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These are some of the most stable carbocations, but what is the relative stability order between them.

According to me it should be: C>D>B>A........ I came to this conclusion because both C and D have equal number of resonating structures i.e.7.However Bend orbital Bond seems to stabalise more than Resonance so C is assigned higher priority.

However, my teacher says that the order should be C>B>A>D........ and he gives priority to Bend orbital bond above anything else.

However, I seem to disagree with his answer. Unfortunately, I don't have the source of the question to verify the answer and couldn't find anything relatable on the internet.

So any help would be appreciated.

(Also, as a side question, which is the most stable carbocation ever isolated, is it tricyclopropylmethylcarbocation or tropyllium ion or any other. If anyone knows the answer might help)

(Found several other questions like mine on cyclopropyl metgy carbocation. However the answers either are using Molecular Orbital Theory Or some other concepts which I haven't learnt(I am still in school). So I wish for a more simplistic approach to the question. Also they are not able to explain the exact position of tropylium ion in the order)

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The answer is D>C>B>A.

This can be explained with the circumstance that the fourth structure (D) is an aromatic compound and therefore has extraordinary stability compared with standard carbocations. The mentioned property arises from: planarity, a circular structure and the fact that the structure fulfills the 4n+2 rule. That carbocation (sp2-hybridized C) is required in order to allow a circular electron flow

The remaining three carbocations are just stabilized by hyperconjugation, which rises with the number of alkylsubstituents bonded to the central carbon.

Here is a reference to the cyclopropenylium cation you mentioned: Breslow, R.; Yuan, C.; J. Am. Chem. Soc. 1958, 80, 5991-5994. DOI: 10.1021/ja01555a026

And a paper about stable carbocations: Journal of the American Chemical Society / 94:6 / March 22, 1972 (Here you can find the carbocation D from above again)

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  • $\begingroup$ I know D is a very stable carbocation due to aromaticity. However, the cyclopropylmethyl carbocation is exceptionally stable deu to its bend orbital bond i.e. a non conventional resonance(popularily called dancing resonance). However you dont seem to have accounted for it in your answer......... $\endgroup$
    – Arpit Kumar
    Commented Feb 11, 2019 at 11:36
  • $\begingroup$ en.m.wikipedia.org/wiki/Tropylium_cation $\endgroup$
    – Alchimista
    Commented Feb 11, 2019 at 11:45
  • $\begingroup$ Given the evidence in the linked post, I'd say this is not correct in its entirety. $\endgroup$
    – Martin - マーチン ♦
    Commented Feb 11, 2019 at 18:58
  • $\begingroup$ I inserted a link to the paper I could find, I don't have the time to look for the other 'reference'. I found two candidates be Olah et.al.; DOI: 10.1021/ja00761a041; 10.1021/ja00761a042 $\endgroup$
    – Martin - マーチン ♦
    Commented Feb 11, 2019 at 19:11

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