[lakealer96]

Hi!
The real question is: why a vinylic carbocation is less stable than an alkyl carbocation?
Everyone says it is because of the hybridization: in a vinylic carbocation, the positive carbon is sp hybridized; it means more s character, so the electrons are closer to the nucleus so it is less stable.
But why? Why should the fact that the electrons are closer to the nucleus make the carbocation instable?

I mean: okay, there is more s character, and the electrons are closer to the nucleus, but why does this make the vinylic carbocation less stable?

I hope you'll understand me, my English is not so good. Thanks!

[orgopete]

Since no one has replied, I'll offer two opinions on this topic.

For those wishing the answer, you should accept the rationale as an hypothesis to explain why a vinylic carbocation should be less stable (or harder to form). The facts are vinylic carbocations are less stable. That cannot be changed or rationalized differently.

For a second opinion, I'll offer that some parts of our theory are less than perfect. In this case, I do agree that the electrons of the 'so-called' sp-orbital are closer to the nucleus. The bond lengths are shorter. Hence, the force between the nucleus and the electrons should be greater. The theory making that prediction seems less than certain. A graph of the s and p-orbitals shows the p-orbital should actually be closer. You may find an earlier discussion of this in the forum beginning here: http://www.chemicalforums.com/index.php?topic=70089.msg253705#msg253705
(I'm not trying to collect flags for offering wrong ideas, I'm just offering my thoughts similar to the poster asking a question.)

[pgk]

Apart the separate atomic hybridization, there is also a bond hybridization.
What is the double bond hybridization, σ or π?
What is the influence of the carbocation to the double bond hybridization, in both cases?
Think about stabilization (or destabilization) of the overall hybridization, via conjugation, too.

[lakealer96]

@orgopete: thanks for the answer! I'm not sure I understood what you really meant. I know, hybridization is just a theory to explain something that could not be explained in other ways. Maybe hybridization is just an attempt to rationalize something we will never understand... But, even if they are just hypothesises, they must have a logic! Then,  if you say that the electrons closer to the nucleus destabilize the vinylic carbocation, you are supposed to explain why they make the carbocation not stable. They are closer to the nucleus so they destabilize the cation: but why? Yes, they are just hypothesises, but they should be rational hypotheses...

@pgk: thanks for the answer, but I can't understand. What do you mean when you say 'double bond hybridization'? I know a double bond consists of a π bond, made up by the two 2p orbitals in the vynilic carbocation.
Now, I know about the hyperconjugation ; I know that it can't occur in a vinylic carbocation... But my teacher said this was only one of the reasons why a vinylic carbocation is less stable ; he said the other reason is the fact that the electrons are closer to the nucleus, but the question is: why does this make the carbocation less stable?

Thanks!

[orgopete]

... if you say that the electrons closer to the nucleus destabilize the vinylic carbocation, you are supposed to explain why they make the carbocation not stable. They are closer to the nucleus so they destabilize the cation: but why?

I find that to be a correct explanation. You must think about the words. If the electrons are closer to the nucleus, the force between the electrons and the nucleus will be larger. It will be more difficult to form a carbocation. If we consider this effect on the carbocations, it will be better able to attract electrons to it. This should make it appear to be "less stable" or more reactive.

The difficulty that I found was how or why the electrons should be closer to the nucleus. The graph in the earlier link showed the 2s-orbital to be further from the nucleus than the 2p. The pi-hybridized orbital would also suggest an electronic repulsion to an addition of a pair of electrons to the carbocation.

I found it necessary to reject the hybridization model, but not for the same reason as the quantum chemistry purists who argue that there are no hybridized emissions/absorptions. We should all reject hybridized orbitals, but how can we explain the chemistry? Now, if you are a student, you just cannot make those arguments. It isn't in your textbook or lectures. I suggest you just accept what you are being told even if it may not seem entirely sensible.

[pgk]

Let’s take ethylene as a vinyl example:
The carbon atom in ethylene has sp2 hybridization. This means three hybrid orbitals that derive from 1 s and 2 p orbitals. Thus, the carbon atom has 4 free electrons that are distributed in these there hybrid orbitals.  The one hybrid orbital participates by donating one electron in the formation of σ molecular orbital of the C-C bond and the rest two hybrid orbitals participate in the formation of σ molecular orbital of the C-H bonds, by donating one electron each. The remaining electron of the p (degenerated) orbital participates in the formation of the π molecular orbital of the C=C double.
The carbocation has sp3 hybridization orbitals that derive from 1 s and 3 p orbitals but the one hybrid orbital is empty and has three electrons that are distributed in the rest two hybrid orbitals. The empty hybrid orbital can accept the electrons of the π molecular orbital. But this destroys the double bond and it is not permitted due to quantum mechanics rules, except if being excited by high amounts of energy.
Let’s take propene as allylic example:
The carbons of the double bond have sp2 hybridization and the alkyl carbon has sp3 hybridization (1 s + 3 p orbitals) and 4 free electrons distributed one per each hybrid orbitals that form 4 σ molecular orbitals, one C-C and three C-H bonds.
In case of allylic carbocation that is sp3, the empty hybrid orbital can accept the electrons of the π molecular orbital forming a new double bond and a new carbocation and vice-versa. Thus, the overall system is stabilized by conjugation.
If the compound is saturated, there are no neighbor π electrons that the empty hybrid orbital of the carbocation can accept and thus, the system is relatively stable (however, less stable than the conjugated system).
Sorry for the orbital mistakes, before. The issue is already complicated and I have errors in counting.
Sorry, again and hoping that have helped you