[Sis290025]

In the synthesis of phenylmagnesiumbromide from bromobenzene and magnesium metal, sometimes iodine is utilized. Exactly how does iodine act as an activator for this reaction (in reference to the magnesium metal)?

Considering that phenylmagnesiumbromide would be fabricated without the addition of iodine, does iodine change the rate of the reaction or yield significantly?

Thank you for your time.

[stevet]

As far as i remember, the iodine removes any magnesium oxide which may cover the magnesium metal, which would prevent its reaction with the halide.

If you get the reaction going without the addition of the iodine, it is not required, as the reaction is quite vogourous as is, though if you are struggling to initiate your Grignard, add a small crystal of iodine, and the reaction should proceed.

Steve

[moov]

Ya,you are absolutely correct stevet...i agree that,one more thing  Sis290025 why can't you try Dibromoetane(activator) instead of Iodine.

[neelam]

hellow

instead of trying with iodine and dibromoethane you try with methyl magnesium chloride, which will remove magnesium oxide and activate the Mg. if you use dibromoethane, high temperature is required for initiation which leads to impurities. using MeMgCl even at low temp itself you will get initiation. iodine is also ok but fixing the specfic quantiy some what critical. only you have add pinch. whereas in case of MeMgCl you can fix a specific quantity to initiate Mg.

for reference please go through prof. Knochel papers.
 

[orgopete]

I don't understand how an MgO would be converted to Mg^o with I₂.

[uvcyclotron]

I don't understand how an MgO would be converted to Mg^o with I₂.

I would support orgopete by saying that I don't understand this conversion either,and would love an explanation.

Also, I am not aware of the MeMgBr method as pointed out by neelam.
AFAIK the reaction is carried out with the Mg metal, in diethyl ether (D.E) or THF (TetraHydro Furan) as solvent, THF is preferred for aryl halides as it has a higher B.P. than D.E.
The ethers as solvent stabilize the the Grignard Reagent (G.R) formed, while also providing an inert reaction medium.

The metal can follow either/both an ionic and free radical mechanism for the G.R. formation

UV

[neelam]

for reference please go through prof. Knochel papers