The following structures are two cis/trans of pent-2-ene. Which is the cis isomer?

In the context of alkenes, the terms "cis" and "trans" refer to the relative positioning of substituents around the double bond. For pent-2-ene, the double bond is located between the second and third carbon atoms.

Identification of Cis and Trans Isomers

1. Cis Isomer: The substituents (in this case, the alkyl groups) attached to the double bond are on the same side.
2. Trans Isomer: The substituents are on opposite sides of the double bond.

To identify which structure is the cis isomer of pent-2-ene:

• You need to look for the arrangement of groups attached to the double-bonded carbons (C2 and C3).
• The structure that has the two methyl (–CH₃) groups on the same side of the double bond is the cis isomer.

Summary

In the case of pent-2-ene, the structure where the two methyl groups are positioned on the same side of the double bond is the cis isomer. The other structure, where they are on opposite sides, is the trans isomer.