MyCompetitiveExams Dynamic 180-question full-length simulator mixed dynamically from all 6 years for 2027 & 2026 aspirants.
Which amongst the following will be most readily dehydrated under acidic conditions?
Acid-catalysed dehydration proceeds via a carbocation intermediate (E1). The nitro group (–NO₂) is a ferocious electron-withdrawing group (–I, –M) that severely destabilizes adjacent positive charge, raising the activation energy drastically. Options (1), (2), and (4) ALL contain a strongly destabilizing –NO₂ group, whereas Option (3) completely lacks –NO₂! Option (3) dehydrates fastest in 5 seconds!
Acid-catalysed dehydration of alcohols occurs via an E1 elimination pathway. Protonation of the hydroxyl group yields an alkyloxonium ion, which loses water in the rate-determining step to generate a carbocation intermediate:
The rate of dehydration is directly proportional to the thermodynamic stability of the intermediate carbocation. A nitro group (–NO₂) is an extremely powerful electron-withdrawing group via both inductive (–I) and resonance (–M) effects. Placing a positive charge near –NO₂ results in severe electrostatic destabilization:
Options (1), (2), and (4) all possess a strongly electron-withdrawing –NO₂ group on the carbon skeleton. In contrast, Option (3) possesses no –NO₂ group; its carbocation is stabilized by adjacent alkyl electron donation (hyperconjugation and +I effect):
Diagnose every choice: Know exactly why the right option works and why distractors fail.
Contains a strong electron-withdrawing –NO₂ group which destabilizes the carbocation intermediate.
The –NO₂ group is in close proximity to the hydroxyl carbon, severely retarding ionization.
Accurate: Free from the strongly destabilizing electron-withdrawing –NO₂ group, allowing unhindered carbocation formation and rapid dehydration.
Contains an –NO₂ group and a primary hydroxyl group, giving the lowest dehydration rate.
Students often only look at alcohol degree (1° vs 2°) and forget that an electron-withdrawing group like –NO₂ near a carbocation increases the activation energy barrier so dramatically that dehydration becomes sluggish.
"The ease of dehydration follows carbocation stability; electron-withdrawing groups strongly destabilize carbocation intermediates and retard dehydration."
Test your speed and accuracy in the authentic 200-minute timed NTA simulator.
Spotted a mistake in this solution? Report an error (support@mycompetitiveexams.com)
Cover all 1,140 official NEET questions in 285 days at your own pace. Progress is saved on your device. No login needed.