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 1
The correct answer is Option (1) → 2-methylpropene
In Williamson synthesis an alkoxide ion displaces a halide ion by an \(S_N2\) attack, giving an ether. An alkoxide is a good nucleophile, but it is also a strong base. With a primary halide substitution wins. With a tertiary halide the back-side attack on carbon is blocked by the three methyl groups, so the alkoxide removes a β-hydrogen instead, and elimination gives an alkene.
Here the halide is tert-butyl bromide, a tertiary halide. The methoxide ion pulls off a hydrogen from one of the methyl groups, a C=C bond forms and bromide leaves:
\( \displaystyle \ce{(CH3)3C-Br + CH3O^- Na+ -> CH2=C(CH3)2 + CH3OH + NaBr} \)
No ether is formed. The major product is 2-methylpropene, the structure in Option (1).
Why the other options are incorrect
Option (2) tert-butyl alcohol — Incorrect. This is the product of hydrolysis of tert-butyl bromide by water or aqueous alkali (an \(S_N1\) reaction); with methoxide in the absence of water no alcohol of this kind is formed.
Option (3) tert-butyl methyl ether — Incorrect. This ether is made by the reverse choice of reagents, sodium tert-butoxide and methyl bromide; with a tertiary halide elimination takes over completely.
Option (4) 2,2-dimethylpropane — Incorrect. This would need a new C–C bond between the tert-butyl and methyl groups, which an alkoxide cannot form.
NCERT (2026-27 edition):
Williamson Synthesis – …
