Grignard reagent with acid

I have a question asking me to draw the mechanisms of Ph-MgBr (grignard reagent) with various carbonyl compounds followed by mild acid work up. This bit was fine but then it says what would happen if the acid were added at the beginning with the grignard reagent.

I know that in the presence of water you get:

Ph-MgBr + H2O >> PhH + Mg(OH)Br

So assuming the acid is H3O+ would you get:

Ph-MgBr + H3O+ >> PhH + Mg(OH2)Br ?

Thanks

Reply 1

username913907

10

Original post by DonnieBrasco

I have a question asking me to draw the mechanisms of Ph-MgBr (grignard reagent) with various carbonyl compounds followed by mild acid work up. This bit was fine but then it says what would happen if the acid were added at the beginning with the grignard reagent.

I know that in the presence of water you get:

Ph-MgBr + H2O >> PhH + Mg(OH)Br

So assuming the acid is H3O+ would you get:

Ph-MgBr + H3O+ >> PhH + Mg(OH2)Br ?

Thanks

Does it matter for undergrad? Surely the only important bit is the ph-H bit.

Reply 2

DonnieBrasco

OP

7

The question say what would happen when the acid is added with the reagent so I guess it does. Should I just say that the grignard reagent breaks down forming PhH and something else? I guess the important fact is that the reagent can no longer react with the carbonyls

Reply 3

Kyri

15

The magnesium will form an ionic interaction with the conjugate base of the acid you used. So if you used HCl you'll get MgClBr. You'll probably see this more easily if you write HCl instead of H3O+ as water can't be a counterion for Mg2+.

Grignard reagent with acid

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