Sodium borohydride reductions and chromatography

In summary, the conversation discusses a lab experiment where benzophenone was dissolved in ethanol and reacted with sodium borohydride. Chromatography was used to monitor the reaction at different time intervals. The TLC plate results show that as time passes, the spots become lower. The person attempts to explain this by suggesting that the metal alkoxide product is more polar than benzophenone. However, it is pointed out that this assumption may be incorrect and the person is asked to consider the role of solvent, water, and moisture in the reaction.
  • #1
PoisonCupcake
6
0

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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  • #2
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.
 

1. What is sodium borohydride and how is it used in reductions and chromatography?

Sodium borohydride is a chemical compound with the formula NaBH4. It is commonly used in reductions, a process in chemistry where a compound is reduced to a simpler form. It is also used in chromatography, a laboratory technique used to separate and analyze mixtures of chemicals.

2. What types of compounds can be reduced using sodium borohydride?

Sodium borohydride is commonly used to reduce aldehydes, ketones, esters, and carboxylic acids. It can also reduce certain metals and metal ions.

3. What is the principle behind using sodium borohydride in chromatography?

In chromatography, sodium borohydride is used as a reducing agent to convert metal ions in a mixture to their respective metals. This allows for better separation of the components in the mixture.

4. How does sodium borohydride compare to other reducing agents?

Sodium borohydride is a milder and safer reducing agent compared to others, such as lithium aluminum hydride. It is also more selective, meaning it only reduces specific compounds and does not react with others in the mixture.

5. What safety precautions should be taken when handling sodium borohydride?

Sodium borohydride should be handled with caution, as it is a strong reducing agent and can react violently with water. Protective gear, such as gloves and goggles, should be worn and it should be stored in a dry and cool place. It should also be kept away from acids and oxidizing agents.

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