Hydration of an alkyne template

Propyne with HgSO₄ in aqueous H₂SO₄: Hg²⁺ activates the triple bond, water adds to the inner carbon, H⁺ replaces Hg²⁺ and the enol tautomerises to propanone.

Mechanism step by step

Hg²⁺ and H₂SO₄ add water Markovnikov; the enol formed tautomerises to a ketone

1. Hg²⁺ adds – The π bond attacks Hg²⁺ (drawn open; really a bridged π complex)
2. Water adds – At the more substituted carbon (Markovnikov)
3. Deprotonation – A mercury-substituted enol
4. C-Protonation – H⁺ adds to the carbon carrying Hg
5. Loss of Hg²⁺ – The catalyst leaves: the neutral enol
6. Tautomerism 1 – The enol C=C takes H⁺
7. Tautomerism 2 – Loss of H⁺ from O: the ketone

Key points

• Markovnikov: terminal alkynes give methyl ketones; only ethyne gives an aldehyde (ethanal).
• The enol is not isolated: the keto form is far more stable.
• Hg²⁺ is regenerated, so it is a catalyst.
• Hydroboration–oxidation of a terminal alkyne with a bulky borane (disiamylborane or 9-BBN) gives the aldehyde instead (anti-Markovnikov).

Drawn with 87 atoms, 23 arrows, 9 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 19 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 15 Textbook