protonating the carbonyl

[crazyasian]

Are there any rxns for the mcat that we need to know, where the Oxygen on the carbonyl is protonated before the Carbonyl carbon is attacked? Or is there a way I know when the Oxygen on the carbonyl needs to be protonated for a rxn to occur?

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[JFalc]

They may ask about some polymerization reaction that occurs through protonation of the carbonyl oxygen. Regardless, it's big with amines. Just be familiar with the reaction of carbonyls and primary amines in the presence of acid.

 

[orgohacks]

Are there any rxns for the mcat that we need to know, where the Oxygen on the carbonyl is protonated before the Carbonyl carbon is attacked? Or is there a way I know when the Oxygen on the carbonyl needs to be protonated for a rxn to occur?

Here's a few:

1) Transformation of acid to ester using acid and alcohol (Fischer esterification).
2) formation of acetals
3) hydrolysis of esters, amides, and nitriles

Also, imine formation sometimes can be drawn as proceeding first through carbonyl protonation, but it's not always necessary.
hope this helps - James

 

[Rabolisk]

Almost any carbonyl reaction on the MCAT that is catalyzed by acid, like those orgohacks listed, would proceed through carbonyl protonation.

 

[TwoPaddles]

Almost any carbonyl reaction on the MCAT that is catalyzed by acid, like those orgohacks listed, would proceed through carbonyl protonation.

True. Carboxylic acids and amides are also protonated at the carbonyl oxygen because there is resonance stabilization and you form a more stable product.

 

[Holderlin]

A carbonyl oxygen will always get protonated under acidic conditions.

The purpose for this in almost all cases is to make the carbonyl carbon more elecrophilic through resonance, or orbital mixing if you're applying a more complex model.

So whenever you see acidic conditions with a nucleophile and a carbonyl this is the mechanism through which the rxn proceeds.

 

[seep]

Remember also that the keto-enol tautomerism involves self-protonation of the carbonyl:

The bottom arrow should be longer as the ketone is far more stable than the enol.

 

[Holderlin]

Remember also that the keto-enol tautomerism involves self-protonation of the carbonyl:

The bottom arrow should be longer as the ketone is far more stable than the enol.

This is under basic conditions where you get alpha carbon deprotonation for rxns like aldol condensations.

*EDIT*

Forgot to mention that the alpha carbon hydrogens are more acidic because electron density on the alpha carbon is delocalized towards the carbonyl oxygen.

 

[nuD]

I wouldn't try to memorize specific reactions for this. Just know (as mensioned by some) that under acidic conditions, a carbonyl oxygen will be protonated making the carbonyl more susceptible to attack.

This isn't special to carbonyls. In general, the O (or N etc.) in compounds will be protonated under acidic conditions.

 

[seep]

Holderlin, they're tautomers. A bottle of acetone (for instance) will contain quintillions of the enol tautomer at any one time.

This tautomerism opens the door to many acid- and base-catalyzed reactions.