Carbocations in general are stabilized by hyperconjugative interactions where po

Question

Carbocations in general are stabilized by hyperconjugative interactions where possible. The standard picture of this stabilization is illustrated below left (structure 1), with the empty p orbital mixing with an aligned sp3 hybrid C-H bond. The vinyl carbocation 2 presents a variant of this hyperconjugative interaction. Now, an sp2 hybrid is mixing with the empty p orbital. Hyperconjugative interaction 2 differs fro the standard model 1 in two important aspects: (1) The hybrid orbital used by the carbon of the C-H bond is different (sp3 in 1 vs. sp2 in 2), and (2) the angle between the C-H bond and the p orbital is different (~ 109 in 1 vs. ~ 120 in 2).

What is the relationship between bond angle and carbocation stability?

Explanation / Answer

Compare to sp3 orbitals, sp2 orbitals have less 'p' character. Therefore, the contribution of p-character to the positive charge on in the empty p-orbital of carbocation carbon is less.

Therefore, the sp2 hybrid orbital in vinyl carbocation is less stabilzes the positive charge.

The stability of carbocation dectrases ss the bond angle increases.

As bond angle increases, interaction of electrons of sigma bond of adjacent bond (C-H) with positive charge of carbocation decreases. So stability decreases.

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