Organic Chemistry 2 · Synthesis roadmaps: reagents, protecting groups and multistep planning
Hex-1-yne to hexan-2-one
Conditions: Hex-1-yne (shown) must be converted into hexan-2-one, CH₃(CH₂)₃COCH₃.
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The question
Which reagents accomplish this?
Reveal the answer and explanation
Answer and explanation
Correct answer H₂O, H₂SO₄, HgSO₄
Mercury(II)-catalysed hydration adds OH to the internal carbon (Markovnikov); the enol tautomerises to the methyl ketone.
Not this one Disiamylborane, then H₂O₂ and NaOH
A bulky borane puts boron, and then OH, on the terminal carbon; the enol tautomerises to hexanal.
Not this one H₂, Lindlar catalyst
Lindlar hydrogenation stops at the alkene, hex-1-ene; no oxygen is introduced.
Not this one NaNH₂, then CH₃I
This alkylates the acetylide, giving hept-2-yne; it makes a C–C bond, not a C=O.
Worked solution
Both alkyne hydrations pass through an enol that tautomerises to a carbonyl compound. The regiochemistry decides which: HgSO₄ in aqueous acid gives the methyl ketone (Markovnikov), a bulky borane followed by oxidation gives the aldehyde (anti-Markovnikov).
What you'll practice
Choose the reagent that hydrates a terminal alkyne to a methyl ketone rather than an aldehyde.
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