Grignard reaction

[bigballer27]

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i know that the grignard reagent can act as a nucleophile and a base. but how do we know what it does?

ex: ROOH+R'MgBr --> ROOMgBr

where it acts as a base, rather than converting the carbonyl to an alcohol+R' as it would when it acts as a nucleophile


my book says, always consider the base properties of a grignard before nucleophile properties, especially in the case where you have an acidic proton

so does it hold true for (almost) all carb acids to act as a base?

 

[loveoforganic]

Protons are literally just a proton. They bounce around from molecule to molecule incredibly quickly, generally much more quickly than two molecules can have the chance to come together. For that reason, if there's an acidic proton, it will generally be removed by whatever stronger base is present before that stronger base acts as a nucleophile.

An exception would be grignards, in that they don't form enolates from ketones/aldehydes, but instead act as nucleophiles. Not sure of the exact reason. Probably becase that proton is not that acidic, and it's proximal to the electron deficient carbonyl.

 

[MLT2MT2DO]

Protons are literally just a proton. They bounce around from molecule to molecule incredibly quickly, generally much more quickly than two molecules can have the chance to come together. For that reason, if there's an acidic proton, it will generally be removed by whatever stronger base is present before that stronger base acts as a nucleophile.

An exception would be grignards, in that they don't form enolates from ketones/aldehydes, but instead act as nucleophiles. Not sure of the exact reason. Probably becase that proton is not that acidic, and it's proximal to the electron deficient carbonyl.

So the reaction that bigballer wrote can not actually take place, correct?

 

[bigballer27]

that is the exact reaction that takes place in one of my practice problems though?

it is: butanoic acid+ CH3MgBr --> (dont know the name) but 4 carbon chain CH3CH2CH2C(=O)-OMgBr, which indicates that it removed the proton from the butanoic acid

but it could have also formed 2 methyl 2-butanol as well, right?
or would that be not in this rxn but as a grignard acting on a ketone, or a 1-alcohol on an aldehyde

sorry, just test tomorrow and these little details are trippin me out!

 

[BerkReviewTeach]

that is the exact reaction that takes place in one of my practice problems though?

it is: butanoic acid+ CH3MgBr --> (dont know the name) but 4 carbon chain CH3CH2CH2C(=O)-OMgBr, which indicates that it removed the proton from the butanoic acid

but it could have also formed 2 methyl 2-butanol as well, right?
or would that be not in this rxn but as a grignard acting on a ketone, or a 1-alcohol on an aldehyde

sorry, just test tomorrow and these little details are trippin me out!

Acid-base reactions are faster than nucleophilic substitution reactions, so the Grignard reagent will first deporotonate the acidic hydrogen on the carboxylic acid group. Once that happens, the molecule becomes an anion and the carbonyl group is far less electrophilic, so any remaining Grignard reagent won't attack such a poor electrophile. You get exclusively deprotonation and an alkane side product.

 

[loveoforganic]

So the reaction that bigballer wrote can not actually take place, correct?

? The acid base reaction? That is what would take place

 

[MLT2MT2DO]

? The acid base reaction? That is what would take place

But the MgBr mainly just acts as a cation for stabilization, it doesn't itself become a Grignard. For all intense and purposes it is O- correct, it would just convert back to the COOH in a mild acid, correct?
Sorry I seem to be confusing myself now...

 

[bigballer27]

i think i got it.
keep it simple. R-MgBr would act as a base (faster rxn than nucleophilic attack) to abstract a hydrogen and participate in the reaction i wrote above

or it would act as a nucleophile, attacking the carbonyl carbon, creating a C attached to an OH and the R group from R-MgBr

 

[loveoforganic]

abstract a proton but yes. The product of the written reaction is just a carboxylate salt, which would be reprotonated in a moderate strength acid.