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Acetylene (C₂H₂), also known as ethyne, is the only gas among the given options that reacts with warm dilute sulfuric acid (H₂SO₄) in the presence of mercuric sulfate (HgSO₄) to form acetaldehyde (CH₃CHO). This is a typical hydration reaction of terminal alkynes in organic chemistry.
In this reaction, the triple bond of acetylene undergoes electrophilic addition of water (H₂O), catalyzed by Hg²⁺ ions. The reaction initially forms an unstable intermediate, vinyl alcohol (CH₂=CHOH), which rearranges to a more stable compound — acetaldehyde — through keto-enol tautomerism.
The simplified reaction is:
C₂H₂ + H₂O → CH₃CHO (catalyzed by H₂SO₄ + HgSO₄)
CH₄ (methane) and C₂H₆ (ethane) are saturated hydrocarbons and do not undergo such reactions. C₂H₄ (ethylene) can undergo hydration to form ethanol, not acetaldehyde, under acidic conditions. Therefore, they do not produce acetaldehyde in this setup.
This reaction is important in the industrial production of acetaldehyde and illustrates how functional groups are introduced in hydrocarbons through controlled chemical transformations. The Hg²⁺ ion plays a key role in activating the triple bond and stabilizing intermediates, making the reaction possible under mild conditions.
Understanding this conversion is essential in organic chemistry, especially in the study of alkyne reactions and aldehyde synthesis pathways. It also highlights the role of reaction conditions and catalysts in determining the type of product formed.
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