Reactions

Mechanistically, the reaction begins with protonation of the triple bond, generating a vinyl cation intermediate. For unsymmetrical alkynes, protonation occurs in a manner that places the positive charge on the more substituted carbon atom, leading to Markovnikov regioselectivity. Water then attacks the electrophilic vinyl cation, forming a protonated enol. Subsequent deprotonation yields the enol intermediate. Under the acidic reaction conditions, the enol rapidly undergoes keto-enol tautomerization.

It is important to note that, in practice, alkyne hydration is typically carried out using aqueous sulfuric acid in the presence of a mercury(II) catalyst (Need a refresher? Check the Alkyne oxymercuration sheet), as protonation of an alkyne alone is generally too slow due to the high instability of the resulting vinyl cation.