Carbocation on Benzene ring?

[RSD2014]

---

Members don't see this ad.

 

[meant2be10]

Where is this problem from?

 

[RSD2014]

Where is this problem from?

Not sure but destroyer and chad touched on the concept of carbocation on benzene ring. Destroyer says it is unstable, but didn't explain why. Chad said benzene carbocation is less stable than Primary carbocation, also did not say why. To start with, I don't even know what to call it.

 

[meant2be10]

Maybe that carbon with the + is no longer sp2, so the benzene ring is no longer aromatic, therefore not stable?

 

[RSD2014]

Maybe that carbon with the + is no longer sp2, so the benzene ring is no longer aromatic, therefore not stable?

I do understand that aromaticity is destroyed by having the positive charge on the benzene ring. But why is it less stable than primary carbocation?

 

[orgohacks]

I do understand that aromaticity is destroyed by having the positive charge on the benzene ring. But why is it less stable than primary carbocation?

It's called an aryl or phenyl carbocation. look closely - Aromaticity is *not* affected. The p orbital will be orthogonal (at right angles) to the p electrons of the ring - i.e. the empty p orbital will be where the hydrogen is.

It's more unstable than methyl because carbocations become less stable as you go from sp3 to sp2 to sp.
I would think that it will be more unstable than the vinyl carbocation drawn because there isn't a C-C bond to stabilize the carbocation through hyperconjugation, but it's subtle.

 

[RSD2014]

It's called an aryl or phenyl carbocation. look closely - Aromaticity is *not* affected. The p orbital will be orthogonal (at right angles) to the p electrons of the ring - i.e. the empty p orbital will be where the hydrogen is.

It's more unstable than methyl because carbocations become less stable as you go from sp3 to sp2 to sp.
I would think that it will be more unstable than the vinyl carbocation drawn because there isn't a C-C bond to stabilize the carbocation through hyperconjugation, but it's subtle.

Correct me if I'm wrong, there is no loss of aromaticity?

 

[newmexomfs]

its called arrenium ion/wheland intermediate
http://en.wikipedia.org/wiki/Arenium_ion

 

[RSD2014]

its called arrenium ion/wheland intermediate
http://en.wikipedia.org/wiki/Arenium_ion

 

[orgohacks]

Correct me if I'm wrong, there is no loss of aromaticity?

to answer the latter part first, yes, the alkyl group stabilizes the positive charge.

I think the sp2 vs. sp distinction is kind of semantic in this case. the geometry of the carbon atom (120 degrees) isn't changing.(if it was true sp it would be linear). It's just a situation where you have an empty p orbital in the plane of the ring. The other p orbital (the one involved in aromaticity) isn't affected.

 

[DQLEUCD]

the more the carbocation can resonate, the more stable it is because the charge can be delocalized with the other forms. so benzene is most stable then the alkene carbocation and then the methane carbocation.

 

[DQLEUCD]

The way you drew it, it is a stable carbocation since it still has a ring and it obeys the aromacity rule of 2n+2. I believe that it is still an sp2 carbon even with the loss of the electrons since it still has the double bond. If a nitrogen took it's place and lost it's pair of free electrons, it would not affect the aromicity of the compound.

 

[loveoforganic]

the more the carbocation can resonate, the more stable it is because the charge can be delocalized with the other forms. so benzene is most stable then the alkene carbocation and then the methane carbocation.

No resonance stabilization fir the carbocation here - the empty p orbital is in the plane of the ring, not above it

 

[Ibraiz]

Question is asking for Carbocation stability and Carbocation is most stable if its tertiary > secondary > primary > methlyl

Simply looking at the question we can come up with the answer III (Benzene b/c secondary carbon is carbocation) > II (Ethene b/c primary carbon is carbocation) > I (methyl b/c its methyl & the least stable carbocation)

OP, please advise what is the correct answer in the source where you got the question from?

 

[RSD2014]

Question is asking for Carbocation stability and Carbocation is most stable if its tertiary > secondary > primary > methlyl

Simply looking at the question we can come up with the answer III (Benzene b/c secondary carbon is carbocation) > II (Ethene b/c primary carbon is carbocation) > I (methyl b/c its methyl & the least stable carbocation)

OP, please advise what is the correct answer in the source where you got the question from?

I > II > III (this is not the same as Benzylic carbocation)
sp3 sp2 sp

 

[lex1489]

RSD, that makes sense. Where do 1*, 2*, 3* fall in with sp, sp2, and sp3?

 

[orgohacks]

RSD, that makes sense. Where do 1*, 2*, 3* fall in with sp, sp2, and sp3?

3° > 2° >1° > methyl > sp2 > sp

By the way, sp carbocations you will never, ever see.

 

[RSD2014]

RSD, that makes sense. Where do 1*, 2*, 3* fall in with sp, sp2, and sp3?

In the case with 1*, 2*, 3*, we look at the number of Alkyl group (EDG) that are donating e- to the positive charge to stabilize the carbocation.
More EDG (Electron-Donating Groups) more stable. Thus, 3* > 2* > 1*

 

[Ibraiz]

RDS, I can't recall reading/learning sp3>sp2>sp1 for carbocation. This is a new concept for me as far as carbocation is concerned. Can you please elaborate on this concept more and show us how please?