Q1 / 180 · COMEDK 2025 (10 May Shift 3) · Chemistry
Identify the Carbonyl compound which will not be formed when hydration of Alkynes is carried out with dil. $\mathrm{H}_2 \mathrm{SO}_4 / \mathrm{Hg}^{2+}$ at 333 K .
Acetone, Butanal, Ethanal, Butanone.
- Correct answerAButanal
- BButanone
- CAcetone
- DEthanal
Solution
Hydration of alkynes in the presence of dil. $\mathrm{H}_{2}\mathrm{SO}_{4}$ and $\mathrm{Hg}^{2+}$ follows Markovnikov addition of water to the triple bond, forming an enol intermediate which tautomerizes to a carbonyl compound.
For ethyne ($HC \equiv CH$), hydration yields ethenol, which tautomerizes to ethanal ($CH_{3}CHO$).
For propyne ($CH_{3}C \equiv CH$), hydration yields prop-1-en-2-ol, which tautomerizes to acetone ($CH_{3}COCH_{3}$).
For but-1-yne ($CH_{3}CH_{2}C \equiv CH$), hydration yields but-1-en-2-ol, which tautomerizes to butanone ($CH_{3}COCH_{2}CH_{3}$).
For but-2-yne ($CH_{3}C \equiv CCH_{3}$), hydration yields but-2-en-2-ol, which tautomerizes to butanone ($CH_{3}COCH_{2}CH_{3}$).
Butanal ($CH_{3}CH_{2}CH_{2}CHO$) is an aldehyde with four carbon atoms. It would require the anti-Markovnikov addition of water to but-1-yne or the hydration of a terminal alkyne at the terminal carbon, which is not the product of the acid-catalyzed hydration of alkynes using $\mathrm{Hg}^{2+}$ catalyst. Therefore, butanal is not formed.
Answer: Butanal
For ethyne ($HC \equiv CH$), hydration yields ethenol, which tautomerizes to ethanal ($CH_{3}CHO$).
For propyne ($CH_{3}C \equiv CH$), hydration yields prop-1-en-2-ol, which tautomerizes to acetone ($CH_{3}COCH_{3}$).
For but-1-yne ($CH_{3}CH_{2}C \equiv CH$), hydration yields but-1-en-2-ol, which tautomerizes to butanone ($CH_{3}COCH_{2}CH_{3}$).
For but-2-yne ($CH_{3}C \equiv CCH_{3}$), hydration yields but-2-en-2-ol, which tautomerizes to butanone ($CH_{3}COCH_{2}CH_{3}$).
Butanal ($CH_{3}CH_{2}CH_{2}CHO$) is an aldehyde with four carbon atoms. It would require the anti-Markovnikov addition of water to but-1-yne or the hydration of a terminal alkyne at the terminal carbon, which is not the product of the acid-catalyzed hydration of alkynes using $\mathrm{Hg}^{2+}$ catalyst. Therefore, butanal is not formed.
Answer: Butanal
Hence, the correct answer is option: 'A'.