Read the passage carefully and answer the questions.
Replacement of a hydrogen atom in a hydrocarbon by an alkoxy or carboxyl group yields a class of compounds known as ethers. Ethers are classified as symmetrical or unsymmetrical on the basis of groups attached to the oxygen atoms. Diethyl ether, a symmetrical ether, has been widely used as an inhalation anesthetic. Ethers can be prepared by acid catalyzed intermolecular dehydration of alcohols and Williamson's synthesis. Acid catalyzed dehydration of alcohols is not generally preferred as it gives a mixture of elimination and substitution products. In Williamson's synthesis, an alkyl halide is allowed to react with sodium alkoxide. Ethers containing substituted Alkyl groups may also be prepared by this method. The C-O bond in ether is weakly polar and is cleaved under drastic conditions with excess of hydrogen halides. In electrophilic substitution, the alkoxy group deactivates the aromatic ring and directs the incoming group to ortho and para positions.
Which is most reactive hydrogen halide for cleavage of ethers?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → HI
Ethers are cleaved by hydrogen halides in two stages. The acid first protonates the ether oxygen, and the halide ion then attacks a carbon atom next to it as a nucleophile:
\( \displaystyle \ce{R-O-R + HX -> R-OH + R-X} \)
Both stages are easiest with HI. It is the strongest acid of the four, so it protonates the oxygen most readily, and \( \ce{I-} \) is the best nucleophile among the halide ions.
HF, with its very strong H–F bond and poorly nucleophilic \( \ce{F-} \), is the least reactive, so the most reactive hydrogen halide is HI.
Note: It seems that there is a mistake in the original NTA paper, which prints alkoxy or carboxyl group when it should read alkoxy or aryloxy group, and deactivates when it should read activates.
NCERT (2026-27 edition):
Cleavage of C–O Bond in Ethers – …
