Question

You are asked to run a chromatographic column for separation of a mixture of compounds: CH3CH2OCH3, CH3(CH2)3OH, CH3CO2H, CH3(CH2)8CH3. Two solvents have been provided i.e. acetone and hexane. Which eluting solvent would you use first and predict the order of elution for the compounds.

Answer 1

In Column chromatography there will be two phases present one is stationary phase (usually silica or alumina) and the second one is mobile phase (usually it a mixture of solvents having different polarities).

The separation of the compounds in the Column chromatography is based on the interactions of the given compound with the stationary phase (let say here it is silica gel).

The compounds which have more polar groups like OH, NH₂ will interact more with the absorbent silica gel through intermolecular forces. The compounds which do not have polar groups have fewer interactions with the absorbent silica gel.

When the column chromatography elute in a mobile phase (here in the given question the solvents are acetone and hexane) Acetone is more polar than the hexane.

For the given compounds the polarity order is like this : CH₃(CH₂)₈CH₃ is the most nonpolar among all, followed by CH₃CH₂OCH₃, followed by CH₃(CH₂)₃OH then CH₃CO₂H which more polar.

If the elution of the column started with less polar solvent (hexane) followed by the gradual increase in the percentage of polar solvent (acetone), the compound which is non polar will elute fast and t whereas the compounds having polar groups due to intermolecular forces with stationary phase (silica gel) they will not elute fast and will come slow when compare to non-polar compounds.