Topic 20: Organic chemistry - 20.3 Stereoisomerism

Nature of science:

Transdisciplinary-the three-dimensional shape of an organic molecule is the foundation pillar of its structure and often its properties. Much of the human body is chiral. 4.1


Essential idea: Stereoisomerism involves isomers which have different arrangements of atoms in space but do not differ in connectivity or bond multiplicity (ie whether single, double or triple) between the isomers themselves

Stereoisomers are subdivided into two classes - conformational isomers, which interconvert by rotation about a s bond and configurational isomers that interconvert only by breaking and reforming a bond. Configurational isomers are further subdivided into cis-trans and E/Z isomers and optical isomers.

Cis-trans isomers can occur in alkenes or cycloalkanes (or heteroanalogues) 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 does not equal R2, R3 does not equal 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 inactive.

Applications 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.


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 (ie R1 or R2) is compared with the group of highest precedence attached to the other (ie 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.

Wedge-dash type representations involving tapered bonds should be used for representations of optical isomers.