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Topic: Quenching a Grignard  (Read 6425 times)

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Offline cchave3

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Quenching a Grignard
« on: October 18, 2011, 03:06:49 PM »
So I am trying to make an aryl grignard by adding iPrMgCl (2M soln).  But now I want to quench the reaction after the grignard is made to see both the aryl grignard made as well as how much iso-propyl groups I have in the reaction to see if I have an excess of grignard reagent.  I have heard that you can do this by quenching with I2 and then just adding some sort of sulfide to get rid of the color.  I think Na2S?  How exactly can I approach this problem?  I cant seem to find anything in the literature about this

Offline Honclbrif

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Re: Quenching a Grignard
« Reply #1 on: October 18, 2011, 05:39:17 PM »
If your reagents aren't precious, I'd just stop worrying and run the reaction, after workup pop it on the NMR and see how much product you made. If its not satisfactory, then you can start trying to diagnose what's wrong.

Can you form your Grignard in situ with Mg metal? That way you don't have to worry about conversion from iPrMgCl. The I2/sulfide method sounds impractical and smelly.
Individual results may vary

Offline fledarmus

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Re: Quenching a Grignard
« Reply #2 on: October 18, 2011, 07:15:05 PM »
I've found the easiest way to check the extent of grignard formation is to quench an aliquot in water, strip it down, and run an NMR. Any of your aryl halide which did not form grignard reagent will remain and the halide for any material that did form the grignard will be turned into a proton. You should be able to quantify at least one of the aromatic peaks will enough to get a good ratio of starting material to product.

Offline stewie griffin

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Re: Quenching a Grignard
« Reply #3 on: October 18, 2011, 07:41:29 PM »
In a similar sense to what fledarmus suggested, I found that I can just "spot the gringard" and run a TLC against the starting halide. Moisture in the air and your spotter will convert the grignard to the proton compound instead of the halide and in my experience there's been enough of an Rf difference to tell the difference between the proton compound and the starting halide. TLC is just a bit quicker that NMR assuming there's a line for your department's NMR.

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