Oxymercuration of Alkynes

Description: Alkynes treated with mercury (usually HgSO4) and water will be hydrated to give ketones, via an enol intermediate.
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Notes: This reaction follows Markovnkov’s rule. Only a catalytic amount of mercury is required in this reaction. Acid is also important. Often mercuric sulfate (HgSO₄) is used in conjunction with sulfuric acid (H₂SO₄), although this exact combination is not crucial.

Examples:

Notes:  Note for that the third example a mixture of ketones is obtained because no clear preference can be established using Markovnikoff’s rule (i.e. both carbons are equally substituted)

Mechanism: For the purposes of Org 1/Org 2 this mechanism is generally not considered “important” due to the weird “de-mercuration” step. Mercury adds to the alkyne (Step 1, arrow A) giving the mercuronium ion. This is then attacked by water at the more substituted carbon (“Markovnikoff selectivity”) (Step 2, arrows B and C) to give the mercurated “enol” after deprotonation (Step 3, arrows D and E). Removal of the mercury happens in two steps: first, protonation of the enol at carbon (Step 4, arrows F, G, H) gives the protonated carbonyl; attack at mercury by solvent (or other nucleophile) regenerates the enol (Step 5, arrows I, J K). Then, the enol is converted to the more stable ketone through tautomerization (Step 6, arrows L and M).

Notes: 

• The ketone is formed because it is more stable than the enol. This process is called keto-enol tautomerism.
• Note that there’s actually quite a few ways this mechanism could be written and this is just one suggestion. The key steps to really be concerned with are steps 1, 2, and 6.

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