Grignards and diatomic halogens

Can Grignards react with diatomic halogens as to perform an substitution reaction?

I'd imagine that the first part of making the reaction work - polarizing the halogen as to prepare to pull it apart - would be easy for a Grignard. Hell, even alkene pi bonds can polarize halogens sufficiently; why can't a Grignard?

So in essence, can phenylmagnesiumbromide react with $\ce{Cl2}$ to make chlorobenzene? The other product of this reaction would be $\ce{Cl-}$ ... which I could see combining with the $\ce{MgBr+}$ in some way.

• I don't know the answer to this, but why would you do this? The Grignard is made from the corresponding halide and now you want to go back to the halide. Oct 25 '14 at 0:30
• It's a theoretical exercise. The question was how to go from a bromide grignard to a chloride grignard Oct 25 '14 at 0:40
• I am not sure how good it works but if it does it could be a nice way to exchange a halogen. For example how many ways are there to go from an aromatic/heteroaromatic bromide to its chloride?
– K_P
Oct 25 '14 at 0:41
• So you know it works, just not how well? Oct 25 '14 at 0:42
• No I am afraid. I was commenting on the practicality of the transformation after @jerepierre 's comment. It looks possible maybe even with a different Cl source like N-chlorosuccinimide. Have a look at some literature. Is it a question from school or your own question?
– K_P
Oct 25 '14 at 0:48

Can Grignards react with diatomic halogens as to perform an substitution reaction?

Yes, but it looks like a mixture of products results, including something explosive. Here is a link to an abstract of some early work that reports,

"Chlorination of Grignard and Iotsich reagents, RMgX, (where X = Br, I) gives considerable quantities of RI and RBr together with RCl. Bromination of RMgI results in RI and RBr. Bromination of RMgCl leads to RBr, and iodination of RMgX (X =Cl, Br), to RI."

An "Iotsich" reagent is an acetylenic Grignard ($\ce{RC#CMgBr}$)

Judging by the fact that 1) the abstract suggests that product mixtures of the various possible alkyl\aryl halides are often formed and 2) no further reports claiming synthetic utlity have emerged, I suspect that the reaction is of limited synthetic value. Nonetheless, your initial suspicion that reaction should occur seems supported.

One further note, many MSDS's and "Hazardous Laboratory Chemicals Disposal Guide, Third Edition" report that when phenylmagnesium bromide is reacted with chlorine, a solid is formed that explodes when shaken. The reference provided is the same J. Organomet. Chem. paper cited above. Given the lack of detail provided on the solid, my guess is that it wasn't characterized.

• what does this mean? "The abstract that you found, which is an alternative to your original claim, I believe is what is known as transmetallation. It would be helpful to see the full paper to understand yields but I don't think it is worth \$30 to find out. Transmetallation is the very reason why productive alkyl substitution does not work well with Grignards and alkyl halides." Oct 25 '14 at 15:21
• I'm confused because why is there transmellation in the reaction of a Grignard with an alkyl halide .... there's only one metal cation (Mg2+) ... is he saying that the Mg(2+) switches places or something? Plus I think the paper is characterizing something best described as trans-halogenation (hence RI, RBr, and RCl as products). Oct 25 '14 at 15:33
• I'm not following, 1) "what does this mean" is "this" referring to the abstract, what don't you follow, 2) what was my "original claim", 3) I'm also confused as to what you mean by "transmetallation".
– ron
Oct 25 '14 at 15:38
• Sorry, that was my teacher's response to me. I gave him the abstract you gave me and told him it looks like chlorination should work. The last line is what I don't understand. Transmetallation occurs with Grignards and alkyl halides? Oct 25 '14 at 15:56
• The last line in the abstract is confusing because they omit the triple bond between the 2 carbons. However that line is not relevant to your initial question. My understanding of transmetallation involves ligand exchange between 2 different metals. It does not occur here.
– ron
Oct 25 '14 at 16:01