Oxymercuration–Demercuration Reaction

Reagents:

  • Step 1: Hg(OAc)₂ / H₂O
  • Step 2: NaBH₄

Overall Reaction:

Alkene → Alcohol (Markovnikov addition, no rearrangement)

Example:

CH₃–CH=CH₂ → CH₃–CHOH–CH₃

(Propene → 2-propanol)

Key Features (VERY IMPORTANT)

1. Markovnikov Addition

  • –OH goes to more substituted carbon

2. NO Carbocation Rearrangement ❌

This is the most asked point in JEE

Unlike acid-catalyzed hydration:

  • No hydride shift
  • No methyl shift

3. Anti Addition (mostly)

  • OH and HgOAc add from opposite sides initially

4. Intermediate Formation

  • Forms mercurinium ion (cyclic intermediate)
  • More stable than carbocation → prevents rearrangement

Mechanism (Simplified)

Step 1: Formation of mercurinium ion

Alkene + Hg(OAc)₂ → cyclic intermediate

Step 2: Nucleophilic attack

H₂O attacks more substituted carbon

Step 3: Demercuration

NaBH₄ replaces Hg with H

Important Comparison (JEE trap)

ReactionRearrangementMarkovnikovIntermediate
Oxymercuration❌ No✔ YesMercurinium ion
Acid hydration✔ Yes✔ YesCarbocation

Special Case (VERY IMPORTANT)

With Alcohol instead of Water:

Hg(OAc)₂ / ROH → Ether formation

👉 Alkene → Ether (Markovnikov)

Tough Concept Question

Q: Why no rearrangement?

Answer:

Because intermediate is mercurinium ion (bridged), not free carbocation

Memory Trick:

Mercury locks structure”
→ No rearrangement possible

Exam Trap

Given:

3° carbocation possible after rearrangement

Leave a comment