methanol can be used as polar solvent, but only up to 10 percent of the mixture. More than 10 percent methanol can dissolve
the silica gel.
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Dichloromethane can dissolve compounds better, but it will take longer to run through the silica.
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Benzene is sometimes useful as the non-polar component, but is usually avoided because of toxicity.
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If your compound is sensitive to acid, put
1-3% triethylamine in your solvent system to neutralize acid in the silica gel. The Rf of your compound may increase
a little bit, check first.