Grignard Reaction

François Auguste Victor Grignard enjoyed his heyday in the late 19th and early 20th centuries. In his spare time, when he and Franz Haber weren't trying to figure out how to kill each other with phosgene, he played around with Mg and halogenated reagents. The ubiquitity of this reaction is attested to in the quote "...every chemist has carried out the Grignard reaction at least once in his lifetime..." by M. S. Kharasch and O. Reinmuth in Grignard reactions of Non-Metallic Substances (Prentice-Hall, New York, 1954). It was true in the '50s and is still true today. Although the Grignard reagent is a very useful synthetic tool for organic chemistry, it is actually most chemistry student's first attempt at becoming an organometallic chemist. Things usually go poorly, in part because the TA's are organic chemists themselves, ill trained at how to handle the air and moisture sensitive reagents, but mostly because the organic chemistry laboratory is ill-equip to handle such sensitive manipulations. These experiments, often set up for failure, are the last of many student's attempts to become interested in organometallic chemistry. This is unfair, due mostly to the great amount of voodoo required to get a desirable reaction in the first place.

I have spent a considerable amount of time recently trying to make my ligand into a Grignard reagent so as to assist in metallating with cobalt. Things have not been going well. This reaction has been attempted at least 10 times now with varying levels of success. The two most successful attempts have occurred January 6th and previously on December 8th. Suspiciously, these days coincided with a moon phase of waxing gibbous. I am not proposing that the phase of the moon determines the outcome of this reaction, but I am not willing it rule it out yet either.

The typical reason for a Grignard reaction to not proceed as desired is a lack of "activity" of the Mg. This is usually directly related to the amount of exposed surface area of Mg in the zero oxidation state. The same principle is true of making contacts in electronics; anyone who has torn a flashlight apart can see the contact where the circuit of the battery is completed. If the contact is dirty (black copper oxide or green copper carbonate) one need simply rub it with some sand paper to expose the copper metal in order to make a good contact. Several tricks for activating Mg turnings have been tried in the work I'm currently attempting.

Pre-treat the Mg: The Mg turnings which exist in our storage cabinet turned out to be pretty awful and disgusting. I wish I had noticed this before I started to use them, but hopefully you can learn from my mistakes. Not really knowing what pure, uncorroded elemental magnesium should look like, I thought the Mg was clean enough, and nothing a crystal of I2 couldn't take care of (see below). Simply looking at a metal to judge the level of corrosion is like having perfect pitch. Some people think they can sing a C note on demand but few people actually can. The safe bet is that any metal which has been sitting around has some tarnish or corrosion to it. I washed a portion of the Mg with 1 M hydrochloric acid to shine it up. This is an exothermic reaction which gets pretty warm (and also evolves H2 gas) so I had a bucket of ice water nearby just in case. After that I gave it a few thorough rinses with deionized water and then anhydrous diethyl ether. Just to be sure this stuff was dry before I took it into the Ar box I left it under vacuum in a sand bath at 130 C overnight. The difference is pretty clear. On the left are the Mg chips as they were in the store room. On the right are the same chips after washing. Like I said, I'm embarrassed to admit I even tried using the unwashed chips. At the time I thought this hadn't worked. More on that later.

I2 crystals: The typical activation reagent used in Grignard reagents is to add a small crystal of iodine. Iodine is used because it is a solid at room temperature, so it is easier to use than bromine or chlorine, although these would presumably have the same effect. The iodine reacts easily with the surface of the magnesium, and after the small amount of I2 is consumed, fresh Mg surface is left exposed to react with your halogenated reagent. Small amounts of preformed Grignard reagent are sometimes used in this same manner, although I did not attempt them myself with this reaction. After using I2 I was still isolating protonated reagent at the end of the reaction, which at the time made me think I was getting some water into the reaction somewhere. I was also having reactivity problems. As I followed the reaction by NMR I noticed that the Grignard product was slow to form, and soon after it did I would see the protonated product.

Heat: In order to speed things along I was heating the reaction and using THF as a solvent so that I could use higher temperatures. (Diethyl ether is the typical reagent, but THF boils about 30 C higher.) Heating a Grignard is not usually advised, as the formation of Grignard reagents is usually exothermic, and therefor performed at room temperature or sometimes in an ice bath.

Drying/Activating Mg: Frustrated with the prospect of water somehow getting into the reaction, I decided to use a qualitative indicator to ensure everything I was adding to the pot was dry. I made up a soluton of sodium naphthalenide. (Simply 1:1 Na metal and naphthalene in THF, about 0.1 M.) After stirring for a few hours all the water will have reacted with the Na and the solution will be dark green. I slowly added the dark green solution to Mg chips stirring in THF until the reaction solution remained pale green, indicating that the Mg was activated and no water existed in the pot. This still gave me slow reactions, and after heating protonated product was still apparent.

At this point I am running out of tricks, but there is one more, to be discussed in another post. As it turns out the Grignard reagent that was forming had the Mg stabilized by a THF when it was isolated, and in solution it was no doubt solvated. Most Grignard reagents are only stable in solution, but probably what happened was the Grignard was activating the alpha proton on THF either at high temperatures or while being isolated. I tried all of the above with diethyl ether as well, but no luck... the reaction does not get hot enough to go to product at the temperatures accessable using ether. The next trick? Rieke Magnesium.