Acid-Catalyzed Hydration

Hydration of Alkenes

Alkene hydration converts an alkene into an alcohol by adding water across the carbon-carbon double bond. Acid-catalyzed hydration operates via a reversible equilibrium governed by Le Chatelier's principle: large amounts of water favor alcohol formation, while concentrated acid and heat favor alkene elimination.

1. Reagents and Equilibrium

The reaction requires an aqueous acid catalyst, typically dilute sulfuric acid ($\text{H}_2\text{SO}_4$) or aqueous hydronium ($\text{H}_3\text{O}^+$). Because the microscopic reverse of hydration is acid-catalyzed dehydration, the reaction is fully reversible. Synthetic utility depends on controlling solvent concentration.

2. Mechanism and Regiochemistry (Markovnikov Addition)

Acid-catalyzed hydration proceeds through a stepwise ionic mechanism that strictly follows Markovnikov's rule: the hydrogen atom adds to the less substituted carbon, placing the positive charge (carbocation) on the more substituted carbon.

1. Step 1 (Protonation): The pi electrons of the alkene act as a weak base, picking up a proton from $\text{H}_3\text{O}^+$ to form the more stable carbocation intermediate.
2. Step 2 (Nucleophilic Attack): A water molecule acts as a nucleophile, attacking the electrophilic carbocation center.
3. Step 3 (Deprotonation): Another water molecule removes a proton from the oxonium ion intermediate, yielding the neutral alcohol product and regenerating the acid catalyst.

3. Rearrangements: Carbocation Pitfalls

Because the intermediate is a discrete carbocation, rearrangements are common whenever a more stable carbocation can be formed. If a secondary carbocation is adjacent to a tertiary or quaternary carbon, a 1,2-hydride shift or 1,2-alkyl shift will occur rapidly before water traps the intermediate, leading to a rearranged major product.

4. Contrast: Oxymercuration-Demercuration

When you want Markovnikov regiochemistry without the headache of carbocation rearrangements, direct hydration fails. Instead, use oxymercuration-demercuration:

• Reagents: Step 1 uses mercuric acetate ($\text{Hg(OAc)}_2$) in aqueous THF, followed by step 2 reduction with sodium borohydride ($\text{NaBH}_4$).
• The Intermediate: Instead of an open carbocation, the reaction proceeds through a cyclic mercurinium ion. This three-membered ring shields the system from skeletal rearrangements while still allowing water to attack the more substituted carbon (due to partial positive charge buildup).
• Result: Clean Markovnikov addition of water with zero skeletal rearrangement.