The reactions which cannot be applied to prepare an alkene by elimination, are
Choose the correct answer from the options given below:

Solution

Chemistry ma’am · 2 steps

Ask what each reagent really does to that substrate: eliminate, substitute, or oxidise.

1. What elimination needs. An alkene forms by elimination when a group leaves along with an H from the next carbon. That needs a leaving group, a β-hydrogen and conditions that favour elimination over substitution or oxidation.

   Reaction	What actually happens	Alkene?
   A: 1-bromo-2-methylcyclohexane, NaOEt	Strong base in alcohol gives E2 dehydrohalogenation to methylcyclohexene	Yes
   B: 2-bromobutane, KOH (aq)	Aqueous OH⁻ acts as a nucleophile, so substitution gives butan-2-ol	No
   C: (CH₃)₃CBr, NaOMe	The tertiary carbon blocks S_N2, so the strong base removes a β-H by E2 and gives 2-methylpropene	Yes
   D: phenol, Na₂Cr₂O₇/H₂SO₄	Oxidation of the ring gives benzoquinone, not an elimination product	No
   E: (CH₃)₃COH, Cu, 573 K	A tertiary alcohol has no H on the carbinol carbon, so it cannot be dehydrogenated. It dehydrates instead and gives 2-methylpropene	Yes

2. So. Only B and D fail to give an alkene, so the answer is (d).

The idea to keep. Alcoholic KOH or alkoxide gives elimination, while aqueous KOH gives substitution. Over hot Cu, primary and secondary alcohols are dehydrogenated, but tertiary alcohols dehydrate to alkenes.

Answer from the NTA final answer key. Solution by PrepWiser.