Which is more stable - benzylic carbocation or benzene - and why?

Benzylic carbocation may have more resonance structure, but these structures are destroying its aromaticity as well. So, how do we compare its stability with benzene?

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    $\begingroup$ Aromaticity lowers the energy of a benzene molecule by a huge amount, which is much larger than any resonance stabilisation possible in the carbocation. So basically, benzene is the stabler one. See here: quora.com/Why-is-the-phenyl-cation-unstable? $\endgroup$ – AbhigyanC Apr 21 '18 at 10:53
  • $\begingroup$ @AbhigyanC you linked to phenyl carbocation, but OP is talking about benzylic carbocation ;) $\endgroup$ – Gaurang Tandon Apr 21 '18 at 14:49
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    $\begingroup$ Comparing a stable molecule to a carbonation (although stabilised) is not the way to think. You might compare other carbonations the to the benzylic one, in these terms. $\endgroup$ – Alchimista Apr 21 '18 at 16:36
  • $\begingroup$ @GaurangTandon nevertheless it's obvious that benzene is more stable #aromaticity $\endgroup$ – Abcd Apr 21 '18 at 20:53

First refer to https://en.m.wikipedia.org/wiki/Chemical_stability to clean up the situation.

There is not much in comparing the stability of individual different molecules which are not among the reactants or the product of a specific reaction or in contact to a specific and same environment.

The question arises from a common confusion for we are used to compare the "stability" of different limiting forms (resonance structures ) of a hybrid of resonance.

Or, differently, but still analyzing entities with the same constitutional formula, the stability of various conformers.

An analysis in the terms meant by the question could be done to compare the stability of different carbocations, for instance, and see if there could be mesomeric effects able to delocalise the charge. In such a sense the presence of the phenyl ring has a stabilising effect as compared to, e.g., a linear alkyl chain.

This state, but anyway answer the question, the number of resonance structure does not even count as for all those of the benzyl cation are to be supposed much less stable than those of benzene, the carbonation is obviously charged so this will be the main difference anyway.

You won't ask what is more stable between methane and ethyl carbocation. The fact that in benzyl cation there is a phenyl ring can only tell you that its formation under a specific mechanism will be favoured as compared to the case of ethane.


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