Acid-Catalyzed Hydration of Alkenes
Acid-Catalyzed Hydration of Alkenes: Addition of H₃O⁺ turns an alkene into an alcohol. Step-by-step mechanism, reagents and practice.
What happens
Addition of H₃O⁺ turns an alkene into an alcohol. The C=C double bond breaks, and H and OH are added across the double bond. The reaction usually follows Markovnikov's rule: H goes to the carbon that already has more hydrogens, while OH goes to the more substituted carbon.
Reaction summary
1-Methylcyclohexene + H₃O⁺ → 1-Methylcyclohexanol (Markovnikov addition)
Reagents and conditions
- Starting material: 1-Methylcyclohexene (alkene)
- Reagents: H₃O⁺
- Products: 1-Methylcyclohexanol (Markovnikov product)
Mechanism
- Protonation: The alkene's π bond attacks H⁺ from H₃O⁺, forming a carbocation.
- Water Attacks the Carbocation: A water molecule's lone pair attacks the carbocation, forming the new C–O bond.
- Deprotonation: A proton is removed from the oxonium intermediate by another water molecule, giving the neutral alcohol.
- Product: The neutral alcohol, 1-methylcyclohexanol.
Selectivity
- Regioselectivity: Markovnikov: H goes to the carbon that already has more hydrogens, while OH goes to the more substituted carbon.
- Stereoselectivity: Not stereospecific: the carbocation is planar, so water can attack from either side. If a new stereocenter forms, a mixture of stereoisomers is expected.
Common mistakes
- Because a carbocation forms, rearrangements can happen, a hydride or alkyl shift may occur if it gives a more stable cation. Don't assume the product is always unrearranged.
Memory trick
Hydration = add water 💧. Across the C=C: H + OH. And remember: OH wants the more substituted carbon.