2-Bromobut-2-ene can exist as cis and trans stereoisomers. Explain why 2-bromobut-2-ene can form cis and trans isomers, and describe how the cis and trans isomers of 2-bromobut-2-ene differ from each other.

2‑Bromobut‑2‑ene contains a C=C double bond, so rotation about the bond is restricted.

Each sp² carbon of the double bond is bonded to two different groups – a methyl group and a hydrogen on one carbon, and a bromine and a propyl group on the other.

Because of the lack of rotation, the relative positions of the two larger groups (methyl and propyl) are fixed. If the two larger groups are on the same side of the double bond, the molecule is the cis isomer; if they are on opposite sides, it is the trans isomer.

Thus the cis and trans isomers differ in the spatial arrangement of the larger substituents across the double bond, giving distinct physical and chemical properties.