Why is a Grignard reaction anhydrous?

1 Answer
anor277
May 27, 2016

Because the natural enemy of a Grignard reagent, and organometallic reagents in general, is water.

Explanation:

#R-X + Mg rarr R-MgX#

AS you know, the reaction is normally performed in dry ether or THF. The oxygen donor on these solvents stabilizes the Grignard reagent, and brings it into solution.

The Grignard reagent is a potent nucleophile, with carbanionic character. It will react with electrophiles, including the hydrogens of the water molecule:

#R-MgX + H_2O rarr R-H + MgOHX#.

Sometimes, if we want to label an alkane with a deuterium, i.e. #""^2H# label, we could make the Grignard reagent from an alkyl halide, and quench the Grignard with heavy water, to give a #d_1-"alkane"#:

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

This would be much cheaper than buying a labelled alkane, as #D_2O# is dirt cheap.

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