Question

Why are Grignard reagents prepared in ether?

Answer 1

Because ethers are aprotic, dipolar solvents in which Grignard reagents have some solubility.

Explanation:

As highly polar organometallic species, Grignard reagents have some carbanionic character: they could be represented as `R^(-)""^+(MgBr)`.

Given this representation, it is clear that they will react swiftly and irreversibly with any protic solvent or water, or even any insufficiently dry non-protic solvent:

`RMgX + D_2O rarr R-D + MgXOD` `(D=""^2H)`

I used `D_2O` here because this is one way to get a deuterium label on a hydrocarbyl chain relatively cheaply.

As you know, however, Grignards are the `C-C` bond making reagent par excellence. They will react dry ice to give carboxylate salts, with ketones and aldehydes to form `3""^@` and `2^@` alcohols after work up. The solvent used to the perform these reactions must be reasonably inert: alcohols do not fit this criterion; ethers (both `Et_2O` and `"THF"`) do, i.e. they allow solubility without reacting with your Grignard.

What structural information that exists suggests that Grignards are oligomeric species, that are solvated by several solvent molecules, which allows the Grignard to get into ethereal solution.