Dear colleagues, please do not be fooled - magnesium oxide does not react with any of these activators - either with iodine nor with methyl iodide nor with dibromoethane.

Activators react with magnesium in those places where the magnesium oxide film has a small thickness or defective, so the mechanical action improves activation. As a result, these places form cavities filled with a magnesium halide (or Grignard reagent by MeMgI-activation), which ones leaches by solvent in subsequent steps.

Especially, it can be seen in the preparation of magnesium alcoholates.

In complicated cases, such as the preparation of the Grignard reagent from pentafluoro-chlorobenzene, dibromoethane activation is needed addition in stoichiometric amounts - to refresh magnesium surface because blocking by products of side reactions. Of course, this amount is much larger than the amount of magnesium oxide on the metal surface.

In some cases, such as a Grignard reagent obtaining from CF3(CF2CF2)nCH2CH2-I, presence of homologues with n>= 4 as impurities block reaction completely.

Activation of metallic magnesium prior to the Grignard synthesis occurs by the formation of MgI2 or MgBr2 surface layer on the metal.

In my opinion, the chemicals that you mentioned, are not the best of the known activators. I think, the best result gives contacting metallic magnesium with bromine vapor, for several hours.

I think, the best result gives prior calcination of POWDERY metallic magnesium in vacuo, contacting POWDERY metallic magnesium with smoll cristal of iodine, 0.1 ml MeJ or 0.1 ml MeMgBr (MeMgJ), and heating, and THF instead (Et)2O. Only 20-30 minute!

Two opinions about of this problem:

1) Hamdouchi C., Walborsky H.M. // In: Silverman

G.S., Rakita P.E. (Eds.). Handbook of Grignard Reagents.

New-York: Dekker, 1996. P. 145–218.

Walborsky H.M. // Acc. Chem. Res. 1990. V. 23.

P. 286–289.

Walborsky H.M., Zimmermann Ch. // J. Am.

Chem. Soc. 1992. V. 114. P. 4996–4998.

2) Garst J.F., Ungváry F. // In: Richey Jr.H.G. (Ed.).

Grignard Reagents: New Developments, Chichester:

Wiley, 2000. P. 185–275.

Garst J.F., Soriaga M.P. // Coord. Chem. Rev.

2004. V. 248. P. 623–652.

Garst J.F., Ungváry F., Baxter J.T. // J. Am.

Chem. Soc. 1997. V. 119. P. 253–254.

Garst J.F., Boone J.R., Webb L., Lawrence K.E.

et al. // Inorg. Chim. Acta. 1999. V. 296. P. 52–53.

As the others have said, add a tiny crystal of iodine to the reaction mixture. Helps to remove some of the MgO protecting the Mg surface thus causing it to react faster. Poking it with a glass rod can also help.

Dear colleagues, please do not be fooled - magnesium oxide does not react with any of these activators - either with iodine nor with methyl iodide nor with dibromoethane.

Activators react with magnesium in those places where the magnesium oxide film has a small thickness or defective, so the mechanical action improves activation. As a result, these places form cavities filled with a magnesium halide (or Grignard reagent by MeMgI-activation), which ones leaches by solvent in subsequent steps.

Especially, it can be seen in the preparation of magnesium alcoholates.

In complicated cases, such as the preparation of the Grignard reagent from pentafluoro-chlorobenzene, dibromoethane activation is needed addition in stoichiometric amounts - to refresh magnesium surface because blocking by products of side reactions. Of course, this amount is much larger than the amount of magnesium oxide on the metal surface.

In some cases, such as a Grignard reagent obtaining from CF3(CF2CF2)nCH2CH2-I, presence of homologues with n>= 4 as impurities block reaction completely.

All procedures in the publications you can follow, but in the beginning use sonicator to initiate the reaction is the best way that I used.

Dr. Chen, you are right, ultrasonics often promote Grignard generation - it is mechanical action that I pointed above. Most impessive results was obtained with ultrasonics and low hardness metals, such as Li, Al and Zn.

However, chemical activation is a paradigm of classical chemistry and is supported with thousands of reliable procedures; access to equipment also play role.

From physical methods is notable an microwave activation, for example see at http://www.sciencedirect.com/science/article/pii/S0040402005015929

Despite of mentions about dielectric heating, in this case we have likely an promotion of single-electron transfer as critical step of metal-organics synthesis.

Stirring dried Mg turnings under Nitrogen or Argon (before adding any solvent) for several hours (ideally overnight) is a very effective way of activating Mg. This works by mechanically abrading the surface exposing clean metal. It requires a good quality inert atmosphere to avoid the cleaned surface re-oxidising. Proceed in the conventional way following this normally has very good results, we have done this routinely for years and with alkyl halides that are known to be stubborn to react. We dry the Mg in an oven (around 100C) for 24 hrs. beforehand.

P.s., magnetic stirring works well, for large scale reactions the magnetic follower can be replaced by an over-head stirrer before starting the reaction.

We too as PeterEdwards above stir Mg turnings under dry Nitrogen before adding any solvent in order to activate Mg and making it react. The Mg also should be well dried before use.

https://www.researchgate.net/post/Does_anyone_know_the_best_way_to_activate_magnesium_for_the_grignard_reagent?cp=re65_x_p3&ch=reg&loginT=v6zmZthpD5pjkWXF06PWdGdiqJXd_eR9Nes23qTZVNZtsnU5wtD-3w%2A%2A&pli=1#view=534299fdd2fd64160a8b4596

Like Peter, I have found that stirring the Mg under inert conditions makes it easy to start a grignard reaction. I have sometimes used ultrasound to get the reaction going, but I find that the best thing to do is to make sure that everything is very dry / anhydrous and then to add a little ether. Then I add some alkly / aryl halide and then stirr for a moment and then stop stirring.

Then I tend to get a little reaction at one spot, I have read that the formation of a grignard is subject to autocataylsis so once a little product forms the rest of the magnesium reacts more quickly.

This process of activating Mg is called entrainment, a very old process. In the case of 1,2 dibromoethane, under the reaction conditions it forms grignard reagent however, from which ethylene escapes thereby the Mg surface is activated, in a sense the top surface is reacted and the surface underneath is free from impurities and the overall reaction is activated. Additionally, the THF solvent is good for the solubility of the grignard reagent formed from aromatic substrates. Iodine and 1,2 dibromoethane is very good than that of methyliodide since the formed Grignard reagent does not take part in the reaction, leaving behind Magnesium iodide and evolving ethylene respectively. Before this process the Magnesium can be acid treated for exposing the active surface. However, in synthetic chemistry the practice that makes the perfect chemist.

It is surface cleaning to remove any magnesium oxide may be chemical or mechanical

I have found that putting the reaction flask in an ultrasonic cleaner does sometimes help to get sluggish grignard preps to go.