Organic Chemistry Chromatography Question

Question

If there is a situation where a student uses a 5:1 pentane/ethyl acetate solvent for both their TLC and column chromatography runs, why is that not a good idea? Why should a different ratio of the solvent system be used for TLC and column chromatography? 

Answer 1

In general for column chromatography, you want to run an eluent that gives you ~0.3 Rf of the target compound on TLC. This ensures that the compound comes out slowly enough through to column to allow for good separation in most cases.

On the other hand, most of the times you're analyzing something by TLC, its preferred to have your target compound elute ~0.5 Rf so that it is easier to analyze the reaction/fractions for purity since as things travel up the TLC plate further, there is more separation.

Therefore, usually people tend to use a more polar solvent system for TLC than for the column. (eg. 5:1 pentane:EA for column and 3:1 pentane:EA for TLC)

Answer 2

There is a possibility that the particular ratio used may result in co-elution of compounds of a mixture. Changing the solvent ratio may result in separating the co-eluting compounds.