Consider the following statements about the preparation of ethers.
(A) The formation of an ether by the acidic dehydration of an alcohol is a nucleophilic bimolecular (SN2) reaction in which an alcohol molecule attacks a protonated alcohol.
(B) The acidic dehydration of alcohols is suitable for the preparation of ethers having tertiary alkyl groups.
(C) In Williamson synthesis, the reaction involves SN2 attack of an alkoxide ion on a primary alkyl halide.
(D) In Williamson synthesis, the reaction of CH3ONa with (CH3)3C–Br gives 2-methoxy-2-methylpropane.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) and (C) only
Both methods form the ether by an SN2 attack, and both work well only with primary alkyl groups. With secondary and tertiary groups, elimination competes and an alkene forms instead.
(A) The formation of an ether by the acidic dehydration of an alcohol is a nucleophilic bimolecular (SN2) reaction in which an alcohol molecule attacks a protonated alcohol. — True. Two alcohol molecules join with the loss of water, for example:
\( \displaystyle \ce{2CH3CH2OH} \xrightarrow[\text{413 K}]{\ce{H2SO4}} \ce{CH3CH2-O-CH2CH3 + H2O} \)
(B) The acidic dehydration of alcohols is suitable for the preparation of ethers having tertiary alkyl groups. — False. The method suits ethers with primary alkyl groups only. Secondary and tertiary alcohols give alkenes, because elimination competes over substitution.
(C) In Williamson synthesis, the reaction involves SN2 attack of an alkoxide ion on a primary alkyl halide. — True. The alkoxide ion is the nucleophile and the halide ion is the leaving group:
\( \displaystyle \ce{CH3CH2Br + CH3O^-Na+ -> CH3CH2-O-CH3 + NaBr} \)
(D) In Williamson synthesis, the reaction of CH3ONa with (CH3)3C–Br gives 2-methoxy-2-methylpropane. — False. With a tertiary alkyl halide no ether forms. The alkoxide is also a strong base, so it removes a proton and gives 2-methylpropene:
\( \displaystyle \ce{(CH3)3C-Br + CH3ONa -> (CH3)2C=CH2 + CH3OH + NaBr} \)
NCERT (2026-27 edition):
7.6.1 Preparation of Ethers – “The formation of ether is a nucleophilic bimolecular reaction (SN2) involving the attack of alcohol molecule on a protonated alcohol…”
“The method is suitable for the preparation of ethers having primary alkyl groups only.” … “However, the dehydration of secondary and tertiary alcohols to give corresponding ethers is unsuccessful as elimination competes over substitution and as a consequence, alkenes are easily formed.”
“The reaction involves SN2 attack of an alkoxide ion on primary alkyl halide.”
“If a tertiary alkyl halide is used, an alkene is the only reaction product and no ether is formed. For example, the reaction of CH3ONa with (CH3)3C–Br gives exclusively 2-methylpropene.” “It is because alkoxides are not only nucleophiles but strong bases as well.”