Alkenes can be transformed into alcohols through hydration reactions. Two key methods are oxymercuration-demercuration and acid-catalyzed hydration, both following Markovnikov's rule for regioselectivity.

Oxymercuration-demercuration offers milder conditions and avoids rearrangements, while acid-catalyzed hydration is simpler but can cause rearrangements. Understanding these processes is crucial for synthesizing alcohols from alkenes effectively.

Hydration of Alkenes

Oxymercuration-demercuration process

• Two-step process converts alkenes to alcohols
  • Oxymercuration step
    • Alkene reacts with mercury(II) acetate (Hg(OAc)2) in water
    • Electrophilic addition of Hg-OH across the double bond forms a mercurinium ion intermediate
  • Demercuration step
    • Sodium borohydride (NaBH4) reduces the mercurinium ion
    • Breaks the mercury-carbon bond and replaces mercury with hydrogen
    • Yields the final alcohol product (ethanol, isopropanol)
• Stereochemistry: Results in overall anti-addition of H and OH groups

Markovnikov's rule in alkene hydration

• Predicts regioselectivity of alkene addition reactions based on carbocation stability
  • More stable carbocation intermediate forms preferentially
  • More highly substituted carbocations are more stable (tertiary > secondary > primary)
• In oxymercuration-demercuration, OH group attaches to the more substituted carbon of the alkene forming the Markovnikov alcohol product
• Example: 2-methylbut-2-ene undergoes oxymercuration-demercuration
  • OH adds to the tertiary carbon yielding 2-methyl-2-butanol

Acid-catalyzed vs oxymercuration-demercuration hydration

• Acid-catalyzed hydration
  • Alkene reacts with water and a strong acid catalyst (H2SO4)
  • Proceeds through a carbocation intermediate yielding Markovnikov alcohol product
  • Advantages
    1. Simple, one-step process
    2. Inexpensive reagents (water, sulfuric acid)
  • Limitations
    • Rearrangements can occur due to carbocation intermediate
    • Not suitable for acid-sensitive substrates (alcohols, amines)
• Oxymercuration-demercuration
  • Two-step process: oxymercuration followed by demercuration
  • Proceeds through a mercurinium ion intermediate yielding Markovnikov alcohol product
  • Advantages
    1. Milder conditions compared to acid-catalyzed hydration
    2. No rearrangements due to the more stable mercurinium ion intermediate
    3. Tolerates acid-sensitive substrates
  • Limitations
    • Two-step process is more time-consuming
    • Uses toxic mercury compounds requiring proper disposal

Reaction Considerations

• Solvent effects: Water acts as both solvent and nucleophile in the reaction
• Regioselectivity: Follows Markovnikov's rule due to the formation of the mercurinium ion intermediate
• Reaction kinetics: Rate-determining step is the initial formation of the mercurinium ion