● Stereoisomers are sub-divided into two classes: conformational isomers, which interconvert by
rotation about an bond, and confi gurational isomers, which interconvert only by breaking and
reforming a bond.
● Confi gurational isomers are further sub-divided into cis–trans and E/Z isomers and optical isomers.
● Cis–trans isomers can occur in alkenes or cycloalkanes (or hetero-analogues) and differ in the
positions of atoms (or groups) relative to a reference plane. According to IUPAC, E/Z isomers
refer to alkenes of the form R1R2C=CR3R4 (R1 R2, R3 R4) where neither R1 nor R2 need be
different from R3 or R4.
● A chiral carbon is a carbon joined to four different atoms or groups.
● An optically active compound can rotate the plane of polarized light as it passes through a solution
of the compound. Optical isomers are enantiomers.
● Enantiomers are non-superimposeable mirror images of each other. Diastereomers are not mirror
images of each other.
● A racemic mixture (or racemate) is a mixture of two enantiomers in equal amounts and is optically
● The term geometric isomers as recommended by IUPAC is now obsolete and cis–trans isomers and
E/Z isomers should be encouraged in the teaching programme.
● In the E/Z system, the group of highest Cahn–Ingold–Prelog priority attached to one of the terminal
doubly bonded atoms of the alkene (i.e. R1 or R2) is compared with the group of highest precedence
attached to the other (i.e. R3 or R4). The stereoisomer is Z if the groups lie on the same side of a
reference plane passing through the double bond and perpendicular to the plane containing the
bonds linking the groups to the double-bonded atoms; the other stereoisomer is designated as E.
Application and skills
● Construction of 3-D models (real or virtual) of a wide range of stereoisomers.
● Explanation of stereoisomerism in non-cyclic alkenes and C3 and C4 cycloalkanes.
● Comparison between the physical and chemical properties of enantiomers.
● Description and explanation of optical isomers in simple organic molecules.
● Distinction between optical isomers using a polarimeter.
Wedge-dash type representations involving tapered bonds should be used for representations of optical