Cis-trans isomerism is a type of stereoisomerism. Explain what is meant by the term stereoisomers and describe the structural feature required for a molecule to show cis-trans isomerism. Use 2-bromobutene as an example to illustrate your answer.

Stereoisomers are compounds that have the same molecular formula, the same atoms bonded in the same order and the same structural formula but differ in the spatial arrangement of their atoms.

Cis‑trans isomerism occurs when a C=C double bond is present. The double bond restricts rotation, so the groups attached to each sp² carbon cannot freely rotate.

For cis‑trans isomerism each carbon of the double bond must bear two different groups. In 2‑bromobutene the double bond is between C2 and C3. C2 is bonded to H and Br, while C3 is bonded to H and CH₃. Because each carbon has two different substituents, the molecule can exist as a cis isomer (Br and CH₃ on the same side) and a trans isomer (Br and CH₃ on opposite sides).