Sodium borohydride reductions and chromatography

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SUMMARY

The discussion centers on the reduction of benzophenone using sodium borohydride (NaBH4) in ethanol, followed by thin-layer chromatography (TLC) analysis. The TLC results indicate a decrease in spot height over time, attributed to the formation of a metal alkoxide product. The absence of HCl workup and potential exposure to moisture are critical factors influencing the reaction outcome and product polarity. The participant concludes that the metal alkoxide's increased polarity is likely due to the presence of heteroatoms, despite initial assumptions about its structure.

PREREQUISITES
  • Understanding of sodium borohydride (NaBH4) reduction mechanisms
  • Familiarity with thin-layer chromatography (TLC) techniques
  • Knowledge of solvent effects in organic reactions, particularly ethanol
  • Basic principles of polarity and molecular structure analysis
NEXT STEPS
  • Research the mechanism of sodium borohydride reductions in organic chemistry
  • Learn about the principles and applications of thin-layer chromatography (TLC)
  • Investigate the effects of solvent choice on reaction outcomes in organic synthesis
  • Explore the role of moisture and workup procedures in organic reactions
USEFUL FOR

Chemistry students, organic chemists, and laboratory technicians involved in reaction analysis and product characterization.

PoisonCupcake
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Homework Statement


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.

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!
 
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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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