During the reaction of acetylene with$\text{ dil. }{\mathrm{H}}_2{\mathrm{SO}}_4 \mathrm{in} \mathrm{presence} \mathrm{of} {\mathrm{HgSO}}_4$, the compound initially formed is

 

oxymercuration-demercuration reaction, and the product is treated with sodium borohydride. The end result is a Markovnikov addition product in which the OH group bonds to the alkene's more substituted carbon atom. The general reaction here is "Oxymercuration-Demercuration." Acetaldehyde is the final product. $\mathrm{HC}\equiv \mathrm{CH}\underset{{\mathrm{HgSO}}_4}{\stackrel{\mathrm{dil}.{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }}\underset{\text{Vinyl alcohol}}{{\mathrm{H}}_2\mathrm{C}=\underset{\mathrm{OH}}{\underset{\mid }{\mathrm{CH}}}}\stackrel{\text{Rearrangement}}{\leftrightarrow }\underset{\text{Acetaldehyde}}{{\mathrm{CH}}_3\mathrm{CHO}}$