Oxymercuration–Hydration of Alkynes
Oxymercuration adds water across the C≡C triple bond of an alkyne. Step-by-step mechanism, reagents and practice.
What happens
Oxymercuration adds water across the C≡C triple bond of an alkyne. The reaction usually uses HgSO₄, H₂SO₄, and H₂O. At first, an enol is formed. The enol is unstable and quickly rearranges into a more stable ketone. For a terminal alkyne, the final product is a methyl ketone.
Reaction summary
R–C≡C–H → R–CO–CH₃ (via an enol); e.g. propyne → acetone
Reagents and conditions
- Starting material: Propyne (terminal alkyne)
- Reagents: HgSO₄; H₂SO₄; H₂O
- Products: Acetone (methyl ketone)
Mechanism
- Hg²⁺ Activates the Alkyne: The alkyne is activated by Hg²⁺.
- Water Adds (Markovnikov): H₂O attacks, resulting in Markovnikov addition of H and OH across the triple bond: H goes to the terminal carbon and OH to the more substituted carbon.
- Enol Formed: An enol is formed. It is only an intermediate: you normally don't isolate it.
- Keto–Enol Tautomerization: The enol undergoes keto–enol tautomerization and rearranges into the more stable ketone.
- Product: The more stable ketone is formed: the final product, acetone. The enol was only passing through.
Selectivity
- Regioselectivity: For a terminal alkyne, hydration follows Markovnikov orientation: H goes to the terminal carbon and OH to the more substituted carbon. A terminal alkyne gives a methyl ketone (R–C≡CH → R–CO–CH₃); an internal alkyne gives ketone(s). Unsymmetrical internal alkynes can produce a mixture of ketones because hydration can occur in either orientation.
Common mistakes
- The biggest mistake is stopping at the enol. The enol is only an intermediate (alkyne → enol → ketone): you normally don't isolate it.
- Don't confuse oxymercuration of alkynes with oxymercuration–demercuration of alkenes. For alkyne hydration, the key reagents are HgSO₄ + H₂SO₄ + H₂O.
Memory trick
Hg²⁺ + alkyne = Markovnikov ketone. And remember the journey: C≡C → enol → C=O. The enol is just passing through; the ketone is the final destination.