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Author: Subject: Crystallisation - truly a dark art
Eddygp
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shocked.gif posted on 20-11-2020 at 02:42
Crystallisation - truly a dark art


Often described as unpredictable, mysterious, based on luck, esoteric practices and with many people reporting their own ritualistic approaches to guarantee good crystals... with that background, there is no doubt there has to be some truth in crystal growing being more magic than science.

However, some of the basic premises we all assume hold true regardless - namely, slow crystallisation yields better crystals than fast removal of solvent.

Imagine my surprise when a complex that had failed to crystallise in over 5 crystallisation attempts, yielding a powder instead, formed huge needles (0.8 mm x 1mm x 5 mm) upon removal of residual solvent on the Schlenk line vacuum. These comparatively huge crystals only grew on the points where the solution was BUMPING. Elsewhere, only a fine white precipitate formed.
I have since got an XRD structure from these.

I was thinking that these apparently unexpected situations may be quite an interesting topic to discuss.
Have you encountered any similar anomalous crystallisation conditions for your products?




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karlos³
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[*] posted on 20-11-2020 at 06:57


I have seen something similar when I precipitated 1-benzylpiperazine as dihydrochloride from an aqueous solution with acetone.
They suddenly formed beautiful needles 3-4mm long, unlike a slow evaporation would yield.
Imagine my surprise!
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njl
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[*] posted on 20-11-2020 at 07:17


I suppose it isn't very special relative to the crystallization you mentioned, but I love seeing nitrostyrene spontaneously come out of solution with a seed crystal
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karlos³
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[*] posted on 20-11-2020 at 08:52


Quote: Originally posted by njl  
I suppose it isn't very special relative to the crystallization you mentioned, but I love seeing nitrostyrene spontaneously come out of solution with a seed crystal

Thats true, the first time I saw it happen accidentally, put a little piece of ice into the postreaction mixture and suddenly several cm long needles grew in a matter of ~30sec.
Truly impressive!
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Heptylene
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[*] posted on 20-11-2020 at 09:13


I've had non-reproducible crystallisations with 4-nitrobromobenzene. First time, the crude product produced 5-mm sized beautiful, transparent yellow crystals. When these were recrystallized again, the crystals were only 1-mm although I used the same procedure (hot ethanol cooled down slowly).

Other problems with crystallisations:
- nucleation and growth on the walls of the container, which makes removal of the product tricky, especially with small amounts and small crystals.
- oiling out for no apparent reason.
- Crystallization that takes days even though thermal equilibrium has been reached and no solvent is evaporating (closed vial).

As you said, it is a dark art...
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monolithic
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[*] posted on 20-11-2020 at 12:37


https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.70...
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Corrosive Joeseph
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[*] posted on 21-11-2020 at 04:17


/CJ

Attachment: J. W. Mullin - Crystallization (Butterworth-Heinemann) (2001).pdf (6.1MB)
This file has been downloaded 116 times





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Heptylene
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[*] posted on 21-11-2020 at 04:28


That was a great read, thanks monolithic!
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