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Chemistry Question 58 – NEET-UG 2025

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Which one of the following compounds can exist as cis-trans isomers?

Cis-trans isomerism (geometric isomerism) arises due to restricted rotation around a bond or within a ring structure.

Step 1: Understand Cis-Trans Isomerism Requirements✦ Active

Cis-trans isomerism (geometric isomerism) occurs in compounds with restricted rotation (e.g., double bonds or cyclic structures). A key condition is that each carbon atom involved in the restricted rotation must be bonded to two *different* groups.

Step 2: Evaluate Alkenes and 1,1-Disubstituted Cycloalkanes○ Expand

Let's examine the given options:

(1) Pent-1-ene (CH2=CH−CH2−CH2−CH3): The first carbon of the double bond (C1) is bonded to two identical hydrogen atoms. Therefore, it cannot show cis-trans isomerism.

(2) 2-Methylhex-2-ene (CH3−C(CH3)=CH−CH2−CH2−CH3): The second carbon of the double bond (C2) is bonded to two identical methyl (CH3) groups. Therefore, it cannot show cis-trans isomerism.

(3) 1,1-Dimethylcyclopropane: Both methyl groups are attached to the *same* carbon atom in the cyclopropane ring. For cis-trans isomerism in cyclic compounds, substituents must be on *different* ring carbons, and each substituted carbon must bear two different groups. Since both methyl groups are on the same carbon, this compound cannot exhibit cis-trans isomerism.

💡 Teacher's Secret Hint

Remember that for alkenes, if either carbon of the double bond has two identical substituents, cis-trans isomerism is not possible.

Step 3: Evaluate 1,2-Dimethylcyclohexane○ Expand

(4) 1,2-Dimethylcyclohexane: This is a cyclic compound with methyl groups on adjacent carbons (C1 and C2). Carbon C1 is bonded to a methyl group and a hydrogen atom (and two ring carbons). Carbon C2 is also bonded to a methyl group and a hydrogen atom (and two ring carbons). Since both C1 and C2 are bonded to two different groups (methyl and hydrogen), and they are on different carbons in a ring, cis-trans isomerism is possible. We can have *cis*-1,2-dimethylcyclohexane (both methyl groups on the same side of the ring plane) and *trans*-1,2-dimethylcyclohexane (methyl groups on opposite sides).

Thus, 1,2-Dimethylcyclohexane can exist as cis-trans isomers.

💡 Teacher's Secret Hint

Cyclic compounds with two different substituents on two different ring carbons can exhibit cis-trans isomerism.

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