Alcohols Phenols And Ethers

Conversion of Cyclohexanol to Cyclohexene

Step 1: Alcohol to Alkyl Bromide (PBr₃ Reaction)

Cyclohexanol + PBr₃ → Bromocyclohexane + H₃PO₃

• Mechanism: Nucleophilic substitution (S_N2-like)
  1. Oxygen lone pair attacks phosphorus in PBr₃
  2. Br⁻ leaves as leaving group
  3. New O-P bond forms while O-H bond breaks
  4. Br⁻ attacks backside, displacing the -OPBr₂ group
• Key Features:
  • Converts -OH (poor leaving group) to -Br (excellent leaving group)
  • Proceeds with inversion of configuration at carbon
  • Phosphorous byproduct is phosphorous acid (H₃PO₃)

Step 2: Elimination to Form Alkene (alc. KOH Reaction)

Bromocyclohexane + KOH (alc.) → Cyclohexene + KBr + H₂O

• Mechanism: Bimolecular Elimination (E2)
  1. Strong base (OH⁻) abstracts β-hydrogen
  2. C-Br bond breaks simultaneously
  3. π-bond forms between α- and β-carbons
• Key Features:
  • Requires anti-periplanar arrangement of H and Br
  • Favored in alcoholic solution (prevents competing substitution)
  • Follows Zaitsev's rule - forms more substituted alkene when possible

Why Other Products Don't Form

Critical Observation: This two-step process (alcohol → alkyl halide → alkene) is a classic method for dehydrating alcohols under mild conditions.

Therefore, the correct final product is A (Cyclohexene).

Given that compounds A and B have the same molecular formula, both will likely be alcohols that react with CH MgBr to form alkylated products. Since compound Y undergoes acidic dehydration more readily, this suggests it has a structure that allows for easier elimination of water. When reacting with Lucas reagent, compound Y also reacts more readily, indicating it is a more reactive alcohol.

The Lucas test typically distinguishes between primary, secondary, and tertiary alcohols, with tertiary alcohols reacting most rapidly. Thus, Y must be a more substituted alcohol, which fits the structure of 2-Methylbutan-2-ol, a secondary alcohol. On the other hand, X is a primary alcohol, pentan-2-ol, which reacts less readily under the same conditions.

Thus, the correct identification for X is Pentan-2-ol and for Y is 2-Methylbutan-2-ol.