# Grignards and anhydrous ethanol

I remember using both 95% ethanol and absolute ethanol quite a few times. You'd need absolute ethanol for a reaction that would fail if water was present, such as a Grignard reaction. I can't remember whether the absolute ethanol was in an ordinary bottle or the type with a rubber septum (where you keep the liquid under nitrogen to avoid contact with air). If the ethanol was to be used as a reagent, a septum bottle would be more appropriate.

http://boards.straightdope.com/sdmb/showpost.php?p=5314683&postcount=10

First, why would one use ethanol for a Grignard reaction? If a Grignard is destroyed by water (as suggested by the poster) then wouldn't a Grignard also be destroyed by ethanol?

I'm guessing that the poster just forgot all his chemistry. Ironic for someone posting on a message board that proclaims to be "fighting ignorance since 1973."

Second (assuming that one could somehow run a Grignard reaction in ethanol), do you really need anhydrous ethanol for Grignard reactions to work? Why not just use a slight excess of Grignard to account for the water present in 95% ethanol? Seems to me that anhydrous ethanol would be pretty expensive given that the usual distillation methods wouldn't get you from 95% ethanol to 100% ethanol.

Finally, is there a reaction in which using excess Grignard to account for acidic protons wouldn't chemically work in some way?

• Absolute ethanol (~99.8%) is not particularly expensive. There are several techniques to skirt around the water-ethanol azeotrope which can be done in very large scale. It's just not quite as common to see for sale, because for most home purposes 95% ethanol is just as good as absolute. – Nicolau Saker Neto May 23 '15 at 11:34

• This is so true. We recently had a question about iodine's role in the activation of magnesium: chemistry.stackexchange.com/q/31824/4945. Therefore I read up on the described method by Lund and Bjerrum. Adding magnesium to the alcohol is used to obtain absolute alcohol, via $\ce{MgEt2}$. Any Grignard would be destroyed by ethanol, just like water would do it. – Martin - マーチン May 23 '15 at 2:58