Which of the following ethers cannot be prepared by bimolecular dehydration of alcohol?
(A) 2-Methoxy-2-methylpropane
(B) 2-Methoxy propane
(C) 1-Propoxypropane
(D) 2-Ethoxy butane
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (B) and (D) only
In bimolecular dehydration, an alcohol heated with concentrated sulphuric acid at about 413 K loses one molecule of water from two molecules. One alcohol molecule is protonated, and a second alcohol molecule attacks it in an \( S_N2 \) step:
\( \displaystyle \ce{2R-OH} \xrightarrow[\text{413 K}]{\ce{H2SO4}} \ce{R-O-R + H2O} \)
The method also gives only symmetrical ethers: a mixture of two alcohols gives a mixture of three ethers. Unsymmetrical ethers are made by Williamson synthesis instead.
(A) 2-Methoxy-2-methylpropane, \( \ce{CH3-O-C(CH3)3} \) — Cannot be prepared. It is unsymmetrical and has a tertiary alkyl group; tert-butyl alcohol with acid gives 2-methylpropene. (It is made from sodium tert-butoxide and methyl bromide.)
(B) 2-Methoxy propane, \( \ce{CH3-O-CH(CH3)2} \) — Cannot be prepared. It is unsymmetrical and has a secondary alkyl group.
(C) 1-Propoxypropane, \( \ce{CH3CH2CH2-O-CH2CH2CH3} \) — Can be prepared. It is symmetrical with two primary groups:
\( \displaystyle \ce{2CH3CH2CH2OH} \xrightarrow[\text{413 K}]{\ce{H2SO4}} \ce{CH3CH2CH2-O-CH2CH2CH3 + H2O} \)
(D) 2-Ethoxy butane, \( \ce{CH3CH2-O-CH(CH3)CH2CH3} \) — Cannot be prepared. It is unsymmetrical and has a secondary alkyl group.
NCERT (2026-27 edition):
By Dehydration of Alcohols – …
