See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Optical Isomerism Analysis **Optical isomerism requires a chiral center** — a carbon atom bonded to four different groups. **Examining each option:** **(1)** $HC \equiv C - \underset{Cl}{\underset{|}{C}} - C \equiv CH$ - Central carbon has: $H$, $Cl$, $C \equiv C$, $C \equiv C$ - Two groups are identical ($C \equiv C$ on both sides) → **Not chiral** **(2)** $HC \equiv C - \underset{Cl}{\underset{|}{C}} - CH_3$ ✓ - Central carbon bonded to: $H$, $Cl$, $HC \equiv C$, $CH_3$ - **All four groups are different** → **Chiral center exists** - This molecule can exist as $d$ and $l$ enantiomers → **Shows optical isomerism** **(3)** $HC \equiv C - \underset{Cl}{\underset{|}{C}} - H$ - Central carbon has: $H$ and $H$ (two identical groups) - **Not chiral** **(4)** $HC \equiv C - \underset{Cl}{\underset{|}{C}} - CH_2$ - Central carbon bonded to: $Cl$, $C \equiv C$, and $CH_2$ - Only three groups (missing fourth) → **Not chiral** **Answer: (2)** is the only molecule with a stereogenic center bearing four different substituents, making it optically active.