There is only one alkyne that forms an aldehyde when it undergoes the mercuric-ion-catalyzed addition of water. What is this alkyne?
The alkyne is ethyne.
The mercury(II) catalyzed hydration of an alkyne first forms an enol, which then tautomerizes to the more stable carbonyl compound.
In the hydration of ethyne, the intermediate product is ethenol.
The tautomeric rearrangement of ethenol results in the formation of ethanal.