Acid Catalyzed Hydration of Alkenes

Introduction

• Reaction Type: Addition reaction
• Description: Adding water across a double bond in the presence of an acid catalyst.

Key Concepts

• Hydration: Adding water (H₂O) across a double bond.
• Catalyst: Helps the reaction occur without being consumed.
• Hydronium Ion (H₃O⁺): Generated by adding an acid (e.g., H₂SO₄) to water.
• Marconikoff Rule: Hydrogen atom attaches to the carbon with more hydrogen atoms.

Mechanism

1. Generation of Hydronium Ion: (H₃O⁺)
   • Water + Sulfuric Acid (H₂SO₄) → Hydronium Ion.
2. Addition of H⁺ to Alkene:
   • Alkene reacts with hydronium ion.
   • Formation of a secondary carbocation.
3. Attack by Water:
   • Water, as a weak nucleophile, attacks the carbocation.
   • Formation of oxonium ion (positively charged oxygen).
4. Proton Transfer: → Alcohol Formation
   • Water removes a proton from oxonium ion to form neutral alcohol.

Rate of Acid Catalyzed Hydration

• Effect of Substitution on Reaction Rate:
  • Tertiary carbocations are more stable and form faster than secondary or primary.
  • Reaction rate increases dramatically with increased substitution.

Equilibrium and Control

• Equilibrium Process:
  • Acid catalyzed hydration is reversible (hydration in forward direction, dehydration in reverse).
• Le Chatelier's Principle:
  • Dilute H₂SO₄: Favors alcohol formation.
  • Concentrated H₂SO₄: Favors alkene formation.
  • Control equilibrium by adjusting proportions of sulfuric acid and water.

Rearrangement and Stereochemistry Concerns

• Carbocation Rearrangement:
  • Intermediate carbocations can rearrange to form more stable products.
• Formation of Stereocenters:
  • Attack from different sides leads to mixtures of stereoisomers (e.g., R and S configurations).

Oxymercuration-Demercuration

• Objective: Avoid carbocation rearrangements.
• Procedure:
  1. Treat alkene with mercuric acetate and water.
  2. Follow with sodium borohydride.
• Outcome: Generates alcohol without rearrangement and follows Markovnikov's rule.

Hydroboration-Oxidation

• Anti-Markovnikov Addition: Regioselective addition to the less substituted carbon.
• Procedure:
  1. Treat alkene with borane (BH₃) in THF.
  2. Follow with hydrogen peroxide (H₂O₂) and base.
• Stereochemistry: Syn addition (both groups added on the same side).

Mechanistic Insights

• Borane Reaction:
  • Concerted mechanism: Addition of hydrogen and boron happens simultaneously.
  • No carbocation formation hence no rearrangement.
• Regioselectivity:
  • Electronic effects and sterics determine position of addition (anti-Markovnikov).

Conclusion

• Three Ways to Synthesize Alcohols from Alkenes:
  1. Acid Catalyzed Hydration: Mixture of products.
  2. Oxymercuration-Demercuration: Markovnikov product without rearrangement.
  3. Hydroboration-Oxidation: Anti-Markovnikov product with syn addition.

Study Tips

• Categorize Reactions by Functional Groups:
  • Make notes on different ways to synthesize alcohols from alkenes.
  • Add new reactions to this category as they are learned.