Question

How would you prepare Grignard reagents in a lab?

Answer 1

Grignards are organometallic reagents, whose natural enemy is water. I can give a few tips.

Explanation:

Inorganic chemists often go to elaborate (and perhaps unnecessary) steps to synthesize Grignard reagents. An organic chemist is much more likely to want to get the job done, and take reasonable steps.

1. Your solvents must be reasonably dry; distillation from a drying agent, or passage thru molecular sieves is recommended.

2. Grignard reactions are heterogeneous. An alkyl/hydrocarbyl halide reacts on the surface of magnesium metal. I would ALWAYS get my magnesium turnings, put them in a mortar, and then give them a good grinding with a pestle. Such a process (at least AFAIK) polishes and scratches the metal surface to give a fresh surface that should be more liable to react with the halide. In your experimental, this also allows you to use the phrase `"freshly burnished magnesium turnings"`.

3. I would not bother distilling the halide. I would, however, ALWAYS pass the halide down a short column of alumina, to dry it, and to remove any residual `HCl`. And you can use a Pasteur pipette stuffed with a bit of glass wool, and then a couple of cm of alumina to serve as the column - such a column is suitable for `1-10*mL` of halide. Often, you might start with unpromising, discoloured halide from an older bottle, and get a water clear liquid off the end of the column.

4. You have probably seen the use of (small quantities) of iodine chips, or ethylene bromide, to start the reaction; use these sparingly, because I always found these to obscure the start of the reaction. Be patient, at the start, squirt some neat halide in, and watch the spot to see if the reaction starts (as evidenced by bubbles and turbidity).

5. If you have problems with starting the reaction (and this is where most problems occur!) try (i) a hot spot with a heat gun (don't ignite the ether tho! and also your supervisor might give you a beatdown if he catches you), or (ii), better, putting your reaction vessel in an ultrasonic bath.

6. Listen to your colleagues. Someone in your department will be very proficient, and will know all these tricks and more. You just have to hunt him or her down and buy them a drink on a Friday night, before picking their brains.

PS I have an idea as to what I talking about, I use to have to make a lot of `Me_2Mg`, which I could isolate as the solid dioxane adduct. I also used a lot of magnesium anthracene as an initiator, and this reagent was truly a pain in the posterior.