Alkyne Reactions: Alkyne Oxymercuration using HgSO4, H2O, H2SO4

Mercuric sulfate in aqueous sulfuric acid promotes the classic Kucherov hydration of alkynes. The alkyne pi bond coordinates to Hg2+, water adds to the more substituted vinylic carbon (Markovnikov orientation), and deprotonation gives an enol-Hg intermediate. Proton-assisted demetalation releases the free enol, which rapidly tautomerizes to the carbonyl product (usually a ketone) under the acidic conditions.

• Carbon-substituted terminal alkynes (RC≡CH) furnish methyl ketones. Ethyne (acetylene, HC≡CH) instead gives ethanal (acetaldehyde).
• Internal alkynes hydrate to ketones; unsymmetrical systems often give regioisomer mixtures.
• The Hg2+ catalyst is regenerated at the end of the cycle, but Hg waste must be handled carefully.

Quick Summary

• Reagents/conditions: catalytic HgSO4 with aqueous H2SO4/H2O, typically 25–80 °C, with gentle heating when needed.
• Outcome: Markovnikov hydration converts the alkyne π bond into an enol that tautomerizes rapidly to the ketone.
• Regioselectivity: OH installs on the more substituted vinylic carbon; RC≡CH gives RCOCH₃ (the terminal carbon becomes the methyl group). Ethyne is the exception and gives CH₃CHO.
• Stereochemistry: The transient enol rapidly tautomerizes; any E/Z information is lost in the ketone.
• Rearrangements: Vinyl cation-like manifolds do not undergo 1,2-hydride or methyl shifts.
• Catalyst: Hg2+ is returned at the end of the cycle (no mercury in the organic products).

Mechanism (7 Frames)

The panels below depict 3-methylbut-1-yne hydrating to 3-methylbutan-2-one.

The alkyne pi system binds Hg2+ (from HgSO4 in acidic water), forming a vinyl mercurinium-like complex that activates the more substituted carbon toward nucleophilic attack.

The activated complex undergoes nucleophilic attack at the carbon attached to the alkyl substituent. The arrows show water attack and opening of the bridged complex.

A solvent water molecule removes a proton from the attached OH₂⁺ group, giving the mercury-containing enol.

The net equation summarizes proton-assisted loss of mercury and regeneration of Hg²⁺; mercury ligands and individual solvent proton transfers are omitted.

The enol is 3-methylbut-1-en-2-ol, CH₂=C(OH)CH(CH₃)₂. It has the same five-carbon skeleton as the starting alkyne.

Protonation at the terminal carbon of the enol gives a protonated carbonyl. Loss of the oxygen-bound proton regenerates acid and gives the neutral ketone.

The methyl ketone is the major tautomer under these conditions. Tautomerization changes bond orders and proton positions while preserving the carbon skeleton.

Mechanistic Checklist

• Markovnikov orientation: OH installs on the more substituted vinylic carbon.
• RC≡CH hydrates to RCOCH₃; ethyne (HC≡CH) gives ethanal rather than a ketone.
• Unsymmetrical internal alkynes can afford mixtures of ketones when both vinylic carbons are comparably substituted.
• No 1,2-hydride or alkyl shifts occur; vinyl cation-like intermediates do not rearrange.
• Hg2+ is catalytic and absent from the organic products (manage Hg waste appropriately).

Worked Examples

Example A - Terminal alkyne hydration (prop-1-yne → propan-2-one)

Propyne gives a methyl ketone via an enol intermediate. Ethyne instead gives ethanal.

Example B - Aryl-substituted alkyne (ethynylbenzene → 1-phenylethan-1-one)

Benzylic stabilization reinforces Markovnikov capture next to the aromatic ring.

Example C - Unsymmetrical internal alkyne (hex-2-yne → regioisomeric ketones)

→

+

Both alkyne carbons bear carbon substituents, so hydration can give hexan-2-one and hexan-3-one. Both products retain all six carbons; no fixed major/minor ratio is assigned here.

Example D - Symmetrical internal alkyne (but-2-yne → butan-2-one)

Multiple Pi Bonds & Selectivity

• Aqueous sulfuric acid can also hydrate alkenes. Do not assume an alkene elsewhere in the molecule survives or that the alkyne always reacts first; chemoselectivity requires substrate-specific conditions.
• If two alkynes are present, the less hindered or more electron-rich triple bond often reacts faster, but forcing conditions hydrate both.
• Benzylic or conjugated alkynes accentuate Markovnikov capture next to the aromatic system.

Practical Tips & Pitfalls

• Medium: Use aqueous H2SO4 with catalytic HgSO4; gentle heating accelerates sluggish substrates.
• Work-up: Mercury is toxic-collect and dispose of Hg-containing waste according to institutional protocols.
• Compatibility: Strong bases or nucleophiles quench the catalyst; enol/ketone products can be acid-sensitive (watch for aldol-type side reactions if additional carbonyls are present).
• Regiochemistry alternative: For anti-Markovnikov hydration of terminal alkynes (aldehydes), choose hydroboration-oxidation instead of Hg2+ catalysis.

Exam-Style Summary

HgSO4/H2SO4/H2O hydrates alkynes through a vinyl mercurinium intermediate, giving Markovnikov enols that tautomerize to ketones. Carbon-substituted terminal alkynes give methyl ketones; ethyne gives ethanal. Unsymmetrical internal alkynes often provide regioisomer mixtures. No rearrangements occur and Hg2+ is catalytic.

Interactive Toolbox

• Reaction Solver - explore available reagents and check the product against the carbon-mapping rules above.
• Mechanism Solver - explore supported reactions and compare their steps with the hydration and tautomerization sequence above.
• IUPAC Namer - confirm ketone names such as acetone, acetophenone, and hexanone regioisomers.