Question

Why is a Grignard reagent prepared in excess relative to an aldehyde?

Answer 1

A Grignard reagent is prepared in excess relative to an aldehyde, because you seldom get a 100 % yield of the reagent.

Explanation:

The Grignard reagent is so reactive that you often get side reactions:

(a) Reaction with water

If your apparatus is not perfectly dry, you will get a reaction with water

`"RMgX" + "H"_2"O" → "RH + Mg(OH)X"`

(b) Reaction with oxygen

The Grignard reagent reacts with oxygen to form peroxides.

`"R-MgX" +"O"_2 → "R-O-O-MgX" stackrelcolor (blue)("H"_3"O"^+)(→)"R-O-O-H"`

(c) Homocoupling

Reactive halides can give coupled products, similar to a Wurtz reaction.

`"R-X + Mg + X-R → R-R + MgX"_2`

All of these reactions reduce the yield of Grignard reagent, so you usually plan to prepare an excess to compensate.