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Homework Help: Sodium borohydride reductions and chromatography

  1. Feb 18, 2012 #1
    1. The problem statement, all variables and given/known data
    Hi, I did a lab where benzophenone was dissolved in ethanol and reacted with sodium borohydride (NaBH4) and chromatography was done on the reaction at different points in time. There was no HCl work up of this though. When I developed the TLC (silica) plate, it looked like this: http://imgur.com/OnGxl

    Lane 1 is before sodium borohydride addition, lane 2 is 10 minutes after addition, lane 3 is 30 minutes after and lane 4 is 60 minutes after.

    I'm wondering why the spots become lower after time has passed.

    3. The attempt at a solution
    I know that because no HCl work up was done that this is just the metal alkoxide that's showing up as a product. Lane 1 is just benzophenone, lane 2 shows both the metal alkoxide and benzophenone and lane 3 and 4 just show the metal alkoxide. I know that compounds have lower elution orders if they're more polar or have more affinity for the silica. I don't think there would be more affinity for the silica because the product doesn't give the compound the ability to hydrogen bond with silica, so I'm left with polarity. The only reason I can think the metal alkoxide would be more polar is due to more heteroatoms, but the way the molcule would look doesn't seem like it would be polar...
    Thanks in advance for your help!
  2. jcsd
  3. Feb 21, 2012 #2


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    Your wrong assumptions as to how the reaction proceeds is preventing you from seeing the answer. Metal alkoxide? In what solvent did this reaction occur? Was water ever used to work up the sample or was the sample ever exposed to moisture? (Did you do your TLC in a glove box?)

    Read this and think about it.
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