[madscientist]

I feel embarassed to ask because it is pretty elementary, but im stuck on the mechanism involved with the Synthesis of Grignard reagents. Cant get my head around how the Mg gets in between the R group and the Br. All i can think of is that it involves the electronegativity difference between C and Mg?

How does the positive Mg bond to the positive Carbon? or is the mechanism i have started in the pic below totally wrong?

Any help appreciated,

cheers.

[Sam (NG)]

Think of the R group as R^- with a counter ion MgBr⁺.  According to my organic chemistry textbook (Clayden, Greeves, Warren, Wothers, Organic Chemistry, OUP) the mechanism of insertion isn't precisely known, but they give the diagram that i have attached.  It is an Oxidative addition mechanism because the Mg goes from (0) to (II).  The certain thing is that the reaction starts between the metal and the halogen.

[edit]  That is meant to look like the arrow coming from the Mg is coming from a lone pair.

[madscientist]

Thanks alot for the quick response Sam, has helped alot.

Cheers,

[movies]

This mechanism is exceedingly difficult to study.

I like to think of oxidative addition as a three-center process where there is a transition state that looks like a three membered ring, first with one real bond between the C and the Br, and two forming bonds between Mg and the C and Br.  In the next part, you swap the forming bond and the real bonds and you have the Grignard reagent.

This interpretation is closer to a radical mechanism than the polar one shown above.

[madscientist]

Thanks movies, appreciate the help.

Cheers