ORGLIST members:

Last week, I sent out a question regarding how one isolates compounds from
the effluent of a preparative reverse phase LC column.  The answers received
have been helpful, and basically fall into two catagories: strip the organic
and water all at once, or strip the organic and freeze dry the remainder
overnight.

One particularly good response asked the direct question: "what is the
problem with stripping the water?"  This is a fair question, and I offer my
response to the list to see if there is any more discussion:
*******************

Dear...

Thanks for the suggestion.  My basic problem is that removal of water
increases the amount of time necessary to isolate the product.  Using the
"old way", one simply strips, say, hexane/EtOAc mixtures on the rotovap at
reasonable temperatures (about 35 - 40 deg C), and isolates the product
directly.  On the other hand, with water in the mixture (maybe as much as
50%), it becomes necessary either to increase the temp on the rotovap to 60
- 70 deg C to remove the water in a reasonable amount of time (leading to
possible loss of a more volatile organic product), or putting the aqueous
phase on a freeze dryer, which requires overnight lyophilization.

I guess I would ask you a similar question (and I ask it out of ignorance,
since I'm pretty new to reverse phase).  Why is reverse phase the method of
choice now, both for analytical and prep work?  For "normal" organic
synthesis (i.e., nonzwitterionic compounds, high organic solubility, etc.)
isn't straight phase chromatography using nonaqueous mixtures just as
effective?
*******************

There's my response...if the collected wisdom of the list has further
suggestions, I would love to hear them.

best regards,

Joe Bozell
Senior Chemist
National Renewable Energy Lab
Golden, CO
USA

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