hydration reaction

Meaning: A hydration reaction adds water across a C=C double bond to give an alcohol. With aqueous acid (H₃O⁺ or dilute H₂SO₄), the alkene is protonated at the less substituted carbon to form the more stable carbocation. Water attacks the carbocation, and loss of H⁺ gives the alcohol, with OH on the more substituted carbon (Markovnikov). Because a carbocation forms, hydride or methyl shifts can occur. The reaction is reversible; its reverse is acid-catalyzed dehydration. In biochemistry, fumarase hydrates fumarate to malate.

Equation: R₂C=CH₂ + H₂O ⇌ R₂C(OH)–CH₃ (H₃O⁺ catalyst; R is an alkyl group; the catalyst is regenerated, and no SI units apply).

On the MCAT: You may be asked to predict the major product from an alkene and aqueous acid, rank alkenes by rate (more substituted alkenes give more stable carbocations and usually react faster), or choose reagents for a target alcohol. Le Chatelier applies: excess water favors the alcohol, while heat and removal of water favor the alkene.

Common confusion: Acid-catalyzed hydration is Markovnikov, not anti-Markovnikov. Hydroboration–oxidation (BH₃, then H₂O₂/NaOH) gives the anti-Markovnikov alcohol by syn addition with no rearrangement. Oxymercuration–demercuration is Markovnikov without rearrangement.