JEE Main 2025 Apr 7 Shift 1, Chemistry Q20: Preparation of Alkenes

Analyse each reaction to decide whether elimination occurs:

A. Bromocyclohexane (2${}^{\circ }$ alkyl halide) with NaOEt (a strong, moderately bulky base) favours E2 elimination $\Rightarrow$ gives cyclohexene. Alkene formed.

B. 2-Bromobutane with aqueous KOH: aqueous conditions favour the nucleophilic substitution pathway (S${}_N$2), giving butan-2-ol. Alcoholic KOH would give the alkene, but aqueous KOH does not. No alkene.

C. tert-Butyl bromide is 3${}^{\circ }$; with NaOMe (strong base) it undergoes E2 readily to give isobutylene (2-methylpropene). Alkene formed.

D. Phenol with acidic Na${}_2$Cr${}_2$O${}_7$ is an oxidation, not an elimination. The product is benzoquinone (1,4-benzoquinone), not an alkene by elimination. No alkene by elimination.

E. tert-Butyl alcohol passed over copper at 573 K: with a 3${}^{\circ }$ alcohol, Cu catalyses dehydration (not dehydrogenation) to give the alkene 2-methylpropene. Alkene formed.

Reactions that do not give an alkene by elimination: B and D.

$\therefore$ Option (4).