Stereoisomerism

Stereoisomers have the same structural formula (the atoms are bonded in the same order) but a different arrangement of the atoms in space.At AS level the type met is geometric isomerism, which occurs in some alkenes.A C=C double bond is made of a σ bond and a π bond. The π bond is formed by sideways overlap of p orbitals above and below the plane of the molecule. Rotating one carbon relative to the other would break this overlap, so there is restricted rotation about the C=C bond. Groups attached to the double bond are fixed on one side or the other.Geometric isomers exist when each carbon of the C=C carries two different groups.Example: but-2-ene, CH₃CH=CHCH₃. Each carbon of the double bond has an H and a CH₃. In cis-but-2-ene the two CH₃ groups are on the same side; in trans-but-2-ene they are on opposite sides.The two isomers cannot turn into each other at room temperature and have slightly different physical properties, such as boiling point.No isomers: but-1-ene, CH₂=CHCH₂CH₃, has two H atoms on carbon 1, so swapping them gives the same molecule. 2-methylpropene, (CH₃)₂C=CH₂, has two identical groups on each carbon, so it has none either.Cis and trans only work when the same group appears on both carbons. The next lesson shows the E-Z system, which names every case.