Oxygen's Effect on Acidity: Electron Donation vs Inductive Effect

[cwpatter]

The addition of the highly electronegative oxygen to an acid will apparently make it more acidic, as it draws electron density away from the acid, making it more electrophilic.

HOWEVER, the addition of electron donating groups to an acid will actually make it WEAKER, as it will INCREASE the electron density. Many electron donators have use the lone pairs around oxygen to increase electron density, making the acid more satisfied and thus less electrophilic.

How do I sort out this paradox?

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[cwpatter]

The only explanation I can put together is that electron donators and electron withdrawers only work their magic when they are incorporated into a conjugated system, and themselves contribute or take away delocalized electrons. But this is conjecture put together by crappy source material.

BUT all the same, you'd still have the inductive effect of an oxygen being attached to the acid. It is still a paradox unless one of the effects: Inductive or Electron Donation, is much stronger than the other.

 

[brood910]

OH is electron donating only when it is attached to aromatic groups.
It's electron withdrawing when it's attached to alkyl groups due to its high electronegativity.
Thus, it increases acidity. It's not only when it's conjugated.
Think about formaldehyde vs acetaldehyde vs acetone. Which one is more acidic?

 

[cwpatter]

Ahhh, I see.

Assuming that all three of those groups are aromatic (having 2 PI electrons, and 4 LP 2+4 = 6 )

I'd say that the acetaladehyde is the most acidic, as the extra methyl group on acetone is electron donating, which would make it a weaker acid. Formaldehyde, if deprotanated, you wouldn't have a sigma bond between the positive charge and the SP2 carbon - so no resonance there to stabilize.

 

[brood910]

Ahhh, I see.

Assuming that all three of those groups are aromatic (having 2 PI electrons, and 4 LP 2+4 = 6 )

I'd say that the acetaladehyde is the most acidic, as the extra methyl group on acetone is electron donating, which would make it a weaker acid. Formaldehyde, if deprotanated, you wouldn't have a sigma bond between the positive charge and the SP2 carbon - so no resonance there to stabilize.

Yep 🙂

 

[cwpatter]

Even though I was right I'm still not 100% about it all, gotta let the info sink I I guess.

Thanks for the help though, appreciate it.