Re: ORGLIST: Reverse phase LC followup

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From: Arthur Cammers-Goodwin (cammers$##$
Date: Thu Oct 28 1999 - 09:10:32 EDT

On Wed, 27 Oct 1999, Bozell, Joe wrote:

> 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:
> *******************
> 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.

Get a "high vacuum rotovap" and spin the round bottom flask while heating
at about 45 degrees. This will keep the water liquid allow evaporation
to occur quickly. It is important not to let the water freeze while
pulling a vacuum on it because subliming the water does take a long time.
Yes, due to water's large and positive enthalpy of vaporization, water
freezes at decreased pressure.

I agree with the previous poster. Just pull off the water . . . it works .
. . I do it all the time.

A. Cammers-Goodwin


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