Which statement describes a trans isomer?

Which statement describes a trans isomer?

• A. On the same side of the double bond
• B. On opposite sides of the double bond
• C. On the same carbon
• D. In a ring system

Answer: (B)

Geometric isomerism around a carbon–carbon double bond arises because rotation is restricted, so two different spatial arrangements can occur when each carbon bears two different substituents. The trans form means the two substituents of interest lie on opposite sides of the plane of the double bond. This opposite-side arrangement is the defining feature of a trans isomer. The other descriptions don’t match this situation: placing the groups on the same side is the cis form, having both groups on the same carbon isn’t a meaningful descriptor for a double-bond stereoisomer, and being in a ring system refers to ring cis/trans relationships rather than a simple alkene. For example, trans-2-butene has the two methyl groups across from each other.

Geometric isomerism around a carbon–carbon double bond arises because rotation is restricted, so two different spatial arrangements can occur when each carbon bears two different substituents. The trans form means the two substituents of interest lie on opposite sides of the plane of the double bond. This opposite-side arrangement is the defining feature of a trans isomer. The other descriptions don’t match this situation: placing the groups on the same side is the cis form, having both groups on the same carbon isn’t a meaningful descriptor for a double-bond stereoisomer, and being in a ring system refers to ring cis/trans relationships rather than a simple alkene. For example, trans-2-butene has the two methyl groups across from each other.