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Why is a Grignard reaction anhydrous?

Answer 1

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

#R-X + Mg rarr R-MgX#

As you are aware, the reaction is typically carried out in THF or dry ether, where the oxygen donor stabilizes and dissolves the Grignard reagent.

Strongly carbanionic in nature, the Grignard reagent can react with electrophiles such as the hydrogen atoms in water molecules.

#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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Answer 2

Zoe Alexander

A Grignard reaction is anhydrous because water can react with the Grignard reagent, which is a strong base, resulting in the formation of an alkane and magnesium hydroxide. This side reaction competes with the desired reaction, reducing the yield of the desired product. Additionally, water can also hydrolyze the Grignard reagent, destroying its reactivity. Therefore, it's essential to carry out Grignard reactions under anhydrous conditions to prevent these side reactions and ensure high yields of the desired product.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.