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Topic: Finding Unknowns  (Read 2841 times)

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Offline DonTom1

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Finding Unknowns
« on: October 11, 2012, 06:30:33 AM »
Hey guys,

I'm having trouble finding an unknown in my chemistry lab. i TLCed  using 70% ether/pentane and stained with Bromocresol Green and it showed me a blue spot on my first dot away from the origin(Carboxylic Acid?) and a yellow spot near the origin. I then used column chromatography to separate the two away from each other. The problem is evaporating my solvent and my NMR. When I rotovap or use my Nitrogen line to evaporate my solvent my unknown also disappears leaving me with nothing. When I leave some solvent in after Nitrogen spraying and took an NMR of my data the only thing that it showed me was my solvents. I have no idea how to get rid of my solvent without getting rid of my unknown. Can anybody help?
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Offline discodermolide

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Re: Finding Unknowns
« Reply #1 on: October 11, 2012, 06:33:46 AM »
A carboxylic acid will not move from the baseline with 70%ether/pentane on silica gel. That will be the spot at the origin.
Must you use chromatography?
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Offline DonTom1

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Re: Finding Unknowns
« Reply #2 on: October 11, 2012, 06:57:02 AM »
I thought that being polar an all the carboxylic acid should still be near the origin. The RF value was between .3 and .34. The TLC stain data sheet said that Carboxylic acids give Blue spots on Bromocresol Green. But still your right. As for the column I could use liquid/liquid extraction to separate the acid. The only thing is I don't know the other two compounds. Is there a better method to separate the two compounds?
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Offline discodermolide

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Re: Finding Unknowns
« Reply #3 on: October 11, 2012, 06:58:18 AM »
Use extraction to separate them. I assume you have covered this?
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Offline DonTom1

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Re: Finding Unknowns
« Reply #4 on: October 11, 2012, 07:02:32 AM »
Yes I have, but are you sure that the other components in my unknown will be extracted out as well if I use acid extraction. That's why I'm a little concerned
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Offline discodermolide

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Re: Finding Unknowns
« Reply #5 on: October 11, 2012, 07:52:36 AM »
Do the extraction with base/ether first. Then separate the layers and run a TLC of the ether layer. See what you get.
Don't throw anything away.
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Offline DonTom1

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Re: Finding Unknowns
« Reply #6 on: October 11, 2012, 07:55:46 AM »
Ok will do and thank you
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Offline fledarmus

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Re: Finding Unknowns
« Reply #7 on: October 11, 2012, 09:02:18 AM »
Do you have any idea what the boiling point of your unknown might be? Is it possible that you are stripping it away with your solvent?

You might want to test the solvent that collects in your receiver and see if you get a spot that runs with and stains with the spot you isolated by chromatography.

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