Which H is more acidic?

[Papi_Chino]

Hey guys, so Mike mentioned that Hydrogen #1 is the most acidic, however I thought that H #2 was more acidic because of benzene's resonance. Does a benzene + Oxygen beat out having two Oxygens when it comes to resonance stabilizing?

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[themuffinman11]

Not 100% sure, but I think it's because the resonance structures are better with the carbonyl groups (putting a negative charge on a Oxygen, which is electronegative and doesn't mind the charge versus carbons, which isn't as great), also you break aromaticity with resonance going into the ring so those resonance structures I think would be less significant.

Also a general rule of thumb, a hydrogen between 2 ketones is very acidic (close to carboxylic acids)

 

[Steeezy]

I don't really know what the purpose of the carboanions are, but assuming they lower pkas we can still follow the rules of any handy pka table. The H-C between the two ketones has a pka of 9-10. The pka of a H-C that is attached to a benzene ring is around 40, and the pka of H-C that can donate to the carbonyl is 19-20. From the pka table, we can see that being in between two ketones would be more acidic than being in between a ketone and a benzene ring.

The more simple way to answer this is to remember that the electronegativity associated with oxygen generally leads to more acidic hydrogens when compared to any conjugated carbon system.

 

[zxe1996]

Resonating between two oxygens is more stable than resonating between a oxygen and a bunch of carbons (benzene) regardless of the fact that you would have more resonance structures with H #2

Classic example:

carboxylate vs phenolate

Phenolate has more resonance but carboxylate is more acidic because the anion is more stable resonating between two oxygens.

 

[Chubz0924]

Not sure if it always applies, but a way my prof taught me was that it is more acidic because it is between two EWG groups