Question

Consider the following reaction: \[ \mathrm{CH_3{-}C{\equiv}C{-}CH_3} \] \[ \xrightarrow[\text{Pd/C}]{H_2} X \qquad \xrightarrow[\text{liq. }NH_3]{Na} Y \] Choose the correct option.

• A. \( X \Rightarrow \mathrm{CH_3CH_2CH_2CH_3} \)
  \hspace*{1.3cm} \( Y \Rightarrow \mathrm{CH_3{-}CH{=}CH{-}CH_3}\ (\text{cis}) \)
• B. \( X \Rightarrow \mathrm{CH_3{-}CH{=}CH{-}CH_3}\ (\text{cis}) \)
  \hspace*{1.3cm} \( Y \Rightarrow \mathrm{CH_3{-}CH{=}CH{-}CH_3}\ (\text{trans}) \)
• C. \( X \Rightarrow \mathrm{CH_3{-}CH{=}CH{-}CH_3}\ (\text{cis}) \)
  \hspace*{1.3cm} \( Y \Rightarrow \mathrm{CH_3{-}CH{=}CH{-}CH_3}\ (\text{cis}) \)
• D. \( X \Rightarrow \mathrm{CH_3{-}CH{=}CH{-}CH_3}\ (\text{trans}) \)
  \hspace*{1.3cm} \( Y \Rightarrow \mathrm{CH_3{-}CH{=}CH{-}CH_3}\ (\text{cis}) \)

Hint:

Remember: \( \mathrm{H_2/Pd{-}C} \Rightarrow \) alkane \( \mathrm{Na/NH_3(l)} \Rightarrow \) trans-alkene (anti-addition)

Answer 1

The Correct Option is A

Concept:

Alkynes undergo different types of reduction depending on the reagent used.
Catalytic hydrogenation with \( \mathrm{H_2/Pd{-}C} \) leads to complete reduction.
Reduction with \( \mathrm{Na/NH_3(l)} \) gives anti-addition, forming a trans-alkene.
Step 1: Nature of the starting compound. \[ \mathrm{CH_3{-}C{\equiv}C{-}CH_3} = \text{2-butyne} \]
Step 2: Reaction with \( \mathrm{H_2/Pd{-}C} \). This reagent causes complete hydrogenation of alkynes to alkanes. \[ X = \mathrm{CH_3CH_2CH_2CH_3} \quad (\text{butane}) \]
Step 3: Reaction with \( \mathrm{Na/NH_3(l)} \). This is a dissolving metal reduction causing anti-addition of hydrogen. \[ Y = \mathrm{CH_3{-}CH{=}CH{-}CH_3}\ (\text{trans-2-butene}) \] Conclusion: \[ X = \text{butane}, \quad Y = \text{trans-2-butene} \]