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銀月遊俠 2012-12-22 05:38 PM

vinylic carbocation

In describing vinylic carbocation, we know the carbon bearing the positive charge is sp hybridised. It has two sp hybridised orbital to form two sigma bonds and one unhybridised p to from pi bond and one unhybrideised p orbital which the positive charge reside. Actually, in a normal carbocation, the positive charge also reside in a unhybridised p orbital. And I have seen some sources explaining like this the more s character of a orbital the less stable when positive charge is in there. But it is not the case that the +ve charge is on sp orbital and sp2 orbital for vinylic and normal carbocation respectively. So how to compare the stability of them in such way?
Does that mean the different hybridization of the carbon atom will affect the property of the unhybridised p orbitals which the +ve charge locate?

what is the resistance preventing this from happening?
[img]http://i1189.photobucket.com/albums/z428/AnthonyCChem/Capture-2_zps92e20028.png[/img]
I noticed there is change in hybridization in this rearrangement and the orbital alignment should be perfect for this to happen.

[[i] 本帖最後由 銀月遊俠 於 2012-12-22 06:58 PM 編輯 [/i]]

jmlo 2012-12-23 01:50 PM

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You are right that in general, the carbocation where the positive charge resides in an orbital possessing more s-character is less stable because of the increased electronegativity of the orbital.

In the present, however, you can't overlook other existing factors that may affect the overall stability of the carbocation. The structure on the left is a secondary carbocation (vinylic-type) while the one on the right is a primary carbocation (allylic-type). The stronger positive inductive effect makes the secondary carbocation more stable. You may argue that resonance may stabilize the primary carbocation, and it is true that allylic carbocation is exceptionally stable.

The proposed rearrangement is apparently not possible technically if you have a closer look on the stereochemistry of the vinyl cation. Since the vinylic carbon is sp-hybridized, the vacant p-orbital is perpendicular to the terminal C-CH3 bond. The C-H bonds are thus deviated from the p-orbital. There will be no effective overlap between the H 1s orbital and the vacant p-orbital on C, and therefore such proposed rearrangement may require a significant degree of structural deformation, and in turn a high activation energy.
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