By a proper choice of reagents, both symmetrical and unsymmetrical ethers can be prepared by Williamson synthesis which involves the reaction between an alkyl halide and an alkoxide ion. The reverse process involving the cleavage of ethers to give back the original alkyl halide and the alcohol can be carried out by heating the ether with HI at 373K.
Which of the following ethers is not cleaved by HI?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Diphenyl ether
When an ether is heated with HI, the ether oxygen is first protonated. Then \( \ce{I^-} \) breaks a C–O bond to an alkyl carbon. An aryl–oxygen bond is not broken: the ring carbon is sp2 and the bond has partial double bond character, so \( \ce{I^-} \) cannot substitute there.
In diphenyl ether, \( \ce{C6H5-O-C6H5} \), both C–O bonds are aryl–oxygen bonds. There is no alkyl carbon for iodide to attack, so HI does not cleave it.
Option (1) Dicyclohexyl ether — Cleaved by HI. Both groups are secondary alkyl groups, so iodide attacks an alkyl carbon:
\( \displaystyle \ce{C6H11-O-C6H11 + HI} \xrightarrow{\text{heat}} \ce{C6H11I + C6H11OH} \)
Option (2) Phenetole — Cleaved by HI. Phenetole (ethoxybenzene) is an alkyl aryl ether. It breaks at the ethyl–oxygen bond to give phenol and ethyl iodide:
\( \displaystyle \ce{C6H5-O-C2H5 + HI} \xrightarrow{\text{heat}} \ce{C6H5OH + C2H5I} \)
Option (3) Di-tert-butyl ether — Cleaved by HI. Both groups are tertiary alkyl groups, so, like any dialkyl ether, it is cleaved at an alkyl–oxygen bond:
\( \displaystyle \ce{(CH3)3C-O-C(CH3)3 + HI} \xrightarrow{\text{heat}} \ce{(CH3)3CI + (CH3)3COH} \)
Option (4) Diphenyl ether — Not cleaved by HI. Both oxygen–carbon bonds are to sp2 ring carbons, which cannot undergo nucleophilic substitution.
NCERT (2026-27 edition):
1. Cleavage of C–O bond in ethers – “… Alkyl aryl ethers are cleaved at the alkyl-oxygen bond due to the more stable aryl-oxygen bond.”
“… The bond between O–CH3 is weaker than the bond between O–C6H5 because the carbon of phenyl group is sp2 hybridised and there is a partial double bond character. … Phenols do not react further to give halides because the sp2 hybridised carbon of phenol cannot undergo nucleophilic substitution reaction needed for conversion to the halide.”