Which statement describes a cis isomer?

Which statement describes a cis isomer?

• A. On opposite sides of the double bond
• B. On different double bonds
• C. On the same side of the double bond
• D. On the same carbon

Answer: (C)

Cis isomerism describes a situation where two substituents attached to the carbons of a C=C bond lie on the same side of the bond. The double bond cannot rotate, so once the arrangement is fixed, those substituents stay on the same side, giving the cis form, as opposed to the trans form where they lie on opposite sides. The description “on the same side of the double bond” precisely captures this arrangement. Other statements either describe arrangements on different parts of the molecule or involving different bonds, which do not define a cis isomer.

Cis isomerism describes a situation where two substituents attached to the carbons of a C=C bond lie on the same side of the bond. The double bond cannot rotate, so once the arrangement is fixed, those substituents stay on the same side, giving the cis form, as opposed to the trans form where they lie on opposite sides. The description “on the same side of the double bond” precisely captures this arrangement. Other statements either describe arrangements on different parts of the molecule or involving different bonds, which do not define a cis isomer.