Alkynes
Hydration of acetylene (ethyne) in the presence of H₂SO₄ and HgSO₄ catalyst at 60 °C initially forms an unstable intermediate that immediately tautomerises to give the final product. The final product is:
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About This Question
- Subject
- chemistry
- Chapter
- hydrocarbons
- Topic
- alkynes
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
Acetaldehyde(ethanal)
Hydration of alkynes follows Markovnikov's rule. Water adds across the triple bond of acetylene (HC≡CH): H adds to one carbon and OH adds to the other, initially forming vinyl alcohol (CH₂=CH–OH), an enol. However, enols are thermodynamically unstable and spontaneously undergo keto–enol tautomerism. The enol CH₂=CHOH tautomerises to the more stable keto form, acetaldehyde (CH₃CHO), by migration of the OH proton to the adjacent sp² carbon. This acid-catalysed tautomerism is rapid and essentially irreversible under the reaction conditions. Therefore, the final product is acetaldehyde (ethanal), not acetic acid (which would require further oxidation).
This medium difficulty chemistry question is from the chapter hydrocarbons, covering the topic of alkynes. It appeared in the 2025 exam.
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