40 votes
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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 ...
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25 votes
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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 ...
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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 ...
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23 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....
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22 votes
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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 ...
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21 votes
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Can ozone have a triangular structure?

Here is the Walsh diagram depicting all the valence molecular orbitals (a diagram showing how individual molecular orbitals change in energy due to bending around the central atom). Oxygen has 6 ...
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20 votes
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Do lone pairs on substituents (e.g. in aniline) count towards Hückel's rule?

It is only the electrons from atoms in the ring that count when applying Huckel's rule. Electrons from substituents on the ring are only cross-conjugated with the aromatic π-system. When you think of ...
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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 ...
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18 votes
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Acidity of substituted phenols

I would appreciate learning whether such reasoning, for these two type of substituted phenols is plausible or whether furthermore solid arguments should be invoked. Your reasoning seems generally ...
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18 votes
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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 ...
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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 ...
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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)
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17 votes
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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 ...
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16 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 ...
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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 ...
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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 ...
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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 ...
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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.
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Is the phenoxide ion aromatic?

I agree absolutely with Max, and while I was still running the calculations, he already provided the answer. The goldbook provides a nice definition for aromatic compounds: In the traditional ...
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14 votes
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How accurate is this polar mechanism for the Diels-Alder reaction?

Disclaimer: This is not a full answer. I was just too long for a comment. I think the suggested mechanism is not entirely wrong, it might be a contributing factor of the whole process, but not a very ...
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14 votes
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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: ...
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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 ...
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14 votes
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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 ...
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13 votes
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Difference between backbonding and hyperconjugation and conjugation

OK, let's step through these terms and let's add "cross-conjugation" and "homoconjugation" for good measure to round out the series. Backbonding: This term is most often used to ...
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13 votes
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Rationalizing the Planarity of Formamide

Most of the amides are planar (due to steric reasons the restriction may be lifted) and so is also formamide. The carbon is obviously $\ce{sp^2}$ hybridised (as this concept is very well applicable ...
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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 ($\...
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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 ...
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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, ...
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13 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 ...
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