Question

What is a Grignard reagent?

Answer 1

Grignard reagents are organometallic reagents in which an hydrocarbyl residue is more or less directly bound to a magnesium metal centre. They offer one of the few means of `C-C` bond formation.

Explanation:

Given a few provisos (i.e. no protic functional groups, exclusion of water) an hydrocarbyl halide, `R-X`, may be metallated directly with magnesium metal:

`R-X + Mg rarr R-MgX`

The `R-MgX` reagent, formally alkyl (hydrocarbyl) magnesium halide, will react as a carbanion, `R^(delta-)""^(delta+)MgX`. Typically the reaction is performed in an ethereal solvent, `Et_2O`, or `"THF"`. The Grignard reagent is usually not isolated, but used directly, and added to aldehydes to give `2""^@` alcohols (after workup),

`R-MgX + R'C(=O)H rarr RC(-OMgX)R'`

to ketones to give `3""^@` alcohols,

`R-MgX + R'C(=O)H rarr RC(-OMgX)R'`

and to dry ice to give a carboxylate salt that is `1` carbon longer than the Grignard residue:

`R-MgX + CO_2 rarr RC(=O)(-OMgX)`

In fact, still the best way to make a carboxylic acid is to pour your Grignard mixture directly onto dry ice, and step back. The Grignard reagent usually fails to react with alkyl halides to give direct `C-C` coupling, but there are reports of the use of copper salts to add to the Grignard to make an organocopper species that is capable of direct reaction with `RCH_2X`.

To conclude, Grignard reagents offer ease of synthesis (of course, you can't be too ham-fisted), prodigious reactivity, and adaptability. We can exploit the water sensitivity of the Grignard if we want to label a hydrocarbon chain with deuterium, `""^2H`, or tritium labels, `""^3H` labels.

`R-MgX + ""^2H_2O rarr R-""^2H + MgO""^2HX`

This is a cheap and efficient way to label a hydrocarbyl chain. And as I have mentioned before it is good way to get your lazy graduate students off the computer and back onto the laboratory bench.