Column Chromatography ketone/silica

In summary, the organic chemist ran a reaction using 2nitrobenzaldehyde and acrylonitrile. After the reaction had gone to completion, they extracted the product with chloroform and then dried it. They then filtered it and rotovaped it. They did a silica gel workup and determined that they had the desired product. However, they think that the acidity of the silica may have protonated the carbonyl in the product, causing it to become a hydroxyl group.
  • #1
I recently ran a baylis-hillman reaction of 2nitrobenzaldehyde and acrylonitrile (DABCO catalyst) and after my reaction had gone to completion (determined by tlc) i extracted with chloroform/washed with brine, dried, filtered, rotovaped etc etc

I ran a crude NMR and determined I did have my desired product.

I ran a silica pipette column with %anhydrous diethyl ether/hexanes and took HNMR again, it indicated that my carbonyl had become a hydroxyl group.

The only thing I can figure is that the silica gel in the column protonated my carbonyl. I know that silica gel is somewhat acidic...does anyone know if this would likely occur or what else could have occurred? )':


MBH product on left, my product after column on right
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  • #2
The acidity of silica depends on the purity of the silica gel. Silica contaminated with heavy metals end up with the metal atoms integrated into the silica molecule and this causes the silanol (Si-OH) groups to be deprotonated far easier. Pure silica isn't very acidic at all. Even if there were protons present to protonate the carbonyl oxygen, there would have to be a nucleophile present to reduce the carbonyl group. That reaction scheme you posted, the nucleophile would have to be a hydride ion. My organic chem knowledge isn't very advanced though, that's the only reaction I know of for reducing carbonyl groups.
  • #3
I would expect the beta hydroxy compound you indicated during silica gel workup. The ketone you wanted (?) would need a mild oxidizing workup of the beta hydroxy compound.

Remember that the DABCO forms an active nucleophilic C from the acrylonitrile, alpha to the CN (a Michael 1,4- adduct)... This then attacks the aromatic carbonyl (like an Aldol reaction), to form an alkoxy anion, that is easily protonated during workup to the hydroxyl.

If your desire is the ketone, a mild oxidant that won't tear up the product (the methylene is doubly activated as a Michael acceptor with an alpha CN and an alpha' CO- the multiple conjugation will also make it prone to polymerization) and a mild workup would be necessary. Dicyclohexyl-carbodiimide (DCC) and stoichiometric base (triethyl amine) could work without isolation of the DABCO reaction mixture as a one pot reaction.

If you have other transformations to perform on the active compound you may want to strategize a bit and see if they can also be done in the same pot with minimal workup-

Good luck!

1. What is column chromatography?

Column chromatography is a laboratory technique used to separate and purify individual components from a mixture. It involves passing a mixture through a column filled with a stationary phase (such as a solid support like silica) and a mobile phase (such as a liquid solvent). The different components of the mixture will travel at different rates through the column based on their interactions with the stationary phase, allowing for separation.

2. What is the purpose of using ketones in column chromatography?

Ketones, such as acetone or methylethylketone, are commonly used as solvents in column chromatography due to their high polarity and ability to dissolve a wide range of compounds. They also have a low boiling point, making them easy to remove from the purified sample after separation.

3. Why is silica used as the stationary phase in column chromatography?

Silica is a common choice for the stationary phase in column chromatography due to its high surface area and ability to interact with a wide range of compounds. It is also relatively inert and inexpensive, making it a practical choice for use in the laboratory.

4. How do you know when a compound has been separated in column chromatography?

Compounds will appear as distinct bands or spots as they travel through the column and are separated from the other components of the mixture. This can be visualized using various detection methods, such as UV light, fluorescence, or chemical staining techniques.

5. What factors can affect the success of a column chromatography experiment?

The success of a column chromatography experiment can be affected by several factors, including the choice of stationary and mobile phases, the size and shape of the column, the rate of flow of the mobile phase, and the purity of the sample being separated. Other factors such as temperature, pH, and the presence of impurities can also impact the results of the experiment.

Suggested for: Column Chromatography ketone/silica