Vinyl cation

A vinyl cation is a positively charged molecule (a cation) where the positive charge is located on a vinyl group (−CH=CH₂).

There is debate on whether a vinyl cation intermediate forms with the addition of a halide (H-X) compound to a terminal alkyne for hydrohalogenation reactions. Alternatively, some believe that the addition of H and Br in this case is actually concerted.

In general, vinyl cations are less stable than alkyl cations due to the difference in hybridization of the carbon bearing the positive charge. Vinyl cations are located on sp carbons while alkyl cations are on sp² carbons. Since sp carbons have less p-character than sp² carbons, they are not able to donate more p-character to the positive charge located in the empty p-orbital, thus destabilizing it.

Conjugation is not an important stabilizing feature in vinyl cations since the pi cloud is perpendicular to the empty p-orbital. However, hyperconjugation can stabilize the positive charge-containing carbon. Consequentely, vinyl cation stability is affected by the degree of substitution. Secondary vinyl cations are more stable than primary vinyl cations. A secondary vinyl cation is 29+ kcal / mol more stable than a primary vinyl cation in the case of the ethenyl cation (CH₂=CH) vs propenyl cation (CH₂=C-CH₃).