Why Does Grignard Addition to Esters Form Ketones?

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In summary: Something to consider: excessive dilution with the solvent will also likely improve yield for this method.In summary, the first textbook way would form an alcoolate and the second mechanism proposed by you would form a ketone.
  • #1
duchuy
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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.

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  • #2
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.
 
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  • #3
chemisttree said:
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?
 
  • #4
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.
 
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  • #5
chemisttree said:
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.
 
  • #6
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.

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FAQ: Why Does Grignard Addition to Esters Form Ketones?

What is the purpose of adding Grignard reagents to esters?

The addition of Grignard reagents to esters allows for the formation of new carbon-carbon bonds, resulting in the synthesis of more complex organic molecules. This reaction is commonly used in organic chemistry to create alcohols, ketones, and other functional groups.

What are the general steps of performing an addition of Grignard on an ester?

The first step is to prepare the Grignard reagent by reacting an alkyl or aryl halide with magnesium metal. The resulting Grignard reagent is then added to the ester in a solvent, typically anhydrous ether. The reaction is then allowed to proceed at a low temperature, followed by quenching with an acid or water to protonate the intermediate and form the desired product.

What precautions should be taken when working with Grignard reagents?

Grignard reagents are highly reactive and can react violently with water and other protic solvents. Therefore, it is important to work with them under anhydrous conditions and handle them with caution. They should also be stored under an inert atmosphere, such as nitrogen or argon, to prevent them from reacting with air.

What factors can affect the yield of the Grignard addition reaction?

The yield of the Grignard addition reaction can be affected by several factors, including the reactivity of the Grignard reagent, the nature of the ester, the solvent used, and the reaction conditions. Additionally, the presence of impurities or side reactions can also impact the yield.

Can Grignard reagents be used to add to other functional groups besides esters?

Yes, Grignard reagents can also be used to add to other functional groups, such as carbonyls, nitriles, and halides. The specific reaction conditions may need to be adjusted depending on the functional group being targeted, but the overall mechanism is similar to that of adding to esters.

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