Alkoxymercuration of Alkenes with Hg(OAc)2, ROH, and NaBH4

Alkoxymercuration–demercuration converts an alkene into a Markovnikov ether in two discrete steps. Step 1 uses Hg(OAc)2 dissolved in an alcohol (we illustrate EtOH, but any ROH can serve) to generate a bridged mercurinium ion that forces the alcohol oxygen to attack the more substituted alkene carbon. Step 2 replaces the C–Hg bond with hydrogen using aqueous NaBH4, releasing elemental mercury and leaving the ether. Because the pathway proceeds through a bridged intermediate instead of a discrete carbocation, rearrangements are suppressed. Compare this ether-forming route with the aqueous oxymercuration guide, acid-catalyzed addition of alcohols, and hydroboration–oxidation, which deliver different regiochemical outcomes.

Introduction

Treating an alkene with Hg(OAc)2 in an alcohol creates a three-membered mercurinium ion in which the HgOAc bridge spans the alkene carbons. Positive character concentrates on the more substituted carbon (Cβ), so the alcohol oxygen attacks there from the face opposite Hg (anti opening). Loss of a proton (often to acetate) delivers the alkoxymercury intermediate. In a separate step, NaBH4 (usually in basic aqueous solution) reduces the C–Hg bond to C–H, releasing Hg(0) and affording the Markovnikov ether. Throughout this guide we distinguish the substituent that originated on the alkene as R(alkene) and the alkoxy fragment as R(alcohol) to avoid ambiguity.

Quick Summary

• Sequence: Step 1 Hg(OAc)2 in ROH (no NaBH4 present); Step 2 NaBH4/NaOH (aq) demercuration.
• Intermediate: Bridged mercurinium ion followed by an alkoxymercury species.
• Regiochemistry: Alcohol adds Markovnikov at Cβ; hydrogen installs at Cα during demercuration.
• Stereochemistry: ROH opens the bridge anti to Hg; after NaBH4 the reduced center is not stereospecific (often racemic).
• Rearrangements: Disallowed—no free carbocation forms.
• Intramolecular variant: Tethered OH can cyclize (5-exo-trig favored; 6-endo possible depending on geometry).
• Scope: Works with a wide variety of alkenes and alcohols (EtOH shown here as the model nucleophile).

Mechanism (Alkoxymercuration–Demercuration)

The alkene π bond donates into Hg2+, forming a three-membered mercurinium bridge. Simultaneously, one acetate ligand departs as AcO⁻. Positive charge resides primarily on the more substituted carbon (Cβ), priming it for nucleophilic attack. NaBH4 must be excluded at this stage or the mercuric acetate will be quenched prematurely.

The ethanol oxygen (representing ROH in general) approaches from the face opposite Hg and bonds to Cβ. The Cβ–Hg bond breaks and the electrons return to Hg, preserving the bridge to Cα. Because the mercurinium constrains the approach, the addition is anti relative to Hg.

The displaced acetate (or solvent ROH/H₂O) acts as base, removing the proton from the oxonium oxygen. The result is a neutral alkoxymercury species featuring Cβ–O–R(alcohol) and Cα–HgOAc.

NaBH4 donates hydride to Hg, while acetate coordinates to boron and the Hg–O bond releases electrons to oxygen. The Cα–Hg bond collapses, installing hydrogen where mercury resided and liberating metallic Hg along with borate/acetate by-products. The stereochemistry at Cα is not controlled during this radical-like reduction, so racemization is common.

The final ether retains the original alkene carbon skeleton, places the OR(alcohol) group on the more substituted carbon, and carries a hydrogen on the less substituted carbon. The relative stereochemistry that existed across the alkene is often lost because the NaBH4 step is not stereospecific.

Mechanistic Checklist

• Identify Cα (less substituted) and Cβ (more substituted). Hg bridges both until the NaBH4 step.
• ROH must attack from the face opposite Hg (anti opening); syn attack is disfavored.
• Keep acetate around—AcO⁻ is a convenient base for deprotonation, though ROH or H2O can serve.
• Note the change in labels: R(alkene) stays on carbon, R(alcohol) rides with oxygen. Do not use a single “R” for both.
• Demercuration is a reduction at mercury, not an SN2 attack at carbon. Draw hydride to Hg, not to the sp3 carbon.
• After NaBH4 the configuration at Cα is not guaranteed; depict racemic mixtures or note the loss of stereochemical information when relevant.

