JEE Main · 2023 · Shift-IImediumALCO-041

Incorrect method of preparation for alcohols from the following is:

Alcohols, Phenols & Ethers · Class 12 · JEE Main Previous Year Question

Question

Incorrect method of preparation for alcohols from the following is:

Options
  1. a

    Ozonolysis of alkene

    ✓
  2. b

    Reaction of ketone with \ceRMgBr\ce{RMgBr} followed by hydrolysis

  3. c

    Hydroboration-oxidation of alkene

  4. d

    Reaction of alkyl halide with aqueous \ceNaOH\ce{NaOH}

Correct Answera

Ozonolysis of alkene

Detailed Solution

Step 1: Check each method for alcohol synthesis

(a) Ozonolysis of alkene: Ozonolysis (O3/Zn or O3/H2O2) cleaves C=C double bonds to give aldehydes and ketones (carbonyl compounds), NOT alcohols. This is the INCORRECT method for preparing alcohols.

This is the answer ✓

(b) Ketone + RMgBr (Grignard) → hydrolysis: \ceR2CO+RMgBr−>R2C(R)(OMgBr)−>[H2O]R2CR(OH)\ce{R2CO + RMgBr -> R2C(R)(OMgBr) ->[ H2O ] R2CR(OH)} → tertiary alcohol ✓ (valid method)

(c) Hydroboration-oxidation: BH3/H2O2/OH- on alkenes → primary/secondary alcohols ✓ (valid method)

(d) Alkyl halide + aqueous NaOH: \ceR−X+NaOH(aq)−>R−OH+NaX\ce{R-X + NaOH(aq) -> R-OH + NaX} → alcohol ✓ (valid SN2 method)

Step 2: Why ozonolysis gives carbonyls not alcohols

Ozonolysis cleaves the C=C bond:

  • Reductive workup (Zn/AcOH): gives aldehydes and/or ketones.
  • Oxidative workup (H2O2): gives carboxylic acids and/or ketones.
  • Neither gives alcohols.

Key Points to Remember:

  • Ozonolysis → carbonyl compounds (not alcohols).
  • Grignard + carbonyl → alcohol (after hydrolysis).
  • Hydroboration-oxidation → alcohol (anti-Markovnikov).
  • RX + aq. NaOH → alcohol (SN2/SN1).

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