Sciencemadness Discussion Board

beginner question in regards to filtering

andre178 - 7-1-2010 at 11:50

It's a silly beginner question, but a reaction I am following asks for the solution to be filtered over a silica plug (6x6cm) with CH2Cl2/EtoAc 90:10 to remove polymeric material and pyridine etc. [I don't want to go in depth about the reaction, but basically I polymerized some octan-1,8-diol OH(CH2)7OH with pyridine in CH2Cl2(dry)].

Would I use a small chrom. column filled with 6cm worth of silica to pass the liquid through? I don't understand how the polymeric material and pyridine would be left behind.


Picric-A - 7-1-2010 at 11:55

My guess would be use silica filter paper the scrape the product off the paper.

If a silica sand plug is nessesary then as you would perform a column chromatography but with only 6cm of sand. Pour in sample to be filtered, apply pressure to expell all liquid but leave solid trapped in silica. Remove solid by dissolving in another pure solvent, drain off. Evaporate solution to leave pure solid.

smuv - 7-1-2010 at 19:03

Take a fritted glass funnel, fill it with silica, the amount depends upon the scale and solubility and how tough the seperation is. Wet the silica completely with hexanes and stir it with a spatula, then apply suction to get a nicely packed plug. Carefully add more hexanes (just enough to fill up to surface of silica) w/o disturbing the surface and load your freshly rotovapped reaction mix (some people put a layer of sand before loading, but I think its silly for a plug) then elute with the CH2Cl2/AcOEt (under suction of course), cutting fractions as you see fit, and running tlc to determine which fractions you should keep, just like a column.


EDIT: TO answer your quesiton, polymeric material will be left behind because it probably elutes much more slowly than your product. The pyridine should stay on the column as well, but don't collect too much of the tails as that solvent system is liable to push pyridine off as well. Anyways you can remove the pyridine at the high-vac or on the rotovap.

[Edited on 1-8-2010 by smuv]

chemchemical - 4-2-2010 at 15:26

Quote: Originally posted by smuv  
Take a fritted glass funnel, fill it with silica, the amount depends upon the scale and solubility and how tough the seperation is. Wet the silica completely with hexanes and stir it with a spatula, then apply suction to get a nicely packed plug. Carefully add more hexanes (just enough to fill up to surface of silica) w/o disturbing the surface and load your freshly rotovapped reaction mix (some people put a layer of sand before loading, but I think its silly for a plug) then elute with the CH2Cl2/AcOEt (under suction of course), cutting fractions as you see fit, and running tlc to determine which fractions you should keep, just like a column.

What he said. You dont need 6cm of silica in a column to do this, but you can use it. Just stuff some cotton in, then sand, then silica. If you dont have a fritted funnel you can use a regular V shaped funnel, stuff some cotton down (to hold silica) and then top with silica. Vacuum filtration would be better if you can, with a column you can put pressure through it. A plug is just what is says, I'm not sure how much polymerization took place but if you can see it the plug is just acting as a filter and you can use a smaller amount, if you can't see it then its more like a column and you'll want a little more just so it doesnt come off with your product.

The polymeric material will be much bigger and less soluble than your material, it will also be slower moving so it is left behind (I know MW doesnt make it slower, but it will increase the number of OH's and their ability to interact with the silica). It will also have more attraction to the silica.
If it just asks to filter you probably wont have to collect fractions and do a tlc, although you might to make sure you collect everything and that your not leaving any behind or taking anything with you.


EDIT: TO answer your quesiton, polymeric material will be left behind because it probably elutes much more slowly than your product. The pyridine should stay on the column as well, but don't collect too much of the tails as that solvent system is liable to push pyridine off as well. Anyways you can remove the pyridine at the high-vac or on the rotovap.

[Edited on 1-8-2010 by smuv]