The hydration of a terminal alkyne with HgSO4/H2SO4 furnishes what product?

• A. Aldehyde
• B. Methyl ketone
• C. Alcohol
• D. Carboxylic acid

Answer: (B)

Hydration of terminal alkynes with mercuric sulfate in acid is a mercury-catalyzed, Markovnikov addition of water across the triple bond that forms an enol, which then tautomerizes to a ketone. For a terminal alkyne of the form R-C≡CH, water adds so that the hydroxyl group attaches to the internal carbon (the one bonded to R) and the hydrogen adds to the terminal carbon. The resulting enol rapidly tautomerizes to the ketone with the carbonyl carbon bearing the R group and a methyl group on the other side, giving the product R-CO-CH3. That is a methyl ketone. (Note: when the alkyne is acetylene itself, the reaction gives acetaldehyde, but for a general terminal alkyne the product is a methyl ketone.)