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Two cyclic ethers are treated with excess HI: (I) Tetrahydropyran (a six-membered ring with one oxygen atom) (II) Chroman (a benzene ring fused with a dihydropyran ring containing one oxygen) Which compound(s) give a di-iodo product on reaction with excess HI?
- I and II both
- II only
- I only
- none
Correct answer: I and II both
Solution
In tetrahydropyran (THP), HI protonates the oxygen, then I- acts as a nucleophile to open the ring at the C-O bond. With excess HI the resulting iodo-alcohol reacts again to replace the OH with I, giving 1,5-diiodopentane. Chroman similarly undergoes C-O bond cleavage at both positions of the heterocyclic ring with excess HI, producing an aryl iodo-ethanol first and then a diiodo product. The aromatic ring is not cleaved. Both compounds give di-iodide.
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