[LunaHart]

Hi,

I could really use some help with the following: which of these compounds is the most stable/has the lowest energy?

At first I thought that it's compound number 3 because oxygen is far more electronegative than nitrogen and thus makes a better acid (lower energy, more stable). But then I looked at the functional groups as a whole and the -NO2 group is deactivating, it withdraws electrons better than the -OEt, which would make compound number 4 a better acid. I am so confused. I am fairly certain that resonance structures play a part in it as well but my mind simply refuses to understand.

All help is appreciated!

[kamiyu]

Compound 4 is more stable. You mention oxygen is more electron-withdrawing than nitrogen is true, but don't forget the nitrogen on the nitro group is +ve charged.

Indeed, nitro is one of the most powerful electron-withdrawing groups.

[LunaHart]

Compound 4 is more stable. You mention oxygen is more electron-withdrawing than nitrogen is true, but don't forget the nitrogen on the nitro group is +ve charged.

Indeed, nitro is one of the most powerful electron-withdrawing groups.

Thank you, but I'm still a little confused. How does the nitro group's positive charge affect the stability?

[kamiyu]

Indeed, this question is about the acidity of phenols which is largely affected by the stability of the conjugate base.

The conjugate base is a phenoxide and if the negative charge on the oxygen can be stabilized by electron-withdrawing group (to decrease the charge density) then it will be stable.

Because the nitrogen atom on nitro group is +ve charged, it strongly electron withdrawing and thus the phenoxide is made stable

BTW, ethoxide group (i.e. OEt) is electron donating. Sometimes, you cannot judge only based on the electronegativity of the atoms.

[AWK]

These structures are not compounds - these are anions. And these are Bronsted bases.
For me - more stable anion means more basic one. Hence the question is: which of these anions is the stronger Bronsted base (or the weaker acid).

which would make compound number 4 a better acid

- you mean parent compound?
K_a·K_b=K_w
stronger acid means weaker base

[NewmanProj]

AWK, the more stable conjugate base will be the weaker one. Think of it this way, what would be more likely to pick up a proton if one were available, 3 or 4?

4 is more stable due to the internal electron sink (to a small degree) but more so due to the extra resonance structures. More resonance = more stability.