Why are vinylic and arylic carbocations highly unstable?

Question

I have checked the internet and read quite a few books, but I still am not able to understand why vinylic and arylic carbocations are highly unstable.     What I found while surfing the internet is: For arylic carbocation, the carbon bearing postive charge cannot participate in resonance with adjacent carbons because it's p orbital is perpendicular to the other orbitals. But, I didn't really understand what this means.

Answer 1

When you have a regular carbocation, say in C-2 of butane, C-2 is Sp3 hybridized. That means all of its orbitals as Sp3 orbitals. In this case, every orbital has 25% S characteristic and 75% P characteristic. In general, the S orbital is more stable, meaning electrons WANT to be in this orbital. This is another way of saying atoms that exhibit a high S orbital characteristic are more electronegative than those that have more P characteristic, because P orbitals don't attract electrons as much, because P orbitals are higher in energy.

Vinylic and allylic carbocations are Sp or Sp2 hybridized, meaning they have more S characteristic than Sp3 carbons. Having a positive charge on an electronegative atom is unstable, so the more S characteristic you have (and therefore the more electronegative your atom), the more unstable the resulting carbocation you will get.