[furaniki18]

my teacher gave us this particular order of stability of carbocations but did not explain why it is so...i am not able to think of any possible effect that could be stronger than resonance...has it got something to do with ring expansion? i am just confused can somebody help..

[CaverKat]

Hmm.  I would have thought the Ph₃C⁺ carbocation to be the most stable (isolatable as well, I think).

Order looks like it's decreasing, then increased then decreasing again.

Ah.  I think it's two separate orders.

The first describes the decreasing stability as you go from tertiary to primary substituted (inductive effects).

The second list is decreasing stability of conjugated carbocations, with cyclic being the highest (not sure why, but cyclic things tend to be happier).  The decreasing as you go from Ph₃ to ₂ to Ph, is definitely the reduced number of resonance forms - decreasing delocalisation, stabilisation.

[furaniki18]

no it's one complete list...i can say this for sure because in many questions in my assignment i had to choose between a cyclopropyl substituted carbocation and the one substituted with phenyl and answers to all of those ques. indicate that cyclopropyl substituted carbocation has greater stability than the one substituted with phenyl...

[CaverKat]

Ok.  I'm stumped.  Sorry 

[Telamond]

This is similiar to your other question, which is also about carbocation stability. Guess your professor has gone through hyperconjugation and all that stuff?

Can you rougly sketch how the carbocation is stabilized in the first structure (with three isopropyl groups) vs. for an example a tertiary carbocation which is stabilized by three sigma C-H bonds?

[furaniki18]

no i cannot...i'd really appreciate if you could help..

[Telamond]

This is how they stabilize the carbocations. Also the steric strains from these are excellent as they "point" the orbitals in the right direction. Compare this to how it'd look like if the cyclopropyl group was a cyclohexyl or two alkyl chains. The most stable conformation wouldn't have been like the one I drew.

[furaniki18]

i'm getting what you are trying to explain..thanks ...do you have any link where i can read about this thing in detail?

[Telamond]

Sorry, I remember this from a lecture hand-outs, not from a coursebook.