8.4 Hydration of Alkenes: Addition of H2O by Oxymercuration

Hydration of Alkenes

Oxymercuration-Demercuration Process

Oxymercuration-demercuration converts alkenes to Markovnikov alcohols in two steps, without the carbocation rearrangements that plague acid-catalyzed hydration. It's one of the most reliable ways to hydrate an alkene cleanly.

Step 1: Oxymercuration

1. The alkene attacks mercury(II) acetate, $\text{Hg(OAc)}_2$, which acts as the electrophile.
2. A three-membered mercurinium ion intermediate forms across the double bond. This bridged ion prevents the kind of carbocation rearrangements you'd see in acid-catalyzed hydration because the positive charge is stabilized by mercury, not sitting on a single carbon.
3. Water attacks the more substituted carbon of the mercurinium ion from the opposite face (anti attack), and subsequent loss of a proton gives an organomercury alcohol.

Step 2: Demercuration

1. Sodium borohydride ($\text{NaBH}_4$) reduces the carbon-mercury bond.
2. Mercury is replaced by hydrogen, yielding the final alcohol product.

Stereochemistry: The overall result is anti addition of H and OH across the double bond, since the nucleophilic water attacks from the face opposite the mercury.

Markovnikov's Rule in Alkene Hydration

Both oxymercuration and acid-catalyzed hydration follow Markovnikov's rule: the OH ends up on the more substituted carbon. The underlying reason is carbocation stability. More substituted carbocations are more stable (tertiary > secondary > primary), so the pathway that builds positive charge on the more substituted carbon is favored.

In oxymercuration specifically, the mercurinium ion is not a true carbocation, but it still has more positive character on the more substituted carbon. That's where the nucleophile (water) attacks.

Example: When 2-methylbut-2-ene undergoes oxymercuration-demercuration, water attacks the tertiary carbon. The product is 2-methyl-2-butanol, the Markovnikov alcohol.

Acid-Catalyzed vs. Oxymercuration-Demercuration Hydration

Both methods give the Markovnikov alcohol, but they differ in mechanism and practical trade-offs.

Acid-Catalyzed Hydration

• The alkene is protonated by a strong acid (typically $\text{H}_2\text{SO}_4$), forming a true carbocation intermediate. Water then attacks as the nucleophile.
• Advantages:
  1. Simple, one-step process
  2. Inexpensive reagents (water and sulfuric acid)
• Limitations:
  • Rearrangements can occur because the reaction goes through a free carbocation. Hydride and methyl shifts will rearrange the skeleton to form a more stable carbocation whenever possible.
  • Not suitable for acid-sensitive substrates
  • The reaction is reversible (equilibrium-controlled), so excess water or removal of product may be needed to push it forward

Oxymercuration-Demercuration

• Proceeds through a mercurinium ion intermediate rather than a free carbocation.
• Advantages:
  1. No rearrangements because the bridged mercurinium ion prevents carbocation shifts
  2. Milder conditions; tolerates acid-sensitive functional groups
  3. Reaction is not reversible in the same way, so yields are generally cleaner
• Limitations:
  • Requires two separate steps
  • Uses toxic mercury compounds that need careful disposal

Reaction Considerations

• Solvent: Water serves as both the solvent and the nucleophile in the oxymercuration step.
• Regioselectivity: Markovnikov, driven by the greater partial positive charge on the more substituted carbon of the mercurinium ion.
• Rate-determining step: Formation of the mercurinium ion is the slow step. Once that bridged intermediate forms, nucleophilic attack by water is relatively fast.
• Comparison to hydroboration: If you need the anti-Markovnikov alcohol, use hydroboration-oxidation instead. Oxymercuration and acid-catalyzed hydration both give only the Markovnikov product.