Question

Why are Grignard reagents prepared in excess?

Answer 1

Because (i) Grignards are reactive beasts that may be deactivated by adventitious water; and (ii) the excess reagent is destroyed during work-up, and removal of the excess is straightforward.

Explanation:

(i) `R-MgX + H_2O rarr R-H + MgXOH`

Water destroys the organometallic reagent. Sometimes we can isotopically label an organic species with `""^2H` or `""^3H` simply by treating the Grignard with heavy or tritiated water.

(ii) Since the product of water work up of excess Grignard is an alkane (most of the time), the excess can be easily removed by suction. It stands to reason that we use excess reagent to make the `C-C` bond reliably (there are so few ways of doing this). Since Grignards are (relatively) cheap, certainly it is cheaper than whatever it is you are working on, it makes sense to use an excess.