The incorrect method for the synthesis of alkenes is
- Treating vicinal dihalides with Zn metal
- Treating of alkynes with Na in liquid NH3
- Heating alkyl halides with alcoholic KOH
- Treating alkyl halides in aqueous KOH solution
Answer
(4) Treating alkyl halides in aqueous KOH solution
Solution
(1) Vicinal dihalide + Zn alkene + ZnX2 (dehalogenation) — gives alkene.
(2) Alkyne + Na in liquid NH3 trans-alkene (partial reduction) — gives alkene.
(3) Alkyl halide + alcoholic KOH, heat alkene (dehydrohalogenation, β-elimination) — gives alkene.
(4) As per NCERT, aqueous KOH supplies OH- as a nucleophile and replaces the halogen: R-X + KOH(aq) R-OH + KX — gives an alcohol, not an alkene.
∴ Method (4) is incorrect.
Answer: (4)