Worked Examples

Example A — 2-Methylpropene → tert-Butyl Ethyl Ether

• Substrate: 2-methylpropene (isobutylene).
• Reagents: Step 1 Hg(OAc)2, EtOH; Step 2 NaBH4, NaOH (aq).
• Outcome: EtO installs at the tertiary carbon (Markovnikov); the hydride from NaBH4 replaces Hg at the methyl-substituted carbon.

Example B — 3-Methyl-2-pentene → 3-Ethoxy-3-methylpentane

• Substrate: 3-methyl-2-pentene (E/Z mixture reacts similarly; Markovnikov dictates the connectivity).
• Reagents: Step 1 Hg(OAc)2, EtOH; Step 2 NaBH4, NaOH (aq).
• Outcome: EtO adds to the more substituted internal carbon; NaBH4 replaces Hg with H on the adjacent carbon, giving an ether that is generally isolated as a racemic mixture because Cα racemizes in the reduction.

When Multiple Alkenes Are Present

Hg(OAc)2 responds fastest to the most electron-rich, least hindered alkene. If several double bonds are available, the one that best stabilizes a bridged cation (often the more substituted or benzylic site) reacts first. Tethered alcohols can compete intramolecularly: a pendant OH that can reach Cβ typically closes a 5-exo-trig ring to give a cyclic ether after NaBH4. Longer tethers may allow 6-endo capture, but yields drop as the geometry becomes less favorable. Map out possible bridges before predicting the major product.

Practical Tips & Pitfalls

• Run it in two steps. Add Hg(OAc)2 to the alcohol solvent (with or without a cosolvent such as THF). After mercuration is complete, work up and then treat the organomercury intermediate with aqueous NaBH4. Mixing NaBH4 into the mercuration step wastes reagent and stops the reaction.
• Control light and temperature. Keep reactions at 0–25 °C and in subdued light; photolyzed mercury salts or elevated temperatures can erode selectivity.
• Bases for deprotonation. The acetate liberated in Step 1 is usually sufficient, but extra ROH or a small amount of water ensures proton transfer if the medium is hindered.
• Waste handling. Collect all mercury-containing aqueous and organic layers for hazardous waste disposal; neutralize NaBH4 carefully to avoid vigorous hydrogen release.
• Label your substituents. Use R(alkene) for the carbon substituent and R(alcohol) for the incoming alkoxy group to avoid confusing stereochemical discussions.

Exam-Style Summary

• Hg(OAc)2 + ROH → mercurinium ion; AcO⁻ leaves.
• ROH attacks anti at Cβ (Markovnikov), giving an oxonium.
• AcO⁻ (or ROH/H₂O) removes the proton → alkoxymercury intermediate.
• NaBH4 reduces C–Hg to C–H; Hg leaves as Hg(0); ether forms.
• No carbocation rearrangements; stereochemistry after NaBH4 is typically not retained.

Interactive Toolbox

• Compare with aqueous oxymercuration to see how changing the nucleophile alters the outcome.
• Explore the Mechanism Solver to rehearse the two-electron arrows for each step.
• Practice ether syntheses in the Reaction Solver alongside acid-catalyzed and Williamson routes.

FAQ / Exam Notes

• Why doesn’t rearrangement occur? The mercurinium bridge prevents the formation of a true carbocation, so 1,2-shifts are disfavored.
• Is the final product stereospecific? No. While ROH opens anti to Hg, the NaBH4 reduction often scrambles the configuration at the carbon formerly bonded to Hg.
• Can I use other alcohols? Yes—MeOH, i-PrOH, t-BuOH, and phenols work. EtOH is shown here for consistency.
• What if the substrate contains an internal OH? Intramolecular attack can compete and usually delivers 5-membered cyclic ethers after demercuration.
• How do I stop at the alkoxymercury stage? Quench before adding NaBH4, then isolate the organomercury species—useful in synthesis but seldom tested in introductory courses.