Oxymercuration of Alkynes
Description: Alkynes treated with mercury (usually HgSO4) and water will be hydrated to give ketones, via an enol intermediate.
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.
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).
- 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.