Question: 84: The incorrect method for the synthesis of alkenes is:
(1) treatment of alkynes with Na in liquid \mathrm{NH}_{3}
(2) heating alkyl halides with alcoholic KOH
(3) treating alkyl halides in aqueous KOH solution
(4) treating vicinal dihalides with Zn metal
Answer: Option (3)
Explanation:
Alkenes are generally prepared by elimination reactions, where small molecules like
\mathrm{H}_{2} or \mathrm{HX} are removed to form a double bond.
In option (1), alkynes treated with sodium in liquid ammonia undergo partial reduction to form alkenes.
Hence, this is a correct method for alkene synthesis.
In option (2), heating alkyl halides with alcoholic KOH leads to beta-elimination (dehydrohalogenation),
resulting in the formation of alkenes. Therefore, this is a correct method.
In option (3), treating alkyl halides with aqueous KOH favors nucleophilic substitution reactions,
producing alcohols instead of alkenes. Elimination does not occur predominantly in aqueous medium.
In option (4), vicinal dihalides react with zinc metal to eliminate halogen atoms and form alkenes.
This is also a correct method.
Thus, the incorrect method for the synthesis of alkenes is treating alkyl halides in aqueous KOH solution.