Sciencemadness Discussion Board

Ether for Grignard

Jacob - 7-10-2019 at 07:09

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?

fusso - 7-10-2019 at 08:05

The rest? Remain as solvent.

12thealchemist - 7-10-2019 at 09:14

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.

Jacob - 7-10-2019 at 09:58

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?

SWIM - 7-10-2019 at 13:57

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?



Jacob - 7-10-2019 at 22:29

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?





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.

DavidJR - 8-10-2019 at 03:08

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.

12thealchemist - 8-10-2019 at 08:53

Quote: Originally posted by Jacob  


Yes, exactly. Ether is not pleasant to work with.



If you want to avoid diethyl ether, THF is a good substitute. It's less flammable and less volatile.