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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?

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  • $\begingroup$ 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. $\endgroup$ – Nicolau Saker Neto May 23 '15 at 11:34
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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?

As well as being excellent nucleophiles, Grignard reagents are also very strong bases because they essentially function as alkyl anions. Therefore they will react with ethanol to form magnesium ethoxide, the alkane, and magnesium bromide.

Grignard reactions are usually carried out in ether solvents such as diethylether and tetrahydrofuran because they are unreactive towards the reagent and they actually stabilize it by coordinating to the magnesium ion to form a tetrahedral complex.

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(image source)

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  • $\begingroup$ so the poster is just looney? $\endgroup$ – Dissenter May 22 '15 at 19:09
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    $\begingroup$ I think so. I don't see any way you could get around the deprotonation. $\endgroup$ – bon May 22 '15 at 19:59
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    $\begingroup$ 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. $\endgroup$ – Martin - マーチン May 23 '15 at 2:58
  • $\begingroup$ @bon - after critically reviewing the quoted text, I note that the poster did not specifically come out and say that ethanol was used as a solvent for Grignard reactions. Is there any case in which ethanol would be useful in the general context of a Grignard reaction? $\endgroup$ – Dissenter May 23 '15 at 23:55
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    $\begingroup$ @Dissenter you might use ethanol to quench the excess Grignard in a reaction vessel at the end of a reaction. $\endgroup$ – matt_black Dec 24 '15 at 23:40

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