Final answer:
HX acids like HCl are not used to catalyze alkene hydration because they dissociate in water, rendering them ineffective as catalysts. Strong acids like sulfuric acid or phosphoric acid are used instead because their conjugate bases are poor nucleophiles, facilitating the hydration process.
Step-by-step explanation:
The reason why HX, such as HCl, cannot be used to catalyze the hydration of an alkene is that gases like HCl react chemically with water. When HCl reacts with water, it dissociates into H+ (aq) and Cl- (aq), thereby not remaining as dissolved HCl molecules which can act as catalysts. For the hydration to occur, a strong acid like sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄) is used because these acids have conjugate bases (HSO₄- for sulfuric acid) that are poor nucleophiles and do not readily react with the alkene. These acids are strong enough to protonate the alkene, leading to a more reactive carbocation intermediate. The abundant water then acts as a nucleophile to attack the carbocation and complete the hydration process.