Animated Solution for Chemistry - Hydrocarbons: Acetylene reacts with hypochlorous acid to form

We are given a reaction between acetylene ($HC\equiv CH$) and hypochlorous acid ($HOCl$). Hypochlorous acid is an interesting reagent because it provides an electrophilic chlorine atom ($C{l}^{+}$) and a nucleophilic hydroxyl group ($O{H}^{-}$).

When acetylene reacts with the first molecule of $HOCl$, the electrophilic $C{l}^{+}$ attacks the electron-rich triple bond, followed by the attack of the $O{H}^{-}$ nucleophile. This electrophilic addition results in the formation of an enol, specifically 2-chloroethen-1-ol:

Second Addition

Enols are generally unstable and tend to tautomerize into their more stable keto forms. In this case, 2-chloroethen-1-ol would tautomerize to 2-chloroethanal ($Cl-C{H}_2-CHO$).

However, in the presence of excess $HOCl$, another electrophilic addition can occur on the double bond of the enol before tautomerization is complete. The second $HOCl$ molecule adds according to Markovnikov's rule (considering the electron-donating effect of the $-OH$ group), leading to the formation of a gem-diol:

Gem-diols (compounds with two hydroxyl groups on the same carbon atom) are highly unstable. They readily undergo dehydration, losing a molecule of water to form a carbonyl group.

Thus, the final major product of this reaction is dichloroethanal, which corresponds to option (a).