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Author: Subject: Grignard successes and failures
Nicodem
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[*] posted on 25-2-2016 at 07:51


Quote: Originally posted by fluorescence  
40 ml 2.5 M fresh n-Butyl Lithium in Hexane same preparation the syringe immediately caught fire the second I got it out of the Nitrogen enviroment. We tried it again, no result, the Chlorocyclohexane simply didn't react to the ltihiated compound.

Since we didn't know if there was any product and the Lithium Chloride formed in the reaction shouldn't be soluble in Hexane we got rid of most the solvent using a vacuum and a cooling trap. Problem was the Cyclohexylchloride didnt react at all and so we had accidentally concentrated the Buthyl Lithium to its maximum concentration.

I never saw an example of Li/Cl exchange. As far as I know it does not work at all. You can even ortho-lithiate chlorobenzenes, after all. This reaction only works for Li/Br and Li/I exchanges (which is by the way among the fastest reaction ever observed, faster even than proton transfers). You would need to use bromocyclohexane with 2 equivalents of t-BuLi. To prepare alkyl lithiums from chloroalkanes you need to do it with metallic lithium just like for BuLi, though I don't see any need for it given that CyMgCl is commercially available.




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[*] posted on 25-2-2016 at 12:51


I've made a lot of phMgBr over the years and never had a problem starting the reaction. It's always good to start with a higher concentration of halide/ether mixture. I hear that THF is a little more dicey a solvent but haven't had to use it. The I2 trick usually overcomes any residual oxide. I have a shitload of Mg turnings btw if anyone needs some. The text book crap about putting the addition funnel on top of the condenser is dangerous and unnecessary. Simply reflux dry ether for a minute or two and no need for inert gas. Put in the first dose of more concentrated halide solution and go. A little heating and stand by with an ice water bath. These mothers run like crazy when started. I abhor the idea of using an LAH soxhlet; why would you do that zed?



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[*] posted on 25-2-2016 at 16:57


I can't add to the expertise of this thread (you guys are smart). Just have a couple of questions @Magpie.

What is the piece of glassware between the addition funnel and the side arm of the claisen? At first I thought it was just an adapter, but it looks like there is some liquid in it.

I don't imagine the water was turned on in your "reflux" condenser, prior to assembling the glassware, right?
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[*] posted on 25-2-2016 at 22:27


It's an adapter, 24/40 to 19/22.

Quote: Originally posted by CaptainPike  

I don't imagine the water was turned on in your "reflux" condenser, prior to assembling the glassware, right?


The water can be turned on any time - the water is in a closed system.




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[*] posted on 26-2-2016 at 03:41


Maybe stupid question, but when is water allowed to get into the grignard again? I have never preformed one, so if one could maybe clear this up for me.
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[*] posted on 26-2-2016 at 09:23


R-MgX + H2O ---> RH + MgXOH

This not only is true for water but for any chemical with a sufficiently labile H, like a carboxylic acid. Even small amounts of water are harmful due to its low molecular wt.




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[*] posted on 26-2-2016 at 09:35


Quote: Originally posted by MrMario  
Maybe stupid question, but when is water allowed to get into the grignard again? I have never preformed one, so if one could maybe clear this up for me.


Water is allowed during workup, not during the reaction. Typically this is as a saturated NH4•Cl solution, or a dilute HCL solution. This is to protonate to the alcohol.
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[*] posted on 26-2-2016 at 09:37


Quote: Originally posted by Magpie  
R-MgX + H2O ---> RH + MgXOH

This not only is true for water but for any chemical with a sufficiently labile H, like a carboxylic acid. Even small amounts of water are harmful due to its low molecular wt.


Don't grignard reagents also react with the protons of alcohols and primary and secondary amines?
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[*] posted on 26-2-2016 at 13:20
… yeah but


Quote: Originally posted by Magpie  
It's an adapter, 24/40 to 19/22.

Quote: Originally posted by CaptainPike  

I don't imagine the water was turned on in your "reflux" condenser, prior to assembling the glassware, right?


The water can be turned on any time - the water is in a closed system.


I just meant that if a cold condenser were lying on your bench, it could conceivably condense water vapor from your lab area, which would run down into your reaction flask. Okay, so maybe I'm paranoid, but it could happen right? And maybe even miniscule amounts of water, added up…
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[*] posted on 26-2-2016 at 14:04


Yes, of course. I don't turn the water on prematurely for that reason. I just had no idea that is what you were getting at.



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[*] posted on 26-2-2016 at 17:53


"I've made a lot of phMgBr over the years and never had a problem starting the reaction. It's always good to start with a higher concentration of halide/ether mixture. I hear that THF is a little more dicey a solvent but haven't had to use it. The I2 trick usually overcomes any residual oxide. I have a shitload of Mg turnings btw if anyone needs some. The text book crap about putting the addition funnel on top of the condenser is dangerous and unnecessary. Simply reflux dry ether for a minute or two and no need for inert gas. Put in the first dose of more concentrated halide solution and go. A little heating and stand by with an ice water bath. These mothers run like crazy when started. I abhor the idea of using an LAH soxhlet; why would you do that zed?"

Best yields were reported for reduction via LiAlH4 ADDITION. Me, I hated the idea.
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