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 ...
27
votes
Accepted
Which compound reacts faster in the Cannizzaro Reaction?
The mechanism of the Cannizzaro reaction is illustrated below.
The first step involves attack by the nucleophilic hydroxide ion on the positively polarized carbonyl carbon to form a tetrahedral ...
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 ...
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....
22
votes
Accepted
What are the correct resonance structures of nitrous oxide?
Firstly, neither of the resonance structures that you drew for your test are possible because they both violate the octet rule. For the structure on the left, the leftmost $\ce{N}$ is in control of ...
20
votes
Accepted
PBP vs TBP geometry?
General Rule #1: Most elements use only s and p orbitals to form bonds, only transition elements and heavier elements use d, f, etc. orbitals in bonding.
General Rule #2: The more s-character in a ...
19
votes
Accepted
Why are the C-C bonds in benzene closer in length to double bonds than single bonds?
one might expect that the bond lengths would be exactly intermediate between a single and a double bond
They actually are. The relevant reference structure, that can provide us lengths of single and ...
19
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 ...
18
votes
Accepted
Is nitrogen of aniline sp² or sp³ hybridized?
The nitrogen in aniline is somewhere between $\mathrm{sp^3}$ and $\mathrm{sp^2}$ hybridized, probably closer to the $\mathrm{sp^2}$ side. We are correctly taught that the nitrogen in simple aliphatic ...
18
votes
Why are the C-C bonds in benzene closer in length to double bonds than single bonds?
Why should they be bang in the middle? Or to better phrase the question: What reasoning are you applying to assume an averaged bond length?
Bonds don’t work as we laymen like to write them, with ...
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)
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 ...
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 ...
15
votes
What are the correct resonance structures of nitrous oxide?
To append Ringo's good answer and to add some more insight into the bonding situation, I performed a calculation on the DF-BP86/def2-SVP level of theory. Since this is a linear molecule, there are ...
15
votes
Accepted
Trying to understand the statement: 'Resonance is not a flickering between the contributing states. '
When it comes to resonance, you should not take the notion of superposition of different states literally. Feynman mentioned it explicitly right from the get go, just read carefully what he says:
...
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.
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 ...
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 ...
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 ...
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 ...
13
votes
Accepted
Why should 2,6-Dimethyl-4-nitrophenol be more acidic than 3,5-Dimethyl-4-nitrophenol
In the 3,5-dimethyl isomer, steric effects come into play and influence the acidity.
Resonance structures like B in the figure below, that stabilize the phenoxide anion explain why 4-nitrophenol ($\...
13
votes
What is a sigma complex?
Yes, it's anionic sigma complex, more precisely Meisenheimer complex. They are much more stable then arenium ions (Wheland intermediates) and can be isolated much more easily.
There was actually ...
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 ...
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, ...
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 ...
12
votes
Accepted
Difference between Resonance Effect and Mesomeric Effect
The IUPAC gold book (IUPAC are the organisation which 'define' standards for things such as chemical nomenclature) describes them as being essentially synonymous. The following definitions are given (...
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,...
11
votes
Is the aromaticity broken in some resonance structures of para-nitro-aminobenzene?
Your teacher is correct, some of the resonance structures in your figure are not aromatic. Still though, as resonance structures they contribute to the overall description of the molecule, just less ...
11
votes
Accepted
Why is it the middle ring of anthracene which reacts in a Diels–Alder?
To provide a reason for the observed regioselectivity, it is helpful to draw anthracene's aromatic π-electron system in alternance of single and double bonds. In this instance, it is more beneficial ...
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