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Topic: separating similar compounds  (Read 14638 times)

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Offline kriggy

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Re: separating similar compounds
« Reply #15 on: February 28, 2014, 04:57:32 AM »
Streaking you mean like the spot on TLC had some kind of "tail"? If so then yes. I addded few drops of Et3N to the mobile phase (probably 3-4 ml of mobile phase total) and it helped with streaking but not with separation. I got one spot for reaction mixture and one for bis-substitued with same Rf. Without Et3N i got also same Rf for mixture and bis but with the tail behind. So it seemed that by using Et3N the compounds which are less mobile are moved together with the more mobile ones. I have pictures im my phone of this so I can upload them when I get home.

Offline TheUnassuming

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Re: separating similar compounds
« Reply #16 on: February 28, 2014, 09:24:15 AM »
Second the toluene suggestion!  I completely forgot about that trick. 
Aromatic solvents like benzene and toluene can pi stack with compounds containing aromatic moieties, often helping with tricky separations.
When in doubt, avoid the Stille coupling.

Offline kriggy

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Re: separating similar compounds
« Reply #17 on: February 28, 2014, 10:27:40 AM »
Im gonna try it next time.
I mean, I already had the mixture on column and doing the CHCl3:MeOH  before I asked here. But I slowly increased the amount of MeOH in mobile phase so it worked OK-ish (IMO much better then 1st time). THe thing is quite large amount of CHCl3 required. I will have MS next week. But I guess I will have to go even more polar because there is still quite a lot left on column.
At least I got some nice crystals of bis-substitued ligand from before  :D
BWT you guys posted so many ideas I dont know If I will have enough time to try them all  ;D

Offline kriggy

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Re: separating similar compounds
« Reply #18 on: March 05, 2014, 04:33:24 AM »
Hi guys I managed to almost separete the compoounds by CHCl3 + MeOH. I tried toluene MeOH 1:1 and acording to miscibilita table they are miscible but on TLC it seems that there are 2 mobile phases when the 2nd one moves with Rf 0,8 compared to the faster one. It seems that the compounds separate well but the 2 fronts of MF are bugging me. Any ideas how they are formed?

Offline TheUnassuming

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Re: separating similar compounds
« Reply #19 on: March 05, 2014, 09:03:39 AM »
So the solvent system looks miscible in a bottle when mixed, but on TLC you are getting two distinct solvent fronts?
When in doubt, avoid the Stille coupling.

Offline Babcock_Hall

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Re: separating similar compounds
« Reply #20 on: March 05, 2014, 10:30:49 AM »
In your drawing is there supposed to be a methylene group in between the nitrogen atoms and the carbonyl group?

Offline kriggy

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Re: separating similar compounds
« Reply #21 on: March 05, 2014, 11:33:00 AM »
So the solvent system looks miscible in a bottle when mixed, but on TLC you are getting two distinct solvent fronts?
Yes. I was told that this could be because one of the solvets in the mixture is evaporating much faster than the other.
In your drawing is there supposed to be a methylene group in between the nitrogen atoms and the carbonyl group?
yes but i can see the drawing being not clear enough

Offline TheUnassuming

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Re: separating similar compounds
« Reply #22 on: March 05, 2014, 11:46:42 AM »
Hrmm... what sort of container do you do your TLC's in?  If you aren't already doing so, try doing it in a sealed vessel of some sort (I use a 30mL glass vial) with a bit of kimwipe up one side.  This usually alleviates any sort of evaporation problems you will have.
When in doubt, avoid the Stille coupling.

Offline kriggy

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Re: separating similar compounds
« Reply #23 on: March 05, 2014, 01:15:10 PM »
I did it in beaker with piece of glass over it so it might evaporate a bit but I didnt have any similar problems with different mobile phases.

Offline Dan

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Re: separating similar compounds
« Reply #24 on: March 06, 2014, 03:42:46 AM »
Hi guys I managed to almost separete the compoounds by CHCl3 + MeOH. I tried toluene MeOH 1:1 and acording to miscibilita table they are miscible but on TLC it seems that there are 2 mobile phases when the 2nd one moves with Rf 0,8 compared to the faster one. It seems that the compounds separate well but the 2 fronts of MF are bugging me. Any ideas how they are formed?

I've never come across that one before, but I've never attempted to run toluene/MeOH. Try swapping MeOH for EtOH, i-PrOH or acetone.
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Offline kriggy

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Re: separating similar compounds
« Reply #25 on: March 11, 2014, 02:34:40 PM »
Hi its me again  :)
Ok because the compounds still doesnt separate well - im getting mostly pure bis-substitued, then some amount of mono (which doesnt yield anything after evaporation so its realy low amounts) and then mixture of mono and unsubstitued I decided to try the prepareative HPLC way. Or at least get some information. I told you that the compounds doesnt separate in analytical HPLC but actualy it wasnt HPLC chromatogram but it was from MS showing us how much of the compound goes into the ionization chamber over time because the sample was measured for minute or longer.
Anyway, the thing is that our preparative HPLC doesnt have gradient elution which is little disapointing. (and doesnt have autosampler so doing 10 cycles is eh.. time consuming I guess ;D)
What are the differences between prep. HPLC and column in terms of elution strenght? I mean for example when I use pure methanol on column all products get elued is it same on HPLC or when I use MF that doesnt elue the compounds at all in collumn, is it possible that it will work in HPLC? How does the high pressure change things compared to normal column?

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