An challenge problem for Organic Students

[WorcesterPHOBoy]

Question: A chemist started to carry out column chromotography on Friday after-noon, reached the point at which the two compounds being seperated were about three-fourths of the way down the column, and then return on Monday to find that the compounds came off the column as a mixture. Sepculate on the reason for this. The column had not run dry over the weekend.


It is so hard to think of a reason =\

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[KUMoose]

Column chromatography is little more then a fancy filtration system. Two elutants, will both eventually make it through the system if the system is kept wet, as the problem suggests.

I would say the most obvious answer is that given the roughly 60 hours between Friday afternoon and Monday morning, both elutants had enough time to drain through the system.

 

[Christie1]

yeah, both compounds will eventually make it down the column with that much time...

 

[usi]

I am a medicinal chemist. I run many many columns every week. Old style, and using all kinds of fancy equipment. So I know with what you proposed you don't have enough information.
First: a column does not run dry if you have enough solvent on the top to compensate for the rate of elusion. YOu are not given either. Examples would be: You have a liter of eluent, and you left the column runing at 1.5 mL per minute. Would your column run dry in x amount of hours?
Second: You could get really fancy and calculate the the Rf of both compounds and include the theoretical plates, rate of elusion etc...
Again, you are not given enough information.

Practically if your compounds did not separate by the time they are 3/4 down the column, they are not going to separate.
Better get them out, rotovap, and run a bigger longer column before your boss comes around asking about it. 😀
Hope it helps. Give me more ino>>>

 

[WorcesterPHOBoy]

I am a medicinal chemist. I run many many columns every week. Old style, and using all kinds of fancy equipment. So I know with what you proposed you don't have enough information.
First: a column does not run dry if you have enough solvent on the top to compensate for the rate of elusion. YOu are not given either. Examples would be: You have a liter of eluent, and you left the column runing at 1.5 mL per minute. Would your column run dry in x amount of hours?
Second: You could get really fancy and calculate the the Rf of both compounds and include the theoretical plates, rate of elusion etc...
Again, you are not given enough information.

Practically if your compounds did not separate by the time they are 3/4 down the column, they are not going to separate.
Better get them out, rotovap, and run a bigger longer column before your boss comes around asking about it. 😀
Hope it helps. Give me more ino>>>

That's the only information the book gave me. I am confused by your answer =\, the others two above seem to be right, are you tryin to say what they are saying too?

 

[usi]

That's the only information the book gave me. I am confused by your answer =\, the others two above seem to be right, are you tryin to say what they are saying too?

Just ignore my getting so detailed.

Both compounds will eventually come out of the column,
and they will not separate because the composition of the mobile phase is not good for the separation (not because you left the column alone
😀 )

 

[pharmeronadell]

Just ignore my getting so detailed.

Both compounds will eventually come out of the column,
and they will not separate because the composition of the mobile phase is not good for the separation (not because you left the column alone
😀 )

So if you use a stronger solvent, you could possibly get them to separate?

 

[WorcesterPHOBoy]

Just ignore my getting so detailed.

Both compounds will eventually come out of the column,
and they will not separate because the composition of the mobile phase is not good for the separation (not because you left the column alone
😀 )

I asked my teacher and she said this to me:

"The compounds do not drain! The chemist stops the flow of solvent, covers
everything, and leaves. Nothing dries out. What is happening in the column
which makes the compounds become mixed?? That is what you have to think
about."

any idea?