Question

Why must grignard reactions be anhydrous?

Answer 1

Because the natural enemy of the Grignard reagent, of any organometallic reagent, is the water molecule.

Explanation:

`RH_2CX + Mg rarr RH_2C^(delta-){""^(delta+)MgX}`

The ipso carbon in a Grignard reagent has carbanionic character; it reacts as would `RH_2C^-`. With water, its reaction wold be rapid and irreversible:

`RH_2C^(delta-){""^(delta+)MgX} + H_2O rarr RCH_3 +MgX(OH)`

Sometimes you can exploit this reactivity. Suppose, for another experiment, you wanted to have the deuterium (i.e. `""^2H=D`) or tritium labelled (i.e. `""^3H`) alkane.

`RH_2C^(delta-){""^(delta+)MgX} + D_2O rarr RCH_2D +MgX(OD);`
`D=""^2H`

Making the Grignard and quenching it with isotopically labelled water would be a convenient method of making the labelled alkane. Deuterium and tritium oxides (heavy waters) are fairly cheap; certainly it would be a lot cheaper than buying the deuterated alkanes.

Note also that when you make a Grignard reagent you go to some unlittle effort in drying your ethers and drying your halocarbon. Typically, an ether would be distilled from sodium or potassium or sodium/potassium alloy.