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Topic: Activating magnesium for Grignard  (Read 8205 times)

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Offline cmaias

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Activating magnesium for Grignard
« on: June 21, 2013, 04:36:22 AM »
Hey everyone,

I am a research student in South-Africa currently trying to get a grignard reaction working with dibromobenzene in THF.
The problem is: the only magnesium powder I have is quite old and stored on a shelf under air...

By any chance, do you have any tip to activate magnesium for Grignard reactions?

Here is what I tried in three successive attempts, without success:
1) Heating at 100C under vacuum for 2 hours and adding a small amount of iodine.
2) Doing the reaction in an ultrasonic bath.
3) Grinding the magnesium powder inside a glove-box just before reaction.

Thank you for your help

Offline Babcock_Hall

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Re: Activating magnesium for Grignard
« Reply #1 on: June 21, 2013, 09:42:13 AM »
I have scratched/crushed the magnesium turnings with a glass rod to expose the shiny surfaces while the reaction is under solvent.  If you can't find any shiny surfaces, then I would question the goodness of the Mg metal.  It can take a little while.

Offline opsomath

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Re: Activating magnesium for Grignard
« Reply #2 on: June 21, 2013, 10:05:39 AM »
Is the magnesium turnings (like chunks the size of grains of rice or so) or is it really powder? If it's the latter, you may be in for a bad time, since the higher surface area means that more of it is coated with oxide and the small particle size means it's hard to chip off the surface by spinning it with a stirrer.

However, 99% of the time the problem is not with the metal, but with the solvent you are using. How have you dried it? If nothing else, you should be letting it stand over molecular sieves for a time before use.

Remember, anything you do to activate the magnesium should be done under inert gas the whole time. Argon is best as nitrogen will react with fresh Mg surfaces a little bit.

Are you adding iodine to the reaction as a catalyst and indicator? That may help. Otherwise, you can try to etch the magnesium in the reaction by adding a small fraction of dibromoethane or even liquid bromine and letting it react before adding your dibromobenzene.

I am sure that I have made this exact same Grignard before and that it gave me no trouble, so definitely check all your reagents.

Offline OC pro

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Re: Activating magnesium for Grignard
« Reply #3 on: June 21, 2013, 02:51:46 PM »
First of all, you really don´t need nitrogen or argon atmosphere. If your THF or ether is fairly dry, the vapor phase of the etherical solvent will keep moisture and oxygen away. Mg turnings can be old, but dibromobenzene as a very fair target will always react. I doubt you need activation. Just add 10% of your dibromobenzene to the Mg and then heat to reflux with a heat gun. It should turn dark very fast. Gratulation, your Grignard has started. Then you can add dropwise the rest of the dibromobenzene. If you are working on larger scale, you even don´t need to dissolve it. You can use it neat as it is. Just cover the Mg with THF and then start.
My first Grignard in student times also was a big disaster. It takes a bit of experience and feeling for this kind of reaction. Once you know the tricks, they will always work.
Of course, you can also use all advises from opsomath. They are also very helpful.

Offline opsomath

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Re: Activating magnesium for Grignard
« Reply #4 on: June 21, 2013, 03:26:50 PM »
I definitely agree that the inert atmosphere is not strictly necessary. I have started Grignards from bromobenzene in the disgustingly humid early fall in Georgia, (USA) using only iodine and ether stored in a bottle over sieves. But some of the tricks like spinning the magnesium dry with bromine or iodine seem to work best with an inert atmosphere.

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