[gandalf]

Hi,
i'm trying to prepare a grignard reagent from benzyl chloride and Mg in diethyl ether. The reaction begins without problems, only handwarmth and a small crystal of iodine does the trick every time, but soon after initiation the Mg gets clogged with a thick white paste and refuses to react further. I guess this particular grignard isn't soluble enough in ether, and my question is: what would you do to enhance its solubility? Add another solvent, raise the temp (but with ether...), try with THF instead of ether? I'm not an expert but my best guess is to add another solvent after the reaction has initiated. What solvent would be a good bet in this case, toluene maybe?

[taurean]

Hi,
i'm trying to prepare a grignard reagent from benzyl chloride and Mg in diethyl ether. The reaction begins without problems, only handwarmth and a small crystal of iodine does the trick every time, but soon after initiation the Mg gets clogged with a thick white paste and refuses to react further. I guess this particular grignard isn't soluble enough in ether, and my question is: what would you do to enhance its solubility? Add another solvent, raise the temp (but with ether...), try with THF instead of ether? I'm not an expert but my best guess is to add another solvent after the reaction has initiated. What solvent would be a good bet in this case, toluene maybe?

Try THF or 1,2-dimethoxyethane.

[T-rex]

raise the temp to about the boiling point of the ether while magnetic stirring bar is moving around.
Keep replacing ether as it evaporates or the grignard product will react with the water in the air. Ethers are pretty usual as far as grignard solvents.

The magnetic stirring is a must or it will make a paste (usually one of those triangular ones if you are using a conical vial do the trick. You can also stick your stirring rod in a burner for a second and then mix around with that as well to break up any real harder spots in the solution.

[Mitch]

If you do the rod on the burner trick and you're using ether, be careful!

[azmanam]

When I make Grignards, I stick a cold finger in the center neck of a 3 neck flask.  suspend the Mg (flame dried) in a minimal amount of solvent.  Add I2, then add the halide.  Add enough to get it initiated, then dropwise just fast enough to keep the reaction refluxing.  The cold finger (cooled w/ dry ice/acetone) will keep your ether in the flask.  I make my Grignard (vinyl Grignard) in ether, but THF or toluene are also common solvents.  T-rex seems to be suggesting the flask is open to air.  I would recommend against that.  Keep the entire system under N2 or Ar if you can.  After the Grignard is formed, I dilute with excess solvent to the concentration I desire.

[T-rex]

I usually use a reflux setup with a drying tube attached, and a rubber top on the claisen adapter so i can add more ether via syringe. (setup attached) I actually got an exact picture out of an organic lab manual.

 But if your solution becomes paste and is very thick... sometimes you gotta do what you gotta do as far as dismantling and mixing everything around.

to me, the constant vaporization of ether is enough to force any water out of the system, especially if a drying tube is attached, unless you are using the product for some exotic application.

also, how do you go about flame drying magnesium without oxidizing it?
or maybe I just recognize this procedure as being called something else.

[azmanam]

Add Mg and stir bar to flask, connect all glassware, septa all outlets, add Ar/N2 inlet, add smaller venting needle.  Start up the bunsen burner and run the flame over all glass surfaces to force residual water out of flask.  Ar/N2 will replace ambient air in flask - system (& Mg) is flame dried.

[diangeloz]

activation of Mg can also be done with dibromoethane (gas evolves), this used to work better for me.  I also prefer THF as the solvent since you can warm it up to much higher temps. Of coarse, as mentioned above, work under dry conditions, that's an absolute prerequisite.

[limpet chicken]

Never tried this personally, but people tell me, that sonication of grignards is a top class way to initiate ones that are being uppity and refusing to start, using an ultrasonic cleaning bath, apparently it greatly increases tolerance of the reaction to water, allowing, seemingly, even ether from half-used cans that have been exposed to the open air to form the grignard.

THF is preferable to ether, as the exposed oxygen atom has a stabilising effect (I think it forms some sort of intermediate complex, but I'm no expert on grignards), dioxane likewise, but dioxane is of course, toxic, hard to source and pricy as blazes.