What is a Grignard reagent?

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.