40 votes
Accepted

What is resonance, and are resonance structures real?

This answer is intended to clear up some misconceptions about resonance which have come up many times on this site. Resonance is a part of valence bond theory which is used to describe delocalised ...
bon's user avatar
  • 15.4k
25 votes

Does benzene structure stand for a single resonance form or the whole molecule?

The answer is you are referring to neither of them. That is because resonance structures don't actually exist in reality. We only use them to give us a rough idea what the actually molecule and bonds ...
Nanoputian's user avatar
  • 7,372
24 votes

Do non-classical carbanions exist?

Interesting question. It is much less studied and reported on than the case of non-classical carbocations, but I did find a few papers. Brown and Occolowitz (Ref.1) reported that deuterated bicyclo[3....
Organic Chemistry Explained's user avatar
20 votes
Accepted

Stability Comparison between the Tropylium and Tricyclopropylcarbinyl Carbocation

It seems likely that the tricyclopropylcarbinyl carbocation would be more stable than the tropylium carbocation for the reasons I'll outline below, but if you have a reference proving this point, it ...
ron's user avatar
  • 84.7k
18 votes
Accepted

Do non-classical carbanions exist?

In addition to the species mentioned in the answer above, I found another one in Organic Chemistry by Morrison and Boyd(Seventh Edition)
Yusuf Hasan's user avatar
  • 3,196
16 votes

What is resonance, and are resonance structures real?

Bon has a very nice example of motion potentially restricted because of a barrier, and although it turns out that this is not the case for the norbornyl cation, there are simpler examples that do show ...
porphyrin's user avatar
  • 30.4k
16 votes
Accepted

Which carbons in pyrrole have the highest electron density?

Interesting question, more subtle than I realized! Theoretical approach: I would turn to resonance and induction as the rationale for electronic properties. I think all of your resonance forms may ...
electronpusher's user avatar
15 votes

Which is the most acidic hydrogen in vitamin C

The correct anion is stabilized by the mighty resonance. That's about it. Hydrogen bonding is irrelevant, since it affects both variants the same way.
Ivan Neretin's user avatar
  • 31.2k
14 votes

Why is meta-nitrophenol less acidic than para-nitrophenol?

The acidity of nitrophenols (or any acid for that matter) is determined by the stability of the conjugate base. In the case of m-nitrophenol and p-nitrophenol, the relative stability can be determined ...
bon's user avatar
  • 15.4k
14 votes
Accepted

What is the reason for the exceptional stability of tricyclopropyl-cyclopropenyl carbocation?

Questions like these always appear simple, when in fact they are not. First of all it has to be clear that stability is no absolute concept. In this case one could ask which of the cations is most ...
Martin - マーチン's user avatar
14 votes
Accepted

Which is the most acidic hydrogen in vitamin C

Compare the $\ce{OH}$ bonds in Vitamin C (ascorbic acid) and decide which one is the most acidic. The most acidic proton in ascorbic acid is the one whose conjugate base is most resonance ...
Chakravarthy Kalyan's user avatar
14 votes

What is the most "important" resonance structure of SCN⁻?

I performed a quick calculation on the DF-BP86/def2-SVP level of theory and analysed it with Natural Resonance Theory (from the Natural Bond Orbital Theory). This results in the following major ...
Martin - マーチン's user avatar
13 votes

Which carbons in pyrrole have the highest electron density?

That is really not a fair question and even with a computational approach the differences are more or less marginal. The approach by electron pusher is certainly the way to go, and here is the ...
Martin - マーチン's user avatar
13 votes

Why is carbonic acid a weaker acid than acetic acid?

Before you ask "why", ask "if". Carbonic acid is not weaker than acetic. In fact, it is stronger, just as one might expect by looking at that extra electron-withdrawing substituent. The trouble is, ...
Ivan Neretin's user avatar
  • 31.2k
12 votes
Accepted

Natural Bond Orbital analysis: Significance of stabilization energy determined by 2nd order perturbation

TL;DR: Lewis $\to$ Non-Lewis $\mathbf{E(2)}$ values have no direct physical significance, are intrinsically un-measurable, and serve only to quantify the extent to which the "real" wavefunction for a ...
hBy2Py's user avatar
  • 17.3k
12 votes
Accepted

How are the hybrid orbitals of sulfur hexafluoride shaped?

