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Resonance structures of aryl or vinyl carbocations

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  1. Nov 24, 2015 #1
    I read that vinyl or aryl carbocations do not have resonance stabilization but I can't understand why that is the case. Below I have drawn some resonance structures for aryl and vinyl carbocations:

    Aryl carbocation: https://imgur.com/QwiHJg8
    Vinyl carbocation: https://imgur.com/rKBs57d

    In both images, the bottom resonance structures results in the carbocation to change in its hybridization (sp to sp2) which seems like its the reason why the delocalization is not possible. However, in benzonitrile the nitrogen at the edge changes from an sp hybridization to an sp2 hybridization as well in the resonance structures it forms (https://qph.is.quoracdn.net/main-qimg-9c9c4891e3c59fe96adbc8caf19efc6a?convert_to_webp=true [Broken]). So why is it okay for benzonitrile to change its hybridization in this manner while this cannot happen for the vinyl/aryl carbocation?

    For the top images, why can't the electrons delocalize in that manner? Would it be because for that to happen, the electrons would have to delocalize into the other p orbital that is empty which is not parallel to the other p orbitals?
     
    Last edited by a moderator: May 7, 2017
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  3. Nov 24, 2015 #2

    DrDu

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    The N in Benzonitrile can be perfectly described using a sp hybridization in all the resonance structures present.
    In the case of vinylic or arylic carbocations, some of the resonance structures you draw are unreasonable if you take the proper geometry of the cation into account (which won't be linear in the case of a vinylic carbocation).
     
    Last edited by a moderator: May 7, 2017
  4. Nov 24, 2015 #3
    Wouldn't the nitrogen have a sp2 hybridization in all but the first resonance structure?

    Also, wouldn't this part of your reply "some of the resonance structures you draw are unreasonable if you take the proper geometry of the cation into account" be relevant for the bottom structures only where the hybridization of the carbon changes from sp to sp2? But even so why can't there be a change in the hybridization?

    Also, what about the top structures? Why can't those resonance structures exist?
     
  5. Nov 24, 2015 #4

    DrDu

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    Hybridization is not a measurable quality of a molecule but is - within certain bounds - free for you to choose. Hence the bonding in the benzonitrile is perfectly well describable with an sp-hybridization on N for all structures.
    I am not sure what you mean with the bottom structures and for which atom exactly you want to change hybridization.
     
  6. Nov 29, 2015 #5
    Oh I see. By the bottom structures I was referring to this: https://imgur.com/wOrj1Q5

    After much thought, it seems like these resonance structures are possible. There isn't any change in hybridization in any atom also. Is this possible though?
     
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