Preparation of Grignard Reagent
Grignard reagents form via the reaction of an alkyl or aryl halide with magnesium metal. The reaction is conducted by adding the organic halide to a suspension of magnesium in an etherial solvent, which provides ligands required to stabilize the organomagnesium compound. Empirical evidence suggests that the reaction takes place on the surface of the metal. The reaction proceeds through single electron transfer: In the Grignard formation reaction, radicals may be converted into carbanions through a second electron transfer.
- R−X + Mg → R−X•− + Mg•+
- R−X•− → R• + X−
- R• + Mg•+ → RMg+
- RMg• + X•- → RMgX
A limitation of Grignard reagents is that they do not readily react with alkyl halides via an SN2 mechanism. On the other hand, they readily participate in transmetalation reactions:
- RMgX + ArX → ArR + MgX2
For this purpose, commercially available Grignard reagents are especially useful because this route avoids the problem with initiation.
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