activation of Magnesium for grignard reaction

Grignard: Initiating the reaction is the tricky part, people have used 1,2-dibromomethane, iodine, TMSCl - but for me the best working initiation technique is to place few equivalents of Mg turnings into an oven-dried flask with a large egg-shaped stirbar, flush it thoroughly with dry argon, add few drops of Br2 and dry-stir the Mg turnings in the Br2 vapors overnight. Then add freshly distilled ether solvent via canula (the bromine color disappears) and then carefully your substrate. I got some Grignards like BrMg(CH2)3MgBr by this technique that are hard to make by other methods (unless you want to mess with Rieke Mg). The Mg turnings are fairly fragile and crushing them in oxygen-free and nitrogen-free environment uncovers a highly-reactive newly-formed surface which is further protected by MgBr2 formation. MgBr2 is soluble in ether. Please note that one has to use Ar because N2 reacts with fresh Mg surfaces, to produce dark Mg nitride.


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