[kal215]

hello,

I don't understand the purpose of Iodine in a grignard reaction with bromobenzene, and magnesium in diethyl ether!

I think I read somewhere that the iodine improves the reactivity  of this reaction? :/
I am not sure how this reaction mechanism would look like..

If anyone has feedback, that would be great.

have a good day!

[Hunter2]

The Iodine activates the magnesium to form the Grignard by building of Mg²⁺. One drop of bromine will also do, but iodine easier to handle.

[kal215]

Hey, thank you for the response.

Interesting.

Hm, I wonder what does the iodine do to activate the Mg?

Ive read also that Iodine is used to indicate if the Grignard reaction has begun. That a iodine crystal dissolved into a bromobenzene solution with magnesium turnings would be a reddish brown, and once the grignard reaction begins the solution will turn clear...

[Hunter2]

It the same thing. Mg + I₂ => Mg²⁺ + 2 I^- Left side brown, right side colorless.

[clarkstill]

I've never quite understood what the iodine does... How does formation of a small amount of Mg(II) facilitate the oxidative addition of more Mg(0) into the C-X bond? Lewis acid activation of C-X maybe? Or does it just break up the surface of the Mg to give a greater surface area? I've also used 1,2-dibromoethane (which presumably forms ethylene) and mechanical activation of the magnesium itself, both of which work very well.