Read the passage carefully and answer the Questions.
Ethers with the general formula \( C_nH_{2n+2}O \) and functional group C-O-C may be divided as symmetrical and unsymmetrical. Lower symmetrical ether may be obtained by the dehydration of alcohols at low temperatures in the presence of protic acids, such as \( H_2SO_4 \) and \( H_3PO_4 \) whereas at high temperature intramolecular dehydration takes place. Formation of ether from primary alcohols is also accompanied by the formation of small amount of alkenes. Due to the steric hindrance, secondary and tertiary alcohols undergo elimination reaction rather than substitution reaction yielding alkene as major product. Ethers can also be obtained by heating alkyl halides with sodium or potassium alkoxides. Substituted Ethers (secondary & tertiary) can also be prepared using this method. If primary halides are replaced with secondary and tertiary halides, alkenes are the major products.
Major product in the following reaction is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → tert-butyl methyl ether
Williamson synthesis makes an ether from an alkyl halide and a sodium alkoxide. The alkoxide ion attacks the carbon carrying the halogen from the side opposite to the halogen and pushes out the halide ion in one step, an \(S_N2\) reaction:
\( \displaystyle \ce{R-X + R'-O^- Na+ -> R-O-R' + NaX} \)
The halide must be primary and unhindered; the alkoxide may be bulky. Here the halide is methyl bromide, the least hindered of all, and the bulky tert-butyl group is on the alkoxide side, so substitution goes smoothly:
\( \displaystyle \ce{(CH3)3C-O^- Na+ + CH3-Br -> (CH3)3C-O-CH3 + NaBr} \)
Methyl bromide has no β-hydrogen, so elimination is not possible. The major product is tert-butyl methyl ether (2-methoxy-2-methylpropane), the structure in Option (4). Q7 of this paper uses the same two groups the other way round, tert-butyl bromide with sodium methoxide, and there elimination gives only 2-methylpropene.
Why the other options are incorrect
Option (1) tert-butyl alcohol — Incorrect. The alcohol would form only if the alkoxide took a proton from water; no water or acid is present, so the alkoxide acts as a nucleophile on methyl bromide.
Option (2) 3-methylbut-1-ene — Incorrect. An alkene would need a β-hydrogen on the halide; methyl bromide has none, and this five-carbon alkene does not have the carbon skeleton of the reactants.
Option (3) 2,2-dimethylpropane — Incorrect. This needs a C–C bond between the two groups; the alkoxide oxygen, not a carbon atom, attacks methyl bromide, so a C–O–C link is formed.
NCERT (2026-27 edition):
Williamson Synthesis
