DAT Destroyer - Grignard in aprotic..?

[ballyhoo]

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Hi,

I'm looking at number 130 of the 2011 version of the DAT Destroyer. It shows 1-propyne (I think I got that right..) reacting first with Ch3MgBr, a Grignard. In the answer key, instead of Ch3- attacking the more substituted C, it attacks the proton of the alkyne.

My question is ... why does the first part not attack the alkyne? Why does it deprotonate and replace the CH3 in the Grignard?

I understand that in a protic solvent this could happen, but acetone is aprotic ...

I've attached the question and the answer here.

Thanks!

 

[znny19]

I think its because the proton on the alkyne is acidic so it would be deprotonated by a base (grigrnard reagent). That's how I understood it.

 

[mario0923]

H3C-CC-H, the terminal alkyne is reactive
so the grignard reagent deprotonates the terminal hydrogen making H3C-CC(-) (-) charge at the terminal carbon.
as you see in the diagram MgBr is (+)
(terminal alkynes' hydrogen is much more acidic than alkane's, but not as acidic as a protic solvent)

CH3 that was in CH3MgBr is now CH4 (it got the proton from the terminal alkyne)

also strong nucleophiles attack the less substituted carbon.

I might be wrong so double check 🙂

in protic solven this will not happen. the Grignard reagent will react with the solvent instead of the molecule that you want to react with because of the acidity difference.

 

[ballyhoo]

Oh okay, that makes sense! So I guess whenever there is a reactive H, the Grignard will deprotonate rather than attack at the less substituted carbon? Or is this exclusively alkynes?

Thanks so much!!

 

[Heller123]

I learned this from destroyer- Grignards are powerful bases and react with acidic protons.
Groups with acidic protons will hinder it's formation (SH, OH, COOH, NH2, Alkynes)