[noiseordinance]

Hey there.

I did an experiment where we oxidized benzoin to benzil. In a nutshell, we combined benzoin with cupric acetate  ammonium nitrate solution. After reacting and isolating the crude product, we did recrystallization, and afterwards, column chromatography. A question asks why we couldn't do it the other way around (chromatography followed by recrystallization).

I understand that the purpose of the column chromatography is to separate away any unreacted benzoin (the hydroxyl group would be attracted to the stationary silica layer). I also see that water is a byproduct of the reaction. If we did the column chromatography first, would water be instead attracted to the silica layer?

Thanks for any suggestions on how I can figure this out.

[nox]

Perhaps byproducts are produced that can't be recrystallized away, so have to column out your stuff and then recrystallize away any remaining junk.

[opsomath]

Usually your product has to be decently pure before you can recrystallize; you have to be able to generate a supersaturated solution of the product _without_ supersaturating any of the impurities. Chromatography, even imperfect chromatography, can get you in the ballpark, and then even impurities very similar in nature to the product can be removed by crystallization.

[noiseordinance]

I'm confused though. In the experiment, we had to recrystallize before we did chromatography. The question suggests that something would go wrong if we did it in the reverse order. What could possibly go wrong if chromatography was done first, followed by recrystallization?

[opsomath]

Oh. Yeah, that is weird. What form was your crude product in? Was it a solid? It seems like it should be possible to do either way. Were there a ton of inorganic salts that you had to remove in the crystallization? Those might have been annoying, since the idea of column chrom. is that you use the minimum amount of solvent needed to get all the solid to dissolve.

[noiseordinance]

Well, when we converted benzoin to benzil, the waste products were water and nitrogen gas. The gas was collected though throughout the experiment, leaving only water and any unreacted benzoin. I know that the column was used to separate unreacted benzoin from the benzil since the benzoin has an alcohol group that would be attracted to the silica layer of the column. No clue why we couldn't do chromatography first though. :/

[Nosterius]

What is your recrystallization solvent? What were your column chromatography solvent(s)?

My guess is that the recrystallization is also a drying step. Especially if you used solvents such as ethanol or isopropanol.

If you poured your mixture directly on top of you silica gel, and used a solvent not miscible with water such as hexanes, you would end up with phase separations inside your column.

All this does not stand if you dried your product during the work-up (with MgSO4 for exemple)

[fledarmus]

I second the "what was your crystallization solvent" question. My guess is that the crystallization is to remove trace metal salts that may be complexed to your starting material and/or products, and which might cause them to streak during the chromatography.

[orgopete]

I would say that chromatography first (if needed) is common. Since recrystallization was performed first, I assume this was done for one (or two) reason. Generally, one absorbs their material on the chromatography column for best results. This usually done with a solvent with low elution power. However, since chromatography was not done first, it may mean the solvent cannot dissolve the sample very well without using a large volume. That would degrade the chromatography. Another method is to dissolve the sample in a larger amount of solvent, add a fraction of absorbent, evaporate the solvent away, and add the absorbed material to the column.

If I remember correctly, benzoin is quite insoluble. Its insolubility can interfere with recrystallization if it will begin to crystallize before all of the benzil can be crystallized, even if a polar solvent were used. If most of the benzoin is first removed, the sample can be dissolved more easily and applied to the column.

If you try to chromatograph directly, the benzoin can interfere with application to the column. If the sample is a suspension, a poorer chromatography will be likely as the benzil may not have been completely dissolved and the insoluble material will now become a stationary phase. Using a larger amount of solvent will also degrade the chromatography or if the preabsorption method were used, increases the amount of time required for the operation.