Among the following ethers, which cannot react with HI ?

The correct answer is Option (3) → diphenyl ether

Ethers are cleaved by concentrated HI (or HBr) under drastic conditions. The ether oxygen is first protonated, and then \( \ce{I^{-}} \) attacks a carbon attached to the oxygen and breaks that C–O bond. In an alkyl aryl ether the alkyl–oxygen bond breaks, because the aryl–oxygen bond is stronger: the oxygen lone pair is in resonance with the ring, giving the Ar–O bond partial double-bond character, and \( \ce{I^{-}} \) cannot attack an sp² ring carbon. So such ethers give a phenol and an alkyl iodide.

In diphenyl ether (\( \ce{C6H5-O-C6H5} \)) both C–O bonds are aryl–oxygen bonds. There is no alkyl carbon for \( \ce{I^{-}} \) to attack, so diphenyl ether is not cleaved by HI.

Why the other options are incorrect

• Option (1) ethoxybenzene (phenetole) — Incorrect. The ethyl–oxygen bond is cleaved:

      \( \displaystyle \ce{C6H5OC2H5 + HI -> C6H5OH + C2H5I} \)

• Option (2) methoxybenzene (anisole) — Incorrect. The methyl–oxygen bond is cleaved:

      \( \displaystyle \ce{C6H5OCH3 + HI -> C6H5OH + CH3I} \)

• Option (4) cyclohexyl phenyl ether — Incorrect. The cyclohexyl carbon is sp³, so the cyclohexyl–oxygen bond breaks:

      \( \displaystyle \ce{C6H5OC6H11 + HI -> C6H5OH + C6H11I} \)

NCERT (2026-27 edition):
Cleavage of C–O Bond in Ethers – “Ethers are the least reactive of the functional groups. The cleavage of C-O bond in ethers takes place under drastic conditions with excess of hydrogen halides. The reaction of dialkyl ether gives two alkyl halide molecules. Alkyl aryl ethers are cleaved at the alkyl-oxygen bond due to the more stable aryl-oxygen bond. The reaction yields phenol and alkyl halide.”