hydration

Meaning: Hydration is the addition of H and OH across a C=C bond, converting an alkene to an alcohol. In acid-catalyzed hydration, the alkene is protonated to form the more stable carbocation, water attacks it, and loss of H⁺ gives the alcohol. The OH therefore ends up on the more substituted carbon (Markovnikov). Because a carbocation forms, hydride or methyl shifts can occur, and a new stereocenter typically forms as a racemic mixture. A second sense: water adds to a carbonyl C=O to give a gem-diol, an equilibrium that favors the diol more for formaldehyde than for ketones.

Equation: RCH=CH₂ + H₂O ⇌ RCH(OH)CH₃, with H₃O⁺ as the catalyst; R is an alkyl group.

On the MCAT: You may be asked to predict the major product, draw the carbocation intermediate, or rank alkenes by rate (more substituted alkenes hydrate faster because they give more stable carbocations). Hydration is reversible: excess water favors the alcohol, while concentrated acid and heat favor dehydration back to the alkene.

Common confusion: Students mix up acid-catalyzed hydration with hydroboration-oxidation, which gives the anti-Markovnikov alcohol by syn addition with no rearrangement. Oxymercuration-demercuration gives the Markovnikov alcohol, also without rearrangement.