Question

Why is Grignard dry?

Answer 1

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

Explanation:

Grignard reagents, though not as water-sensitive as lithium reagents are reactive to water. Dry ethers are generally used in their synthesis.

`R-X + Mg rarr R^(delta-)-MgX^(delta+)`

Once the Grignard has been prepared, quenching with water gives back the hydrocarbon:

`R^(delta-)-MgX^(delta+) + H_2O rarr R-H + MgXOH`

Sometimes, we can exploit that reactivity. Suppose, for another experiment we needed the deuterated alkane, we could quench the Grignard with heavy water:

`R^(delta-)-MgX^(delta+) + ""^2H_2O rarr R-""^2H + MgXO""^2H`

Deuterium oxide, `D_2O`, is quite cheap as labelled solvents go.