Which one of the following carbonyl compounds cannot be prepared by addition of water on an alkyne in the presence of HgSO₄ and H₂SO₄?

Solution

Chemistry ma’am · 3 steps

Markovnikov hydration puts the C=O on the inner carbon, so only ethyne can ever give an aldehyde.

1. How hydration works. With HgSO₄ and H₂SO₄, water adds across the triple bond by Markovnikov's rule. The OH goes to the carbon that carries fewer hydrogens. The enol formed then tautomerises to a carbonyl compound.
2. Terminal alkynes. In R-C≡CH the OH lands on the inner carbon, the one bonded to R. The product is always a methyl ketone, R-CO-CH₃, never an aldehyde. The only exception is ethyne, where R is H, so the product is CH₃CHO.

   Product	Alkyne needed	Possible?
   (a) CH₃COCH₂CH₃	1-butyne or 2-butyne	yes
   (b) cyclohexyl methyl ketone	ethynylcyclohexane, C₆H₁₁-C≡CH	yes
   (c) CH₃CH₂CHO	propyne gives CH₃COCH₃ instead	no
   (d) CH₃CHO	ethyne	yes

3. So. Propanal would need the OH on the terminal carbon of propyne, which is the anti-Markovnikov position. This route cannot give it, so the answer is (c).

The idea to keep. Acid and Hg²⁺ hydration of an alkyne gives a ketone every time, except for ethyne, which gives acetaldehyde. To get a higher aldehyde, use hydroboration–oxidation instead.

Answer from the NTA final answer key. Solution by PrepWiser.