Re: ORGLIST:crystallization

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From: Jacob Zabicky (zabicky$##$bgumail.bgu.ac.il)
Date: Tue May 21 2002 - 09:08:09 EDT


Hello Kees,

Some suggestions not in order of incresing or decreasing importance, but
all stemming from self experience. You make your own decisions about the
safety measures that fit the case.

1. SCRATCHING
Organic chemists strongly believe that placing a solution in a test tube or
Erlenmayer flask and scratching the vessel with a glass rod will cause
nucleation. Possible disadvatages: a) you scatched the vessel until it
became opaque and useless, and ended up with a suspension of fine glass
instead of fine crystals; b) involuntarily you hit the bottom of the vessel
with the rod and break it.

2. SEEDING
Supersaturated solutions will crystalize when a few grains of the same material
is thrown in. I understand there's a catch to that, because the compound is
new and this is the first time since the creation of the universe anybody
will have a glimpse at such crystals if you succeed. Well, no worry . . .
sometimes crystals of a similar compound, or any crystals at that, will do
the trick. It is said that old organic chemists carry sufficient seeds in
the sleeves of their lab-robes to make crystallyze anything. So, a good
practice is never send the robe to the laundry to start collecting the
valuable seeds. Possible disadvantages: Old lab-robes tend to smell if you
dont wash them.

3. COOLING
Perhaps your solution has not yet reached saturation and the impuries add
to that, lowering the mp of the stuff you are looking for. Gradually cool
the stuff, possibly with some scratching. Possible disadvantages: an oily
goo separates and never crystallizes.

4. SLIGHTLY CLEANING UP
Perhaps the impuries don't let the stuff crystallize. You can clean up the
solution with a bit of active charcoal, active alumina or silica. Possible
disadvantages: your stuff clings stronger to the solid agent than the
impurities.

5. DRY COLUMN CHROMATOGRAPHY
Run your solution througha dry column until it reaches the bottom, cut out
the various sections (UV lamp detection, discoloration or some simple
reactions on little samples taken from it), extract the compound and try
again. Possible disadvantages: nobody knows how to make a good dry column.

6. FORGETTING ABOUT IT
Place the solution uncovered in a dark cupbouard and forget about it. After
a few days you open the cupboadr looking for something else, and lo and
behold! the most beautiful crystals formed. Possible disadvantages: a) the
think doesnt work because you cannot forget about it and keep openning the
cupboard every few hours, disturbing the delicate mechanism; b) you realy
forgot about it for a long time and the thing dried up and grew moulds that
ate the stuff.

7. FORCING SYNTHESIS
If possible, place the reagents in a solvent that dissolves them only
slightly, and shake the suspension overnight (overday is no good). If the
reagents can't help it, they'll react in solution and the product
precipitates. Possible disadvantages: the thigng doesn't work and you
spoiled good reagents and wasted long nights.

8. MISCELLANEOUS
All these worked on one occasion or other: Adding bad solvents to promote
precipitation, salting out, slightly heating, praying, buying it from
Aldrich (somebody prepared it but didn't publish it, just sold it), etc.
Possible disadvantages: none, because by now you've hopelessly given up,
and, if there's the slightest hope, welcome!

All the best,

Jacob

At 21:30 20/05/2002, Kees Bakker wrote:
>Dear all,
>
>one of the biggest problems that I face in organic chemistry is: how to
>crystallize a new organic compound. Especially when you expect the compound
>to be crystalline.
>So I am looking for all kinds of tips and tricks on crystallization.
>
>Thanks for help.
>
>Kees
>
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Prof. Jacob Zabicky
Institutes for Applied Research
Ben-Gurion University of the Negev Private: POB 12366, Beer-Sheva 84863
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