Question

Acetylene reacts with dil. $$H_2SO_4$$ in presence of mercury salts to give acetadehyde but with dil. $$HCl$$ under similar conditions, it gives vinyl chloride. Explain why?

Answer 1

First of all mercury ions form a complex $$(I)$$ with acetylene. Since $$H_2O$$ is more nucleophilic than $$SO_4^{2-}$$ ion, it attacks the complex to form first vinyl alcohol which then tautomerises to give acetaldehyde.

In case of dil. $$HCl$$, since $$Cl^-$$ ions is more nucleophilic than $$H_2O$$, it reacts with complex $$(I)$$ to form vinyl chloride.