Answer
🧪 Conversion of Phenylacetylene to trans-Styrene
Reaction Overview
C₆H₅–C≡CH → C₆H₅–CH=CH (trans)
🔬 Strategy
The conversion is achieved in two sequential steps:
- Step A: Partial hydrogenation of the alkyne to form the cis-alkene
- Step B: Isomerization of the cis-alkene to trans-alkene
Step A: Partial Hydrogenation to cis-Alkene
Reagents: H₂ / Lindlar’s Catalyst
Lindlar’s catalyst allows syn-addition of hydrogen across the alkyne to stop at the cis-alkene stage.
C₆H₅–C≡CH → C₆H₅–CH=CH (cis)
Step B: Isomerization to trans-Alkene
Reagents: Na / NH₃ (liquid ammonia)
A dissolving metal reduction converts the cis-alkene to the trans-isomer through a radical anion intermediate.
C₆H₅–CH=CH (cis) → C₆H₅–CH=CH (trans)
✅ Reagents Summary
| Step | Transformation | Reagents Used |
|---|---|---|
| A | Alkyne → cis-Alkene | H₂ / Lindlar’s Catalyst |
| B | cis-Alkene → trans-Alkene | Na / NH₃ (liq.) |
🧠Summary
- Use H₂ with Lindlar’s catalyst for selective syn-addition to form the cis-alkene.
- Follow with sodium in liquid ammonia to convert the cis-alkene to a stable trans-alkene.
- This sequence provides high stereoselectivity for the trans-styrene product.