[The Guy]

Hi

I read  lot about using TLC to develop column chromatography.

If the retention factor of the product on solvent mixture is 0.3, what does that mean in column chromatography? How much solvent should I use to elute the desire product, based on the rf?

If there is two other component has RF value close to my product, and I used less polar mixture -lets say 20% EtOAc:Hex- and that would move my product also, then I will use the more polar solvent to elute my product. Still in that case confused how to deal with it. Tracking it using TLC wouldnt work since the concentration is low and I can distinguish between all the byproduct.

Any expert can guide me through this??

[Archer]

First of all what sort of Column Chromatography are you doing? Normal of Flash?.


Secondly the closer the RF of your impurities the more silica you need relative to your crude product.

Please provide the RF of all of your TLC findings.

[orgopete]

Yes, TLC is a good preview of column chromatography. Regarding the questions about chromatography, there should not be any mystery. By its nature, it is about differential solubility and one of the phases is stationary. There are other factors to be considered though.

If you had a compound with no retention on the stationary phase, it would elute with one column volume and its width would depend on the width or thickness of the band as first applied. If it were very narrow, the band would be sharper. If you added a single component to a chromatography column, even with high loading, but a low Rf for that solvent, the low solubility in the mobile phase will result in a broad band.

If you think about this, there are trade off now matter how you run the column. Clark Still's paper simply gave a good practical recipe to give a good result, but it is still about differential solubilities. If TLC showed something at the origin, you may be able to remove it by adding an equal weight or even less of silica to a solution of your compound and filtering. This simply assumes the partition to the silica is much higher that your solution-solute phase. A highly concentrated solute may behave quite differently that one that is dilute.

I don't recall the number of column volumes and perhaps someone can supply this information, but I think an Rf of 0.3 will come off in 2-3 column volumes and will still be a reasonably narrow band. Again, check in Still's paper for details. If you reduce the Rf, it will require more solvent and the bands become broader. If you use more silica, you will need more solvent and silica. Depending on the amount you must separate, both may become issues. The actual separation will remain a function of the differential solubilities though. The magnitude of the differential solubility by changing the Rf may not be very large, but the practicality of the separation may improve. If the Rf is high, the fraction may be well resolved and in narrow bands, but if your fractions are too large, you may unknowingly combine them. If low, they are easier to collect separately, but in larger volumes.

[The Guy]

First of all what sort of Column Chromatography are you doing? Normal of Flash?.


Secondly the closer the RF of your impurities the more silica you need relative to your crude product.

Please provide the RF of all of your TLC findings.

Flash

in the 50% mixture, 0.24 for product, 0.3 and 0.35 by products.

Thank you for your contribution, I am more interesting on knowing how to apply this finding to the volume should be used on the column.

[The Guy]

Yes, TLC is a good preview of column chromatography. Regarding the questions about chromatography, there should not be any mystery. By its nature, it is about differential solubility and one of the phases is stationary. There are other factors to be considered though.

If you had a compound with no retention on the stationary phase, it would elute with one column volume and its width would depend on the width or thickness of the band as first applied. If it were very narrow, the band would be sharper. If you added a single component to a chromatography column, even with high loading, but a low Rf for that solvent, the low solubility in the mobile phase will result in a broad band.

If you think about this, there are trade off now matter how you run the column. Clark Still's paper simply gave a good practical recipe to give a good result, but it is still about differential solubilities. If TLC showed something at the origin, you may be able to remove it by adding an equal weight or even less of silica to a solution of your compound and filtering. This simply assumes the partition to the silica is much higher that your solution-solute phase. A highly concentrated solute may behave quite differently that one that is dilute.

I don't recall the number of column volumes and perhaps someone can supply this information, but I think an Rf of 0.3 will come off in 2-3 column volumes and will still be a reasonably narrow band. Again, check in Still's paper for details. If you reduce the Rf, it will require more solvent and the bands become broader. If you use more silica, you will need more solvent and silica. Depending on the amount you must separate, both may become issues. The actual separation will remain a function of the differential solubilities though. The magnitude of the differential solubility by changing the Rf may not be very large, but the practicality of the separation may improve. If the Rf is high, the fraction may be well resolved and in narrow bands, but if your fractions are too large, you may unknowingly combine them. If low, they are easier to collect separately, but in larger volumes.

Thanks alot for the detailed post.
I've read Clark Still's paper and I couldn't find the relation between rf and eluent volume. I need the equation to deal with different rf value.

[Archer]

According to  Advanced Practical Organic Chemistry by J. Leonard et. al.
http://books.google.com/books/about/Advanced_Practical_Organic_Chemistry_Sec.html?id=aP88FuFO5QUC

Your Rf values suggest somewhere between a 30:1 and 50:1 silica to sample ratio.

From that you should be able to work out your column volume and work from there.