Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: nj_bartel on July 31, 2009, 03:14:16 PM
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A question states: A student uses thin layer chromatography on silica plates to monitor the progress of the reaction below. Does the product have higher or lower Rf than the starting material?
The reaction below is treatment of an a,b-unsaturated ketone treated with sodium borohydride to produce an a,b-unsaturated alcohol.
The answer given is: Lower, because the product is more polar than starting material. I put: Higher, because the product is less polar than the starting material.
Why would an alcohol be more polar than a ketone/aldehyde? It doesn't display resonance effects and the dipole vectors of the oxygen bonds don't sum completely additively. Alcohols can hydrogen bond which would stretch that O-H bond somewhat, but this effect would probably be pretty minimal in a nonpolar TLC solvent.
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Alcohols can hydrogen bond which would stretch that O-H bond somewhat, but this effect would probably be pretty minimal in a nonpolar TLC solvent.
what else is interacting with your molecule? chromatography can be thought as a series equilibrations between the mobile and stationary phases, you have a nonpolar solvent, but what about the plate itself? Keep in mind that hydrogen bonding is generally stronger than dipole dipole interactions.
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Ah, silica, so SO2, which is only going to be undergoing significant attraction with molecules with a protic hydrogen, which would make sense with the answer. Is this what you were getting at?
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On a silica gel slide, alcohols are always strongly retained and thus elute way more slowly than aldehydes and ketones.