A Level Chemistry OCR Practice Exam

Which of the following describes the cis isomer in cis-trans isomerism?

1. Hydrogen atoms are on opposite sides of a double bond

2. Hydrogen atoms are on different sides of a double bond

3. Hydrogen atoms are on the same side of a double bond

4. Hydrogen atoms are absent from the molecule

The correct answer is: Hydrogen atoms are on the same side of a double bond

In cis-trans isomerism, the cis isomer is characterized by having substituents, such as hydrogen atoms, on the same side of a double bond. This spatial arrangement influences the physical and chemical properties of the compound, such as boiling points and solubility. The term "cis" comes from the Latin word meaning "on this side," indicating that the same groups or atoms are located on one side of the molecule. For instance, in a simple alkene with a double bond between two carbon atoms, if both hydrogen atoms attached to the carbons are aligned on the same side of the double bond, the isomer is classified as the cis form. This often leads to different interactions and reactivity compared to the trans isomer, where the hydrogen atoms would be on opposite sides of the double bond. Thus, the definition of cis isomerism accurately pertains to the arrangement of hydrogen atoms or substituents being on the same side of the double bond, which is why this statement is recognized as correct.