See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Analysis of Geometrical Isomerism in Dichloropentenes Geometrical (cis-trans) isomerism requires a **C=C double bond with two different groups attached to each carbon**. **Option (1): 1,2-dichoro-1-pentene** - Structure: $CH_2=C(Cl)-CHCl-CH_2-CH_3$ - C1 of double bond: has $H$ and $Cl$ (different) - C2 of double bond: has $Cl$ and $-CH(Cl)CH_2CH_3$ (different) - ✓ **Shows geometric isomerism** (cis/trans possible) **Option (2): 1,3-dichloro-2-pentene** - Structure: $ClCH_2-CH=C(Cl)-CH_2-CH_3$ - C2 of double bond: has $-CH_2Cl$ and $CH_2CH_3$ (different) - C3 of double bond: has $Cl$ and $CH_2CH_3$ (different) - ✓ **Shows geometric isomerism** (cis/trans possible) **Option (3): 1,1-dichloro-1-pentene** - Structure: $CCl_2=CH-CH_2-CH_2-CH_3$ - C1 of double bond: has **two identical $Cl$ atoms** attached - ✗ **Cannot show geometric isomerism** (both groups on C1 are the same) **Option (4): 1,4-dichloro-2-pentene** - Structure: $ClCH_2-CH=CH-CH(Cl)-CH_3$ - C2 and C3 of double bond each have $H$ and carbon groups (different) - ✓ **Shows geometric isomerism** (cis/trans possible) **Answer: (3) 1,1-dichloro-1-pentene does NOT show geometrical isomerism** because the double bond carbon bearing both chlorines lacks the required two different substituents.