Resonance structures of aryl or vinyl carbocations

In summary, the resonance structures of vinyl and aryl carbocations are different because of the way the nitrogen is positioned. However, benzonitrile can still have an sp2 hybridization on its nitrogen because it has other atoms that balance out the hybridization.
  • #1
sgstudent
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3
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?
 
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  • #2
sgstudent said:
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?
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).
 
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  • #3
DrDu said:
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).

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?
 
  • #4
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.
 
  • #5
DrDu said:
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.

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?
 

What are resonance structures?

Resonance structures are different Lewis structures that can be drawn for a molecule, showing the different possible locations of electrons. They are used to explain delocalization of electrons and stability of molecules.

How do resonance structures affect the stability of aryl or vinyl carbocations?

Resonance structures play a crucial role in stabilizing aryl or vinyl carbocations by delocalizing the positive charge across multiple atoms. This results in a more stable overall structure compared to a single, localized positive charge.

What is the difference between aryl and vinyl carbocations?

Aryl carbocations have a positive charge on a carbon atom attached to an aromatic ring, while vinyl carbocations have a positive charge on a carbon atom attached to a double bond. This difference in bonding affects the stability and reactivity of these two types of carbocations.

How does the number of resonance structures affect the stability of a carbocation?

The more resonance structures a carbocation has, the more stable it is. This is because delocalizing the positive charge across multiple atoms distributes the charge and reduces the overall energy of the molecule.

What are some common methods for synthesizing aryl or vinyl carbocations?

Aryl or vinyl carbocations can be synthesized through various methods such as Friedel-Crafts alkylation, electrophilic aromatic substitution, and elimination reactions. These reactions involve the formation of a positively charged carbon atom, resulting in the formation of the desired carbocation.

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