[kekie]

If so, why?

[Archer]

You need to do some work here.

What do you think?

What does SP2 mean?

[kekie]

?
Sp2 hybridization is when the 2s orbital and the 2p orbitals hybridize to form three sp2 orbitals. It leaves one p orbital untouched, so it can form a pi bond.

I saw an example where a carbocation was assumed to be Sp2 hybridized, and I was wondering if that was necessarily the case.

[Archer]

Think about the orbitals which you are describing. What is orbiting?

Draw out an SP2 hybridised carbon then draw a carbocation with its Orbitsl hybridisation What's the difference between the two?

What are the bond angles for sp3 and sp2 orbitals?

[kekie]

"with its Orbital hybridization" Whats orbital hybridization? Sp2? I don't really understand what you are asking.

Anyway, I already noticed that a Sp3 hybridized carbon with an electron missing (carbocation) looks suspiciously like an Sp2 hybridized carbon, but with no p orbital. In fact, it might have trigonal planar structure like Sp2 as well...
Is this what you're trying to get me to realize?

Edit:
And Sp2 carbocations look like sp hybridization.

As for the bond angles, I know off the top of my head Sp3 has a angle of 109.5, or something, and Sp2 is in a plane with all three prongs trying to be as far away from each other as possible, so it would have 360/3=120 degree bond angles.

[Archer]

Yes so your orbitals are your electrons, so in the absence of a p orbital, logically it should be sp2.

But what about aryl carbocations are they sp hybridised? and alkynyl carbocations?

[kekie]

Okay, so if you have one too few electrons (1+) then you can only go to Sp2, but with no double bond/p orbital.

Benzene rings? You'd be missing a pi bond.
Alkynes? They seem weird - they'd have to be terminal.

[Archer]

What about an alkenyl carbocation (not resonance stabilised) would this be SP hybridised? i.e. would you see a scrambling of geometry around the cationic carbon?

Terminal alkynyl carbocations are extremely unstable and quite rare. I have no idea what the hybridisation would be. Just S maybe? I am sure someone will shed some light on this for us.

[kekie]

What?

I would think that it would be sp hybridized - though that depends on weather the missing electron was from a sp hybridized orbital - as I have been assuming, one less sigma bond - or from a p orbital - one less pi bond.

The original issue was this - in a (mostly) conjugated heptane ring with a carbocation, (hence mostly) the carbocation is said to be sp2 hybridized, therefore having a p orbital, letting the electrons delocalize.

Then passes Hückel's rule, so it's aromatic, but that's irrelevant.

[kekie]

Oh, no wait, if you took a Sp2 hybridized carbon, and removed an electron, wouldn't that electron have to be one of the ones in a sp2 orbital, not the one in the p orbital? Then this would fix everything.

So, having a sp3 hybridized carbocation (Which is basically sp2 but with one less p orbital) would be higher energy than just having a sp2 hybridized carbocation (Which is basically sp but with one less p orbital).

...Maybe? Am I correct?

[Archer]

This guy explains carbocation stability quite well.

http://www.masterorganicchemistry.com/2011/03/11/3-factors-that-stabilize-carbocations/

[Dan]

The way I would look at this is to consider the relative energies of sp³ and sp² orbitals - which is higher?

Draw molecular orbital (MO) diagrams for the two cases:

sp³ hybridised (i.e. 4 x sp³ orbitals)
sp² hybridised (i.e. 3 x sp² orbitals + 1 x p orbital)

Now fill the orbitals with the 6 electrons on the carbocation and consider which configuration is more stable.

[kekie]

Sp3 is higher. So, is my post that starts with 'Oh, no wait' correct?

[Dan]

Sp3 is higher. So, is my post that starts with 'Oh, no wait' correct?

I'm not sure, I can't follow it - can you explain?

[TheOrganic]

The people who have replied before me have correctly, tried to explain why a carbocation should be sp2 hybridized and not sp3. But since you are still inclined to believe that a carbocation should be sp3, here is my try at teaching..

Consider the fact that s-orbitals are more concentrated around the nucleus than p-orbitals. If so, which one would be at a higher energy?

Also consider, non-hybridized orbitals are higher in energy than hybridized ones.

If so, which one would you prefer to keep empty: a hybrid orbital or a non-hybrid orbital?