Jacob
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Ether for Grignard
Hi there! I was wondering how much ether is needed per mole of organohalide to produce grignard reagent. It is know that two moles of Et2O stick to a
single RMgX, but how about some excess ether?
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fusso
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The rest? Remain as solvent.
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12thealchemist
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Diethyl ether or tetrahydrofuran (another ether) are often used as solvents in the Grignard reaction. This is because they can coordinate to the
magnesium metal centre and help solvate the Grignard reagent.
Since the amount of solvent is typically many times the molar scale of the reaction, it is usually in huge excess and so the quantity of ether that
coordinates to the Grignard reagent is irrelevant. The other advantage of using a low boiling point solvent like diethyl ether is that it is very low
boiling (~35°C) and so can easily dissipate and regulate the temperature of the reaction - the formation of the Grignard reagent is typically quite
exothermic.
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Jacob
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Quote: Originally posted by 12thealchemist  | Diethyl ether or tetrahydrofuran (another ether) are often used as solvents in the Grignard reaction. This is because they can coordinate to the
magnesium metal centre and help solvate the Grignard reagent.
Since the amount of solvent is typically many times the molar scale of the reaction, it is usually in huge excess and so the quantity of ether that
coordinates to the Grignard reagent is irrelevant. The other advantage of using a low boiling point solvent like diethyl ether is that it is very low
boiling (~35°C) and so can easily dissipate and regulate the temperature of the reaction - the formation of the Grignard reagent is typically quite
exothermic. |
Yes, temperature control is important, specially for preventing Wurtz coupling side reactions. But that can be taken care of with cold water reflux
and ice bath. So, how much excess is enough?
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SWIM
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You seem to be asking what the minimum amount of ether is that you can use successfully for, not a specific Grinard reaction but for any and all
Grinard reactions.
I doubt that there is a valid answer to that question.
At least not a single valid one.
Can you tell us what reaction you are actually attempting?
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Jacob
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| Quote: Originally posted by SWIM | You seem to be asking what the minimum amount of ether is that you can use successfully for, not a specific Grinard reaction but for any and all
Grinard reactions.
I doubt that there is a valid answer to that question.
At least not a single valid one.
Can you tell us what reaction you are actually attempting?
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Yes, exactly. Ether is not pleasant to work with.
Tried for phenyl bromide and benzyl chloride, 3 to 1 molar ratio works just fine. The usual black mix without using iodine.
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DavidJR
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THF is a lot safer than diethyl ether if you can get it.
Minimum amount you can get away with depends on the solubility of the particular Grignard reagent, so nobody is going to be able to give you an exact
molar ratio for every case.
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12thealchemist
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If you want to avoid diethyl ether, THF is a good substitute. It's less flammable and less volatile.
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