Why is vinyl carbocation unstable?

A positively charged molecule, or cation, with the positive charge on a vinyl group (−CH=CH2) is called a vinyl cation.

Vinyl carbocations are sp hybridized. Linear geometry and sp hybridization are preferred for carbon that has two additional atoms attached to it.

Vinyl cations are found on sp hybridized carbons; because sp hybridized carbons have less p-character, they are not able to donate more electrons to the positive charge located in the empty p-orbital, destabilizing it. This difference in hybridization of the carbon bearing the positive charge causes the vinyl cations to be less stable.

Vinyl carbocations lack hyperconjugation due to the absence of adjacent C-H bonds, leading to increased instability. The conjugation in vinyl systems is weaker compared to allylic or benzylic systems, resulting in a higher energy state for vinyl carbocations.