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 pairs of reagents, when heated, will give a good yield of an ether?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Sodium tert-butoxide and ethyl bromide
In the Williamson synthesis an alkoxide ion attacks an alkyl halide by SN2. An alkoxide is also a strong base, so it can pull off a β-hydrogen instead and give an alkene. Substitution wins only with a primary alkyl halide; the bulky or branched part of the ether must therefore come from the alkoxide.
Sodium tert-butoxide with ethyl bromide puts the hindered tert-butyl group on the alkoxide and uses an unhindered primary halide, so ethyl tert-butyl ether forms in good yield:
\( \displaystyle \ce{(CH3)3CONa + CH3CH2Br} \xrightarrow{\text{heat}} \ce{(CH3)3C-O-CH2CH3 + NaBr} \)
Why the other options are incorrect
Option (1) Isopropyl bromide and sodium isopropoxide — Incorrect. Isopropyl bromide is a secondary halide, and the branched isopropoxide acts mainly as a base, so elimination to propene competes strongly and the ether yield is poor.
Option (2) Isopropyl bromide and sodium ethoxide — Incorrect. The halide is again secondary. Ethoxide, a strong base, removes a β-hydrogen, so much of the halide becomes propene instead of ethyl isopropyl ether.
Option (3) Bromobenzene and sodium phenoxide — Incorrect. Bromobenzene is an aryl halide. The C–Br bond to the sp2 ring carbon has partial double bond character, so phenoxide cannot replace the bromine and diphenyl ether is not formed this way.
NCERT (2026-27 edition):
2. Williamson synthesis – “Better results are obtained if the alkyl halide is primary. In case of secondary and tertiary alkyl halides, elimination competes over substitution.”