Ethyne reacts with dil.`H_2SO_4` in presence of `Hg^(2+)` salts to give acetaldehyde, but with HCl, under similar conditions , it gives vinyl chloride. Account for such observation.

Explanation

In the first step ,ethyne reacts with `Hg^(2+)` in to form cyclic complex (I) . This is then attacked by more nuleophilic `H_2O` rather than weakly nucleophilic `SO_4^(2-)` , to form unstable vinyl alcohol which then tautomerises to give acetaldehyde.

It HCl is used instead of `H_2SO_4` then the complex (I) is attacked by more nucleophilic `Cl^-` rather than weakly nucleophilic `H_2O` to give vinyl chloride