JEE Main · 2025 · 29 Jan 2025 · Shift 2 · Alcohols Phenols and Ethers

Cleavage of an alkyl aryl ether by HBr proceeds through protonation of the ether oxygen followed by $S_{N}2$ attack of $B{r}^{-}$ at the less hindered alkyl carbon — the aryl–oxygen bond is never broken because it carries partial double-bond character from conjugation with the ring and cannot support backside attack or a stable aryl cation. For phenyl ethyl ether (phenetole, $C_6{H}_5-O-C_2{H}_5$), bromide attacks the ethyl carbon, expelling the phenoxide, which is then protonated to phenol, while the ethyl group leaves as ethyl bromide: $C_6{H}_{5}O{C}_2{H}_5+HBr\to C_6{H}_{5}OH+C_2{H}_{5}Br$. This ether is the one that yields phenol on treatment with HBr.