# Grignard alkene on a carbonyl

I was given the following mechanism to prove today post-lecture:

and my attempt is given below.

Now, when the negative charge on the alkene attacks the carbonyl, the electrons are pushed onto the oxygen - which is understandable. My question is: why do these electrons come back down, rather than just be protonated to give an alcohol?

• This is the Weinreb "amide". In your first rxn., Mg is bound to both oxygens. The complex is stable until aqueous workup causes decomposition to the ketone. Your proposal would cause the ketone to react with the Grignard reagent. Ketones are more reactive than amides. – user55119 Jun 11 '19 at 2:14
• Why don't ketones form geminal diols in water? colby.edu/chemistry/CH242/15-2.pdf – Karsten Theis Jun 11 '19 at 22:50
• @KarstenTheis perfect if you can add that as the answer then I can accept it as the answer! – vik1245 Jun 12 '19 at 16:50
• @Karsten Theis: Not at all. I appreciate your concern and thank you for doing the right thing. Cheers! :-) – Mathew Mahindaratne Jun 12 '19 at 19:02

The nucleophilic carbon on Grignard reagent attaches to carbonyl carbon and gives tetrahedral intermediate as a metal alkoxide complex. But, this alkoxide can collapse to reform strong C=O bond, displacing the amino portion of the amide as a leaving group, in the form of the amide ion, $$\ce{R1R2N-}$$. This produces the ketone as a final product, assuming you have used only one equivalent of Grignard reagent (Channel reactions of Grignard reagent with esters).