Hydration of alkynes follows Markovnikov's rule. Water adds across the triple bond of acetylene (HC≡CH): H adds to one carbon and OH adds to the other, initially forming vinyl alcohol (CH₂=CH–OH), an enol. However, enols are thermodynamically unstable and spontaneously undergo keto–enol tautomerism. The enol CH₂=CHOH tautomerises to the more stable keto form, acetaldehyde (CH₃CHO), by migration of the OH proton to the adjacent sp² carbon. This acid-catalysed tautomerism is rapid and essentially irreversible under the reaction conditions. Therefore, the final product is acetaldehyde (ethanal), not acetic acid (which would require further oxidation).