acx01b
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the role of iodine in RMgX synthesis
I founded two versions:
- magnesium turnings in a flask, add a 1mg (1mm) crystal of iodine, heat a little, you see the purple vapors.
THEN add the ether and the halide.
- magnesium turnings in a flask, add the ether, the halide, and THEN a crystal (1mg) of iodine.
which one is the good one ?
[Edited on 8-5-2004 by acx01b]
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ziqquratu
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As I understand it, either one can be used... Personally, I've only ever done the add ether to magnesium then add a crystal of iodine. Thus I
can't comment about the other way, but this works quite well and is very simple. Just add a crystal of iodine to the magnesium and ether, stir
for a few minutes, then slowly start adding the halide.
Sometimes the reaction will still be stubborn and not proceed, but if you put it in some hot water for a minute or two it should kick off smoothly.
You might want to stop adding halide while you're heaing it, just to be totally safe. You'll know it's going when the mixture begins
to go cloudy and starts to lose the yellow color as the iodine is consumed - eventually the mix should totally lose the yellow.
Another option that I haven't tried, but have been told is extraordinarily effective at activating the magnesium, is to place the mixture in an
ultrasound bath for a short time - should get everything happening quite nicely.
Make sure you keep the addition rate slow but constant, else the reaction may run away or stall (depending on which one you stuff up!). 1 drop every
5-10 seconds is usually ok, particularly if you keep the flask in an ice bath, but keep an eye on it to be sure.
This has never failed to give a good yeild. Just remember to have everything clean and DRY, and be careful, seeing as how just about everything that
goes into a Grignard flask is flammable, toxic or both!
By the way, wouldn't this have fitted quite well in the other Grignard thread that's popular at the moment, over in Chem in General?
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