Why Does Grignard Addition to Esters Form Ketones?

[duchuy]

Homework Statement

x

Relevant Equations

2 mechanisms proposed below

Hi,
I have a question regarding these 2 possibilities: Why does the addition of RMgX on an ester always form a ketone? We could very well have that tetrahedral intermediate without eliminating the alcoolate group no? Since your reaction pretty much always tend towards the formation of a weaker base, the second reaction goes from carbanion like molecule to a neutral molecule, whilst the first textbook way would form an alcoolate?
Please explain to me what is the motor for the 1st reaction so that the reaction would preferably go that way instead of the second one.
Thank you so much for your help.

 

[chemisttree]

For the mechanism you have proposed for #2… this is a hemiketal. It is not a happy camper. You can force it to become a ketal (two -OR groups) under desiccating conditions and in the presence of more alcohol but you have just finished adding water and there is little free alcohol around! Hemiketals decompose back to their ketone origins in the presence of water. Actually there is an equilibrium between the ketone/alcohol and hemiketal but it is heavily weighted toward the ketone/alcohol.

#1 is correct. Don’t forget that the MgBr⁺ is nearby to assist the alkoxide leaving. So the normally poor OR^- leaving group has some help both from the soon-to-be carbonyl oxygen’s lone pair and the metal salt.

 

[duchuy]

For the mechanism you have proposed for #2… this is a hemiketal. It is not a happy camper. You can force it to become a ketal (two -OR groups) under desiccating conditions and in the presence of more alcohol but you have just finished adding water and there is little free alcohol around! Hemiketals decompose back to their ketone origins in the presence of water. Actually there is an equilibrium between the ketone/alcohol and hemiketal but it is heavily weighted toward the ketone/alcohol.

#1 is correct. Don’t forget that the MgBr⁺ is nearby to assist the alkoxide leaving. So the normally poor OR^- leaving group has some help both from the soon-to-be carbonyl oxygen’s lone pair and the metal salt.

But if hemiketal turns back to ketone after I add water, and even ifI have alcohol in the solution, i'd no longer be able to turn that ketone into an alcohol no? Because all of my RMgX will be consumed from the moment I add water in the solution? So is it possible to end up with ketones as side products at the end of the reaction?

 

[chemisttree]

Yes, ketone is possible if one equivalent of grignard is slowly added to the ester. Done the other way around, you get a mixture of the tert alcohol and unchanged ester.

 

[Mayhem]

Yes, ketone is possible if one equivalent of grignard is slowly added to the ester. Done the other way around, you get a mixture of the tert alcohol and unchanged ester.

Something to consider: excessive dilution with the solvent will also likely improve yield for this method.

 

[Mayhem]

Also: ketones are considered more reactive than esters in this context. "The ketone is a more reactive intermediate than the ester itself". Hence, it is favorable for the Grignard reagent to attack the ketone over the ester.