Hydration of alkynes: practice and mechanism
Water adds following Markovnikov and the enol tautomerises at once to the ketone: terminal alkynes give methyl ketones.
Example
Water adds to the triple bond following Markovnikov. The enol formed first immediately tautomerises to the more stable keto form. Terminal alkynes give methyl ketones, not aldehydes.
Mechanism step by step
- 1
Hg²⁺ coordinates to the triple bond and makes it electrophilic.
- 2
Water attacks the more substituted carbon (Markovnikov), giving an enol.
- 3
The enol tautomerises to the ketone: H moves from O to C, and C=C–OH becomes CH–C=O.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- An aldehyde would only form by anti-Markovnikov addition (hydroboration). The Hg²⁺-catalysed hydration follows Markovnikov and gives the ketone.
- That is the enol, only the intermediate. It immediately rearranges to the more stable keto form (keto–enol tautomerism).
- Only one molecule of water adds, not two. The enol becomes the ketone; no diol forms.
Often confused with: Catalytic hydrogenation · Hydroboration–oxidation
Frequently asked questions
How do I get an aldehyde from a terminal alkyne?
With hydroboration–oxidation (a bulky borane, e.g. disiamylborane): anti-Markovnikov, and the enol becomes the aldehyde.
Why is the keto form more stable?
The C=O bond is considerably stronger than the C=C bond of the enol.