# How to determine the order of elution in silica gel column chromatography?

Benzofuranoids ustusoranes A (1), B (2), and E (3) were isolated from Aspergillus ustus 094102. If these compounds were separated by column chromatography on silica gel, what would be the elution order? (top — comes off the column first, bottom — comes off last).

As I get it, column chromatography is like TLC where in a silica gel the nonpolar compound comes out first.

• The comparison with TLC is valid, and your judging of polarity, based on the interaction of the $\ce{OH}$ groups with the silica sounds good. So, yes, I would come to the same conclusion. If you would perform the separation in the lab, you would nevertheless keep all three fractions and determine the structures by independent methods (NMR). Commented Mar 3, 2017 at 9:11

Your conclusion is absolutely correct. Column chromatography can be thought of as three-dimensional version of TLC (and vice-versa). So the most polar compound which interacts with silica gel most elutes slowest and the least polar compound leaves the column first.

I also support your suggested order as an initial starting point. However, As Klaus mentioned in a comment, unless one has performed the same separation multiple times one should never rely on paper-chemical polarity analysis alone and always confirm structures independently — typically that means ‘measure an NMR of all fractions’.

• Why three-dimensional? Commented Mar 3, 2017 at 22:48
• @orthocresol Diffusion in a column can go $x$ and $y$, elution is $-z$. In a TLC, elution is also $z$, but diffusion can only happen in $x$ direction.
– Jan
Commented Mar 3, 2017 at 23:27
• I always thought 2D TLC referred to the thing where you develop along $z$ direction, then rotate it by $90^\circ$, stick it in a different solvent system, and develop along $x$ direction. Commented Mar 3, 2017 at 23:33
• @orthocresol Yes, that’s the method of two-dimensional TLC evolution. But even in standard, single evolution TLCs you get a certain diffusion perpendicular to the elution direction and spot broadening.
– Jan
Commented Mar 3, 2017 at 23:37
• Ah, okay; I wasn't meaning to nitpick, I was just curious as to what you meant. Commented Mar 3, 2017 at 23:39