Question

An alkene, \(X\) \((\mathrm{C_5H_{10}})\) exhibits cis/trans isomerism. Bromination of \(X\) followed by reaction with reagent(s) \(Y\) gave the product \(Z\). What are \(Y\) and \(Z\)?

• A. \(\mathrm{NaNH_2}\) ; \((\mathrm{CH_3})_2\mathrm{CHC\equiv CH}\)
• B. \(\mathrm{KOH(aq)}\) ; \(\mathrm{CH_3CH_2CH_2C\equiv CH}\)
• C. alc.\(\mathrm{KOH,\ NaNH_2}\) ; \(\mathrm{CH_3CH_2C\equiv CCH_3}\)
• D. alc.\(\mathrm{KOH,\ NaNH_2}\) ; \(\mathrm{CH_3CH_2CH_2C\equiv CH}\)

Hint:

Vicinal dibromoalkanes on treatment with \[ \boxed{\text{alc. KOH followed by } \mathrm{NaNH_2}} \] undergo double dehydrohalogenation to give alkynes.

Answer 1

The Correct Option is C

Step 1: Identify the alkene. Among the alkenes with molecular formula \[ \mathrm{C_5H_{10}}, \] the compound showing cis/trans isomerism is \[ \boxed{\mathrm{Pent\mbox{-}2\mbox{-}ene}.} \]

Step 2: Bromination followed by dehydrohalogenation. Pent-2-ene undergoes bromination to give \[ \mathrm{2,3\mbox{-}dibromopentane.} \] Treatment with alcoholic KOH followed by \[ \mathrm{NaNH_2} \] causes double dehydrohalogenation to form the alkyne \[ \mathrm{CH_3CH_2C\equiv CCH_3} \] (pent-2-yne).

Step 3: Write the answer. Hence, \[ Y=\text{alc. }\mathrm{KOH,\ NaNH_2}, \] and \[ Z=\mathrm{CH_3CH_2C\equiv CCH_3}. \] Thus, \[ \boxed{(C)} \] is the correct answer.