[nonlinear]

I have been studying acid-base, and I think that I get the general idea of how things work. My question arose from doing an example question that compared the strength of p-Nitrophenol versus m-Nitrophenol. Basically, it turns out that the electrons on the conjugate base of p-Nitrophenol can be delocalized onto one of the oxygens in the NO2 of the molecule, whereas in m-Nitrophenol, the resonance structures work out so that the electrons can't "get to" those oxygens and (this is just what I'm thinking) are limited to the hydrocarbon ring on m-Nitrophenol. The end result is that m-Nitrophenol is a weaker acid than p-Nitrophenol.

My question is, how "potent"/important is it to localize those electrons onto the oxygens in the NO2 in terms of affecting acid strength? I thought that it mostly mattered that the electrons could be spread out as widely as possible; although I know that generally it's "better" / more stable to have the negative charge on the more electronegative atom, I thought that the electrons would be happy simply to have more room in the molecule.

Can anybody help clarify this for me? Thank you!

[justgladtobeme]

Ok, I'm gunna take a crack at this.  I'm not a chemist by any means... just a biologist stuck in an organic lab.

whereas in m-Nitrophenol, the resonance structures work out so that the electrons can't "get to" those oxygens and (this is just what I'm thinking) are limited to the hydrocarbon ring on m-Nitrophenol.

I think you're pretty much on spot on with what you're thinking.  Since the electrons in m-Nitrophenol can't resonate to the oxygens in the electron-withdrawing group (NO₂), they're left mostly within the benzene ring.  Conversely, in p-Nitrophenol they can, and thus the electron density is localized on the oxygens in the nitro group.  (Thus, electron-withdrawing.)

To answer your question, acid strength (at least in the case of phenols) is directly related to the substituents.  You can think of it in two ways:

First, inductively. Electron-withdrawing groups like nitro pull electrons out of the benzene ring, and thus away from the oxygen in the hydroxy group.  This causes the hydrogen to be more easily donated as it has a "weaker" bond and this, of course, raises its acidity.

You can also look at it in the resonance of the phenoxide ion, like you did.

Take this situation one step further, and analyze o-Nitrophenol.  It falls between meta and para.  It, like para, is able to resonate the electrons to the nitro oxygens.  But in an inductive analysis, you can see the electron density is closer to the O-H hydroxy bond, thus "holding" the hydrogen better.

[nonlinear]

Hi, thanks for your *delete me*

A followup question:

So.. this means that any sort of resonance will be stabilizing, but if you can delocalize the electrons to a more electronegative atom such as oxygen, the effect will be much greater?

For o-Nitrophenol, the NO2 group is close enough to hydrogen bond with its own OH group (so that the OH would not want to give up it's H, I think?) I know H-bonds are pretty strong; does this overpower the inductive effect that the NO2 group has? Which effect is more "potent," and why?



And, one last (and only somewhat related) question:
I read that if an electronegative atom is too close to the hydrogen, its inductive effect will actually be destabilizing. Can anybody tell me why this is, and how close "too close" is? I've done several problems and I've been uncertain as to whether the inductive effect is close enough to be stabilizing or destabilizing

Thank you again!

[justgladtobeme]

So.. this means that any sort of resonance will be stabilizing, but if you can delocalize the electrons to a more electronegative atom such as oxygen, the effect will be much greater?

For the most part, yes.  And especially so in the case of determining an acidic proton.  If you haven't studied it yet, keto-enol tautomerism is an example where a resonance and delocalization of an negative charge isn't stable.  Also, in resonance structures, the structures with negative charges on electronegative atoms are more energetically favorable.

Your point about o-Nitrophenol having intramolecular hydrogen bonding is interesting.  It's not something I learned specifically, but what I can come up with is that the covalent bond being formed as it is losing the acidic proton is stronger than a hydrogen bond.  This fits because, as I'm sure you've learned, covalent bonds are far stronger than hydrogen bonds, as they actually have sharing of electrons rather than interactions of a dipole moments.  (Then again, I have no actual knowledge in this area, and my answer could be a crapshoot.)

As for your last question, I'm not sure what you're getting at.  Stabilizing/destabilizing in regards to acidity?  You might want to check out this link: http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/intro1.htm  What you're looking for will most likely be under reactions of substituted benzenes, I think... Any way, it's a great resource.

Hey, if you have experience in an organic lab, or understand solubility better than I do, would you mind checking out my post?  I'm stumped.  Thanks!

[spirochete]

Edit:  Half of this was aready said   I should read all the posts more carefully.

I was rather surprised when I looked up the pkas and saw that ortho-nitrophenol is slightly less acidic than para-nitrophenol.  Inductively I would always expect that the ortho adduct would be better able to delocalize the negative charge in the conjugate base, making the conjugate acid relatively stronger.

I have never heard anything about induction being less powerful at close distances.  In fact typically induction is much more stabilizing at close distances and falls off exponentially as you move the electronegative element additional sigma bonds away from the negative charge.  If you could give me a source on this statement I'd appreciate it.

Conceptually, you want to consider factors that either stabilize the acid or destabilize the conugate base:

1) Perhaps an intramolecular hydrogen bond stabilize the conugate acid of the ortho adduct.  This would make it less acidic.  

2) Perhaps the negative charge on the nitrogen group experiences some electron-electron repulsion with the anionic oxygen in the conjugate base that destabilizes it. It would be interesting to see if this trend also occurs in the ester substituted analogues in this situation.

Does anyone else have a convincing explanation?

[TSClimber]

1) Perhaps an intramolecular hydrogen bond stabilize the conugate acid of the ortho adduct.  This would make it less acidic.  

2) Perhaps the negative charge on the nitrogen group experiences some electron-electron repulsion with the anionic oxygen in the conjugate base that destabilizes it. It would be interesting to see if this trend also occurs in the ester substituted analogues in this situation.

Does anyone else have a convincing explanation?

I agree with you on this. Especially the intramolecular hydrogen bond theory since that would raise the pKa. I don't have any references to back you up though!.