This question involves identifying the correct reagent for reducing an alkyne to a trans-alkene. The transformation shown is from an alkyne (R-C≡C-R') to an alkene with the hydrogens on opposite sides (trans configuration).

The key concept here is stereoselective reduction. Different reducing agents give different stereochemical outcomes:

• Catalytic hydrogenation (e.g., H₂/Pd) gives fully saturated alkane.
• Lindlar's catalyst (H₂/Pd-BaSO₄, quinoline) gives cis-alkene.
• Dissolving metal reduction gives trans-alkene.

The dissolving metal reduction uses alkali metals (Na or Li) in liquid ammonia (NH₃). This reaction proceeds via a radical anion mechanism that favors the formation of the more stable trans-alkene.

The complete reaction is:

$R-C\equiv C-R^{\prime }\overrightarrow{[1.\mathrm{Na},\mathrm{liq}.{\mathrm{NH}}_3;2.{\mathrm{NH}}_4\mathrm{Cl}]}R-C=C-R^{\prime }\left(\mathrm{trans}\right)$

Now, let's evaluate the options:

1. Na, Liq. NH₃: This is the correct dissolving metal reduction for trans-alkene formation.
2. Zn-Hg/HCl: This is Clemmensen reduction, used for reducing carbonyl groups (C=O) to CH₂, not for alkynes.
3. NH₂NH₂, $\stackrel{⊝}{O}H$: This is Wolff-Kishner reduction, also used for carbonyl reduction (C=O to CH₂), not for alkynes.
4. NaBH₄: This is a mild reducing agent for aldehydes and ketones (reduces C=O to alcohol), ineffective for reducing alkynes.

Therefore, the correct reagent is Na, Liq. NH₃.

Related Topics

Reduction Reactions in Organic Chemistry: Different functional groups require specific reducing agents. Understanding the selectivity of reagents (which groups they reduce and the stereochemistry) is crucial in organic synthesis.

Alkyne Chemistry: Alkynes can undergo addition reactions (hydrogenation, halogenation, hydrohalogenation) and can be reduced to alkenes or alkanes with control over stereochemistry.

Important Formulae/Reactions

Dissolving Metal Reduction:

$\mathrm{Alkyne}\to \mathrm{trans}-\mathrm{Alkene}\text{\hspace{0.17em}\hspace{0.17em}using\hspace{0.17em}\hspace{0.17em}}\mathrm{Na}/{\mathrm{NH}}_3\left(l\right)$

Catalytic Hydrogenation to cis-Alkene:

$\mathrm{Alkyne}\to \mathrm{cis}-\mathrm{Alkene}\text{\hspace{0.17em}\hspace{0.17em}using\hspace{0.17em}\hspace{0.17em}}{H}_2,\mathrm{Lindlar}\prime s\text{\hspace{0.17em}\hspace{0.17em}catalyst}$