Exam Style Question - 4

Question: Explain why but-2-ene has E-Z isomers but but-1-ene does not. Name the isomer of but-2-ene where the two CH₃ groups are on opposite sides of the double bond.E-Z isomerism needs restricted rotation about a C=C bond and two different groups on each carbon of the double bond.But-1-ene is CH₂=CHCH₂CH₃. Carbon 1 has two hydrogen atoms, which are identical, so swapping them gives the same molecule. It cannot show E-Z isomerism.But-2-ene is CH₃CH=CHCH₃. Each carbon of the C=C has one H and one CH₃, which are different, so two stereoisomers exist.On each carbon, CH₃ has higher priority than H because carbon has a higher atomic number. With the CH₃ groups on opposite sides, the isomer is E-but-2-ene.