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Topic: destruction of cmpd while doing rotovap?  (Read 3157 times)

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Offline kriggy

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destruction of cmpd while doing rotovap?
« on: December 05, 2014, 12:41:43 PM »
Hi guys, I was doing this reaction:


It seemed to go realy good (I was using HPLC-MS to monitor the reaction) and when I stopped reflux I did final HPLC and saw only one compound with the correct mass. However, when I evaporated the solvent, I got yelow/orange compound (matches the reported colour) however my molecular peak from MS dissapeared and I got some with lower mass (28 less). It seems very weird that the compound broke up since the temperature of reflux was just much higher than the one on rotovap (40 degrees and 100 mbar vacuum). Or could it be just a mistake in measurment?
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Offline discodermolide

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Re: destruction of cmpd while doing rotovap?
« Reply #1 on: December 05, 2014, 12:47:01 PM »
So the compound just lost CO in the mass spec.
What do your NMR(s) say?
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Offline kriggy

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Re: destruction of cmpd while doing rotovap?
« Reply #2 on: December 05, 2014, 01:10:28 PM »
Quote from: discodermolide on December 05, 2014, 12:47:01 PM
So the compound just lost CO in the mass spec.
What do your NMR(s) say?

Wow..that simple  huh? :-[  Could be. The thing is that the retention time changed by like 10 seconds. Could it be that I dissolved one sample in methanole and the other in acetonitrile (seems unlikely)?
I didnt run NMR, I might be able to do it next week since we are getting new NMR mashine right now.
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Offline discodermolide

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Re: destruction of cmpd while doing rotovap?
« Reply #3 on: December 05, 2014, 01:29:37 PM »
Could be. When did you run the TLCs? Before and after work-up?
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Offline kriggy

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Re: destruction of cmpd while doing rotovap?
« Reply #4 on: December 05, 2014, 01:47:53 PM »
I did run only 1 TLC which showed single spot.
I then run stopped heating and run HPLC-MS which showed me that there is one major compound. Then I wait for it to cool (to see if product precipitates - It did not). Then evaporate the solvent and run another HPLC-MS which showed me:
increase of retention time
loss of carbonyl group
one new little peak

I just find very hard to belive that the compound broke up when I evaporate the solvent. It just doesnt make sense to me.
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Offline discodermolide

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Re: destruction of cmpd while doing rotovap?
« Reply #5 on: December 05, 2014, 02:43:00 PM »
It didn't. The TLC difference is just due to the absence of solvent. The compound exists!!
Get your NMRs.
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Offline kriggy

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Re: destruction of cmpd while doing rotovap?
« Reply #6 on: December 05, 2014, 02:52:31 PM »
Thats good to know.
So to sum it up:
the difference in retention times was cause by the fact that in the 1st measurment, the sample was taken from the reaction mixture directly (including the solvent) while in the 2nd measurment, the solvent was gone?
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Offline discodermolide

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Re: destruction of cmpd while doing rotovap?
« Reply #7 on: December 05, 2014, 03:01:22 PM »
Yes!
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Offline kriggy

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Re: destruction of cmpd while doing rotovap?
« Reply #8 on: December 05, 2014, 03:06:02 PM »
Ok
Thank you for your time.
Have a nice day  :)
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