Alkyne Reactions: Hydration with Hg(OAc)₂ in AcOH/H₂O

Mercuric acetate, Hg(OAc)₂, activates an alkyne toward water addition in aqueous acetic acid. Proton transfers and mercury removal lead to an enol, which tautomerizes to a carbonyl compound. Terminal alkynes R–C≡CH give methyl ketones; ethyne is the exception and gives ethanal. Unsymmetrical internal alkynes can give two ketone regioisomers, so “more substituted” alone does not select the product. The pathway is polar and closed-shell—no radical initiation or carbocation rearrangements. Compare this workflow with hydroboration oxidation if you need an anti-Markovnikov aldehyde instead.

Quick Summary

• Reagents/conditions: Hg(OAc)₂ in water-containing AcOH. HgSO₄ in dilute aqueous H₂SO₄ is the conventional alternative for alkyne hydration; a universal rate or temperature comparison is not implied.
• Outcome: Hydration of C≡C → organomercury intermediates → enol → carbonyl; R–C≡CH gives a methyl ketone, while HC≡CH gives ethanal.
• Mechanism class: Electrophilic mercury activation, water capture, proton transfers, protodemetalation and tautomerization; a polar, closed-shell pathway.
• Selectivity: Symmetrical internal alkynes give one constitutional ketone; unsymmetrical ones may give two. Do not assume a ratio without substrate-specific evidence.
• Safety: Hg(II) salts are toxic and persistent. Work in the hood, wear gloves, and collect Hg waste for licensed disposal.

Mechanism (6 Frames)

Reference substrate: 1-(prop-1-yn-1-yl)cyclohexane → depicted 1-cyclohexylpropan-1-one. The other possible regioisomer is 1-cyclohexylpropan-2-one; no product ratio is assigned.

Frames 1–3 are simplified stages, with mercury speciation and intervening proton transfers omitted. Frames 4–5 show the electron flow for the free enol’s conversion to the ketone.

The drawing identifies the C9 starting skeleton. Electrophilic mercury activation makes the triple bond susceptible to water attack; it does not change the carbon skeleton.

The panel shows one regioisomer using covalent C–Hg–OAc shorthand. Water supplies the hydroxyl group; deprotonation alone does not break the C–Hg bond. Ligand exchange, solvent coordination and E/Z geometry are not specified.

This is a net stage, not one elementary step: the mercurated enol can tautomerize to an α-mercurated ketone before acid-mediated mercury loss. Replacing the carbon-bound mercury fragment with hydrogen yields the depicted free enol. No NaBH₄ workup is needed.

The enol π bond takes a proton from AcOH while an oxygen lone pair forms C=O. Acetate is formed as the acid loses H⁺; no C–Hg bond remains in this panel.

Acetate removes the oxonium proton and the O–H bond electrons return to oxygen. The C=O bond already exists; this step neutralizes oxygen and regenerates AcOH.

Both the alkyne and this ketone contain nine carbons. Hydration adds H₂O, and the product is a ketone rather than cyclohexanecarbaldehyde. The alternative carbonyl position gives 1-cyclohexylpropan-2-one.

Worked Examples

But-1-yne gives butan-2-one: the carbonyl forms at C2. This methyl-ketone rule applies to R–C≡CH; ethyne instead gives ethanal.

Mechanistic Checklist

• Regiochemistry: R–C≡CH gives R–CO–CH₃; HC≡CH gives CH₃CHO. Unsymmetrical internal alkynes can yield either carbonyl position.
• Vinyl-mercury pathway avoids carbocation rearrangements—no 1,2-shifts.
• Closed-shell arrows only; no radical steps are involved.
• Tautomerization is required: these examples give ketones rather than isolated enols; ethyne gives an aldehyde.
• Mercury handling is essential: vent reaction vessels, avoid skin contact, and segregate Hg waste.

Practical Tips

• Water is the nucleophile in hydration. In anhydrous AcOH, acetate addition can instead give acetoxymercuration products; the solvent change is chemically significant. HgSO₄/H₂SO₄/H₂O is an alternative hydration system, with different acidity.
• Acid-labile substituents (acetals, certain protecting groups) and coordinating heteroatoms (thiols, amines) can interfere—mask or remove them beforehand.
• Use hydroboration–oxidation (R₂BH then H₂O₂/NaOH) to obtain anti-Markovnikov aldehydes from terminal alkynes if that outcome is desired.
• Strip mercury from the workup with sulfide precipitation, aqueous chelators, or Hg-specific resins; never discard Hg wastes with organics.

Exam-Style Summary

Hg(OAc)₂ in aqueous AcOH promotes water addition through organomercury intermediates. Proton transfers, mercury removal and tautomerization give a methyl ketone from R–C≡CH; ethyne gives ethanal. Unsymmetrical internal alkynes can give two ketone regioisomers. Do not add a NaBH₄ step: it can reduce the carbonyl product.

Related Guides

• Alkyne Hydration with HgSO₄/H₂SO₄/H₂O (Markovnikov ketones)
• Alkyne Hydroboration–Oxidation (anti-Markovnikov aldehydes)
• Alkyne Oxidation with KMnO₄ (hot) — Oxidative Cleavage

Interactive Toolbox

• Mechanism Solver — browse available mechanisms; this link does not preload or guarantee the illustrated Hg(OAc)₂ sequence.
• Reaction Solver — open the Classic solver and inspect its supported reagents; this example is not preloaded and no regioisomer ratio is promised.
• IUPAC Namer — verify products such as butan-2-one or 2-methylpentan-3-one.