See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Solution: Cis-Trans Isomerism Analysis **Understanding the requirement:** Cis-trans isomerism occurs when there is a **double bond with two different groups on each carbon atom**, preventing free rotation. ## Analysis of each option: **(A) 2-methyl-2-heptene** $$\text{Structure: } CH_3-C(CH_3)=CH-CH_2-CH_2-CH_2-CH_3$$ The double bond carbons are: $C(CH_3)=CH$ - Left carbon has: $CH_3$ and $CH_3$ (same groups) - Since the left carbon has **two identical methyl groups**, cis-trans isomerism **cannot exist** - The two methyl groups cannot be distinguished as "cis" or "trans" **(B) 1,2-dimethylcyclopentane** — The cyclic structure creates stereoisomerism (cis/trans configurations). **(C) 2-heptene** — Structure $CH_3CH=CHCH_2CH_2CH_2CH_3$ has different groups on both carbons of the double bond → **shows cis-trans isomerism**. **(D) Methylcyclopropene** — The double bond has different groups on each carbon → **shows cis-trans isomerism**. ## Answer: **(A) 2-methyl-2-heptene** **Why:** The presence of two identical methyl groups on one carbon of the double bond violates the requirement for cis-trans isomerism, which needs at least two different groups on each doubly-bonded carbon.