While searching the Internet for information about the tricyclopropylmethyl carbocation, I found a site (askiitians.com) where it said that its stability was due to "dancing (superfast) resonance".

What is "dancing resonance"? How does it take place, and what are the prerequisites for it?


1 Answer 1


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.

When explained properly, resonance is a consequence of our unability to draw adequate depictions of what the bonding situation in the molecule truly looks like. For example, take a benzene ring. We can only draw single and double bonds. The structures should be identical whether the double bonds are in 1,3,5 or 2,4,6 positions. Our depictions predict different structures because double bonds are shorter than single ones. The actual benzene structure, however, has equal bond lengths between all carbons which could be described as 1.5-bonds. It is incorrect to think of the double bonds "resonating" between 1,3,5 and 2,4,6; rather, both depictions should be considered as two extreme pictures that by themselves cannot capture the truth.

This may be relatively easily understood in simple cases such as benzene or the cycloheptatrienyl cation, where the resonance depictions are all very similar. It is a lot harder to understand it in the case of a cation such as the tricyclopropylmethyl cation. There are a number of answers here explaining exactly which resonance forms stabilise it; it can and should also be seen as a nonclassical cation of the pentavalent carbonium type. You can, for example, check this question.

However, that does not mean that the resonance in these non-classical cations has a different nature. It is not correct to say that the resonance here is "faster" than in benzene, because resonance is simply not a oscillation or vibration between different structures: there is no "speed" or "frequency" to speak of.

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    $\begingroup$ To add a personal comment: I have never seen any reputable source use the phrase "dancing resonance". $\endgroup$
    – orthocresol
    May 29, 2021 at 14:56
  • $\begingroup$ There's also those people who draw a phenyl moiety as a hexagon with a ring inside it. Come to think of it, that might actually be the better way to do it, too. $\endgroup$
    – Antimon
    May 29, 2021 at 17:07

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