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Author: Subject: Grignard Reagent
RedsAreRaw
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[*] posted on 13-1-2017 at 11:36
Grignard Reagent


I have the question: "Why would one not prepare a Grignard reagent from BrCH2CH2OH?"

Is it because you would get a mixture of products BrMgCH2CH2OH and BrCH2CH2MgOH?

Thanks.
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gdflp
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[*] posted on 13-1-2017 at 12:06


No, not really. The reaction of water with Grignard reagents is a well studied reaction which you should easily be able to find details on. Try to think about the similarities between the protons in water and one of the protons in your substrate and see where that gets you.

[Edited on 1-13-2017 by gdflp]




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Metacelsus
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[*] posted on 13-1-2017 at 12:18


You'd have to use a protecting group for the alcohol (like a silyl ether).



As below, so above.
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[*] posted on 13-1-2017 at 14:21


As gdflp said, the reason this won't work is the same as why Grignards cannot be made in the presence of water.

Grignard reagents, like other organometallic reagents, can be considered a source of R- which is a very strong base and nucleophile (no mesomeric or inductive stabilisation). The proton on an alcohol, like water, is mildly acidic and so will be captured by R- to form the corresponding alkane (or 1-bromoalkane in this case). This will happen much more quickly than your Grignard reagent is formed, so you cannot make Grignards using this substrate.




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[*] posted on 13-1-2017 at 16:12


Quote: Originally posted by Metacelsus  
You'd have to use a protecting group for the alcohol (like a silyl ether).


See the Boord olefin synthesis. I am uncertain if silyl ethers behave similarly, though.
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[*] posted on 16-1-2017 at 07:34


Thanks for the help!

I was only thinking about the initial reaction and not what would follow.
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[*] posted on 16-1-2017 at 13:11


The initial reaction would occur just like any other Grignard synthesis. Your product would be immediately destroyed after formation, though, which is why you can't actually make a Grignard reagent from this substrate. This occurs due to the presence of the slightly acidic hydrogen of the hydroxyl group.

The idea of a "protecting group" is to temporarily deactivate this hydroxyl group such that it can no longer destroy the Grignard reagent. Once the protection is done, you can make your Grignard as normal, use its reactivity how you need it, then de-protect the hydroxyl group (reverse how you protected it). The only caveat, of course, is that the protecting group itself, or the product of the reaction of the Grignard reagent, cannot have their own acidic protons/can otherwise react with Grignards.

[Edited on 16-1-2017 by Hexavalent]




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