[macman104]

A postdoc in our lab just did the nitration of 3-bromopropyl benzene.  I will be helping her with the column tomorrow (since we're splitting the product).  We are only interested in the para product, since the reaction produces about a 50/50 mixture of the ortho and para product. 

Everytime I have done this separation, I have used a column with the bulb top and used air pressure starting with hexane then moving towards a 1:20 Ethyl Acetate/Hexane, and so on.  This method usually produces some pure para product, and a larger amount of ortho/para mixture since they are so close for separation. 

Today, when I was talking with her about it, she said that it was not a good idea to use the air pressure and that we would be doing just a normal gravity column.  Normally, since they are so close this separation takes something like...2-3 hours, I can't imagine how long it's going to take as a pure gravity column.  However, she said that this would provide us with a better separation.

*phew* here's my actual question:  Why is the gravity column going to provide a better separation?  My understanding of using the air pressure was simply to speed up the column, not that it affects the quality of separation.  Secondly, does anyone have any other thoughts on the separation of these two isomers?  Any help in coming up with a speedier solution would be awesome.

[sjh9]

Applying pressure (flash column) will separate your products more effectively.

"A faster flow rate of the eluent minimizes the time required to run a column and thereby minimizes diffusion, resulting in a better separation, see Van Deemter's equation. A simple laboratory column runs by gravity flow. The flow rate of such a column can be increased by extending the fresh eluent filled column above the top of the stationary phase or decreased by the tap controls. Better flow rates can be achieved by using a pump or by using compressed gas (e.g. air, nitrogen, or argon) to push the solvent through the column (flash column chromatography)."

-http://www.answers.com/topic/column-chromatography?cat=health

[macman104]

See...now, she told me that flash chromatography was different than simply applying air pressure above the column, that it was some sort of machine.  You also say that it will separate the products more effectively, so you disagree with her assessment?

[sjh9]

See...now, she told me that flash chromatography was different than simply applying air pressure above the column, that it was some sort of machine.  You also say that it will separate the products more effectively, so you disagree with her assessment?

Yes I dissagree with her.  There are automated flash column systems however they simply replace the air pressure with a pump and the TLC plate with a detector.  Some use plastic disposable columns rather than glass.  Same basic priciples are involved.

[macman104]

Yes I dissagree with her.  There are automated flash column systems however they simply replace the air pressure with a pump and the TLC plate with a detector.  Some use plastic disposable columns rather than glass.  Same basic priciples are involved.

Ah, ok, so we have effectively been doing flash chromatography then.  I guess maybe I'll run her plan by our professor since I'm really not interested in spending hours upon hours waiting for a gravity column (would that be overstepping "roles" and should I just stay quiet?).  Did I mention that it is large scale reaction as well (40g) so lots of product to separate...

Any other thoughts on this separation, any other techniques we might employ (if you don't have any that's fine too)?

[sjh9]

I'm not sure of the dynamic in your lab but if my boss was wrong about something I'd probabaly take the issue up with him by presenting something like that link I provided above.  If he still though he was right, I'd tell him to run the column himself  . 

Unless you find a way to selectively crystallize either product I cannot think of a better way to separate your mixture.  If you had not nitrated the molecule I'd suggest trying sublimation.

[macman104]

I'm not sure of the dynamic in your lab but if my boss was wrong about something I'd probabaly take the issue up with him by presenting something like that link I provided above.  If he still though he was right, I'd tell him to run the column himself  .

Well, she's not my boss.  She's a postdoc in the lab run by my professor.  We're only working together because we are going to split the product for our separate reactions.

Unless you find a way to selectively crystallize either product I cannot think of a better way to separate your mixture.  If you had not nitrated the molecule I'd suggest trying sublimation.

Unfortunately, nitration is key to the overall reaction scheme and is the source of the mixture (the bromo product is pure afterall).  I can't imagine selectively crystallizating the ortho or para nitro product as they seem like they would be too similar, so I guess we're stuck with column.

Thanks for the help and info, greatly appreciated!