8
$\begingroup$

I recently was told that once you find a good solvent mixture for separation on thin layer chromatography (TLC), you should only use 10% of the more polar solvent (for silica) when you run your column. I tried it and it worked great!

Since I'd never heard that before, is that a standard rule of thumb, or did I just get lucky?

$\endgroup$
  • $\begingroup$ There is an article in the Journal of Organic Chemistry that describes this process. I'm having trouble finding it. When I do, I'll make an answer out of it. $\endgroup$ – Ben Norris Jun 19 '12 at 12:14
  • 2
    $\begingroup$ Is this the article you are referring to? Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923–2925. $\endgroup$ – scientifics Jun 19 '12 at 13:24
  • $\begingroup$ That is indeed the article. $\endgroup$ – Ben Norris Jun 20 '12 at 11:12
  • 1
    $\begingroup$ I didn't see anything in there about changes between TLC solvent and column solvent: I thought you used the same solvent in the TLC and column, and that seems to be what the article describes. $\endgroup$ – Canageek Jun 21 '12 at 20:14
  • $\begingroup$ @Canageek, yeah, they seem to use the same solvent mixture. I guess that was a fluke that that tip worked. $\endgroup$ – scientifics Jun 21 '12 at 20:32
3
$\begingroup$

I never heard about the "10% rule" and I never applied it. Transferring TLC (silica on aluminium plates, Merck) results to preparative column chromatography on similar silica usually worked without changes, although the situation is a bit different in the column: the upper part of the TLC plate is rather dry when letting it run, while the silica in the column is not. Note that I was never a friend of the dry packing method and always prefered the slurry method to prepare the columns.

Anyway, I never thought about increasing the polarity.

Bad separation typically was a result of column overload, bad packing or cracking of the stationary phase, e.g., when the diethylether evaporates in the column.

+1 For citing the classic article on flash chromatography!

I can really recommend the technique and I used it ... more than once ;)

$\endgroup$
1
$\begingroup$

I believe the 10% rule only applies to ''strong'' solvents such as methanol or ethanol. These solvents have polarities around 6.6 compared to ethyl acetate which has a polarity of 4.3 or so and hexanes which is what, zero?

Anyways the rumor is that strong solvents will degrade the silica. Here is an interesting article that confirms and denies said rule. http://www.flash-purification.com/does-methanol-really-dissolve-silica-during-flash-column-chromatography/

As with all good chemistry questions it depends and nothing beats running the experiment. The proof is in the NMR.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.