2-hexyne give trans -2-hexene on treatment with 

Solution

To convert 2-hexyne to trans-2-hexene, you need a stereoselective reduction method that preferentially forms the trans-alkene. The Li / Liq. NH₃ system (a form of Birch Reduction) is specifically designed for this purpose. Here’s the detailed mechanism:

1. Electron Addition

Lithium metal reacts with liquid ammonia, generating solvated electrons (e-), which are highly reactive and capable of attacking the π-system of the alkyne.

R-C ≡ C-R + e^- → R-C^* = C^-R

2. Radical Anion Formation

The π-electrons of 2-hexyne interact with the solvated electron to form a radical anion intermediate.

3. First Protonation

Ammonia donates a proton (H+) to the radical anion, forming a radical intermediate.

R-C^* = C^-R + NH₃ → R-CH = C^*R

4. Second Electron Addition

Another solvated electron adds to the radical intermediate, converting it into a carbanion.

5. Second Protonation

A second ammonia molecule donates another proton, resulting in the formation of trans-2-hexene as the product.

R-CH = C^*R + NH₃ → R-CH = CHR

Why trans-2-Hexene is Formed

The Birch Reduction is stereoselective and minimizes steric hindrance. The trans-product (trans-2-hexene) is more stable than the cis-product due to reduced repulsion between substituents.

Final Reaction

CH3CH2C ≡ CCH2CH3 →{Li / Liq. NH3} trans-CH3CH2CH = CHCH2CH3