If you limit consideration to hybridization of atomic orbitals, a good reference to see is On the role of d orbitals in sulfur hexafluoride J. Am. Chem. Soc., 1986, 108 (13), pp 3586–3593. First, it ...
DavePhD's user avatar
  • 40.6k
12 votes
Accepted

Participation of Pi-bonds and lone pairs in resonance

You have probably misunderstood what the sentence actually means. It means that only one, either $\pi$ bond or lone pair will participate in resonance if the atom has both. For example, in pyridine,...
Iceberry's user avatar
  • 1,218
11 votes
Accepted

Which is the most acidic proton of (3Z)-3-ethylidenecyclopent-1-ene-1-carbaldehyde?

If you draw curly arrows for the deprotonations, it becomes clear that only the pathway in green is productive due to the ability to form an extended enolate. If you deprotonated the green protons, ...
NotEvans.'s user avatar
  • 17.1k
11 votes
Accepted

What is "dancing resonance"?

Resonance is not ‘dancing’. Resonance is often confused by undergraduates (and sometimes even professors) as some kind of oscillation back and forth between multiple structures. That is not the case. ...
Jan's user avatar
  • 68k
11 votes
Accepted

Why is the compound 3,4-dihydroxy-3-cyclobutene-1,2-dione more acidic than acetic acid?

This molecule is commonly known as Squaric acid. The Wikipedia entry gives the possible resonance structures of the conjugate base (added here for convenience): You can see that there are four ...
Don_S's user avatar
  • 1,410
11 votes
Accepted

Is steric inhibition of resonance or steric inhibition of protonation dominant in o-toluidine?

The reason for the decreased basicity is essentially the ortho-effect as outlined here: Ortho-effect in substituted aromatic acids and bases. Your question on the other hand asks about a seeming ...
Martin - マーチン's user avatar
11 votes
Accepted

Why hydrogen bonding in some acids make them a stronger acid when it is present even before deprotonation?

Some of the conformers of salicylic acid are given below, all of which are planar in configuration.$\mathrm{^{[1]}}$ Although the compound can adopt several conformations the first conformer(in the ...
Rishi's user avatar
  • 477
10 votes

Does benzene structure stand for a single resonance form or the whole molecule?

I've seen the following drawing used instead of the three double bonds. The circle signifies that the ring is aromatic. It's an alternative that is used to show that all bonds are the same.
MaxW's user avatar
  • 22.2k
10 votes

How are the hybrid orbitals of sulfur hexafluoride shaped?

Hybridisation is determined by geometry! This is the number one thing that you should learn from this answer. If you know the geometry of a molecule then you can work out the hybridisation of the ...
bon's user avatar
  • 15.4k
10 votes
Accepted

Which hydrogen of 3-methylcyclohex-2-en-1-one is the least acidic?

You are correct. Position 2 is the least acidic. Position 3 is not the least acidic hydrogen. It might be the most acidic hydrogen. Position 2 is the least acidic hydrogen of the four marked. What ...
Ben Norris's user avatar
  • 42.9k
10 votes

Stability of α-chlorocarbocations

As Oscar Lanzi suggested, both $+M$ and $-I$ applies here, but $\ce{Cl}$ stabilizes carbocation, meaning $+M$ is more effective than $-I$. This fact was confirmed by this peer-reviewed paper (Ref.1): ...
Mathew Mahindaratne's user avatar
9 votes
Accepted

Why are anti aromatic compounds unstable?

In conjugated-pi ring systems of N atoms, you typically have a single low energy orbital and then paired (degenerate) energy levels until you have N orbitals total. This has to do with the number of ...
R.M.'s user avatar
  • 4,884
9 votes
Accepted

Most stable resonating structure of 1-nitro-4-nitrosobenzene

The answer is not that difficult. You should have a certain number of rules to follow in determining the most stable resonance (not "resonating") structure. It probably starts with check all for ...
orthocresol's user avatar
9 votes
Accepted

Which of these carbocations are more stable?

As we know (to some extent), resonance in a molecule occurs due to sideways overlapping of p-π orbitals. Now in the molecule B, there is high steric hinderance near the carbocation due to which the ...
Vaibhav Dixit's user avatar

Only top scored, non community-wiki answers of a minimum length are eligible