Oxymercuration of Alkenes with Hg(OAc)₂, H₂O, and NaBH₄

Oxymercuration–demercuration hydrates alkenes in two distinct steps. First, Hg(OAc)₂ in water (often THF/H₂O) activates the π bond to give a three-membered mercurinium ion that directs Markovnikov attack by water. After acetate (or solvent) deprotonation, the alkoxymercury intermediate forms. Second, NaBH₄/NaOH (aq) reduces the C–Hg bond to C–H, expelling Hg(0) and revealing the Markovnikov alcohol. Because the pathway proceeds through a bridged intermediate rather than a free carbocation, rearrangements are suppressed and acid-mediated side pathways are minimized. Compare this hydration with acid-catalyzed hydration, alkoxymercuration–demercuration for Markovnikov ethers, hydroboration–oxidation for anti-Markovnikov alcohols, and bromohydrin formation for another bridged-ion anti opening.

Introduction

Hg(OAc)₂ coordinates to the alkene and expels one acetate as AcO⁻, yielding a three-membered mercurinium ion in which HgOAc remains ligated. The more substituted carbon of the bridge bears greater positive character (Cβ), establishing the Markovnikov site for nucleophilic attack. Water then opens the bridge anti to Hg, generating an oxonium that is deprotonated by acetate or solvent base. Finally, NaBH₄/NaOH (aq) reduces the C–Hg bond to C–H and releases Hg(0). The closed-shell sequence avoids free carbocations, so rearrangements do not compete and syn/anti relationships from the original alkene are largely lost after reduction.

Quick Summary

• Reagents: Step 1 Hg(OAc)₂ in H₂O (often THF/H₂O); Step 2 NaBH₄/NaOH (aq).
• Outcome: Markovnikov hydration (OH on the more substituted carbon).
• Mechanism: Mercurinium formation → H₂O anti attack → deprotonation → NaBH₄ demercuration (C–Hg → C–H).
• Rearrangements: None; no free carbocation.
• Stereochemistry: Bridge opening is anti to Hg; the NaBH₄ reduction is not stereospecific, so new stereocenters are formed as racemic mixtures.
• Pitfalls: Do not draw carbocation shifts or NaBH₄ adding across the π bond; depict demercuration as reduction at mercury, not hydride SN2 at carbon.

Mechanism (Oxymercuration–Demercuration)

The alkene π bond donates into Hg²⁺, expelling one acetate to AcO⁻ and leaving HgOAc bound across both carbons. The more substituted carbon (Cβ) stabilizes positive character and sets the Markovnikov site for attack.

Water approaches from the backside of the bridge and bonds to Cβ in an SN2-like fashion. The Hg–C bond to Cβ opens, and the electrons return to Hg, giving an oxonium adjacent to Cα–HgOAc.

The acetate anion generated in Step 1—or water in the medium—deprotonates the oxonium, producing the neutral alkoxymercury intermediate tethered to HgOAc.

NaBH₄ transfers hydride to Hg while acetate coordinates to boron and the Hg–O bond opens. The C–Hg bond collapses to a C–H bond, liberating Hg(0). The reduction does not enforce a particular configuration at the carbon formerly bound to Hg.

The neutral alcohol retains the carbon skeleton of the starting alkene and shows no rearrangement; stereochemical information at the reduced carbon is typically lost (racemic when a new stereocenter forms).

Mechanistic Checklist (Exam Focus)

• Draw the alkene → Hg²⁺ donation and show AcO⁻ leaving while HgOAc remains bridging.
• Label Cα/Cβ and indicate greater positive character at Cβ.
• Show water attacking anti to Hg at Cβ and the Hg–C bond electrons returning to Hg.
• Include deprotonation of the oxonium by AcO⁻ (or solvent).
• Depict NaBH₄/NaOH (aq) as reducing C–Hg to C–H, with Hg(0) expelled—no hydride SN2 at carbon.
• Note explicitly that rearrangements do not occur and the final stereochemistry is not controlled after reduction.

Worked Example — Hydration of 1-Methylcyclohexene

• Substrate: 1-methylcyclohexene.
• Reagents: Step 1 Hg(OAc)₂, H₂O (THF/H₂O); Step 2 NaBH₄/NaOH (aq).
• Pathway: Mercurinium formation → anti attack of H₂O at the tertiary ring carbon → acetate deprotonation → NaBH₄ reduction of C–Hg.
• Outcome: 1-methylcyclohexan-1-ol (Markovnikov) with no rearrangement; the new stereocenter forms as a racemic mixture.

When Multiple Alkenes Are Present

Oxymercuration targets the double bond that forms the more stabilized mercurinium ion and can be opened anti by water. Favorable sites are more substituted, allylic, or benzylic alkenes with less steric hindrance on the backside approach. Analyze each C=C bond by its ability to accommodate positive character in the bridge and by accessibility to the nucleophile.

Practical Tips & Pitfalls

• Run the sequence in two steps. Complete the Hg(OAc)₂/H₂O stage before introducing NaBH₄/NaOH (aq); premixing destroys the mercury reagent.
• Solvent choice. THF/H₂O solubilizes both components; avoid strongly acidic media that reopen carbocation pathways.
• Temperature. 0–25 °C maintains control and suppresses side reactions.
• Variants. Replacing H₂O with ROH furnishes alkoxymercuration–demercuration for Markovnikov ethers without rearrangements.
• Waste handling. Treat all mercury-containing wastes as hazardous and dispose of them according to institutional protocols.
• Stereochemistry. Always show anti opening in Step 2; expect loss of stereochemical information after NaBH₄ reduction.

Exam-Style Summary

• Regiochemistry: Markovnikov hydration (OH at the more substituted carbon).
• Mechanism: Mercurinium formation → anti water attack → deprotonation → NaBH₄ demercuration.
• Intermediates: Mercurinium and alkoxymercury; no free carbocation, so no rearrangements.
• Stereochemistry: Anti opening; overall stereochemistry not fixed after reduction (racemic when new stereocenters form).
• Competing pathways: Acid-catalyzed hydration may rearrange; oxymercuration avoids those issues.

Interactive Toolbox

• Model oxymercuration in the Reaction Solver and contrast outcomes with acid hydration or hydroboration.
• Use the Mechanism Solver to rehearse each curved-arrow step.

FAQ / Exam Notes

• Why is the product Markovnikov? Cβ bears greater positive character in the mercurinium, so nucleophilic water attacks there.
• Do rearrangements occur? No—there is no free carbocation, so 1,2-shifts are suppressed.
• What does NaBH₄ do here? It reduces the C–Hg bond to C–H, ejecting Hg as Hg(0); it does not add across the π bond.
• Is the final product stereospecific? No. The anti opening is stereospecific, but NaBH₄ reduction scrambles the configuration at the carbon that bore Hg, giving racemic mixtures when stereocenters arise.