SUMMARY OF KEY CONCEPTS

•	A molecule that is not superimposable on its mirror image is said to be chiral.

•	A chiral molecule is one that does not contain a plane of symmetry. The most common cause of chirality is the presence of a tetrahedral sp3-hybridised carbon atom bonded to four different groups, this is referred to as a stereogenic center.

•	Compounds that contain such stereogenic centers exist as a pair of non-superimposable mirror image stereoisomers called enantiomers.

•	Enantiomers are identical in all physical properties except for the direction in which they rotate the plane of polarised light.

•	The configuration of a stereogenic center can be described as either R or S by applying sequence rules.

•	Diastereoisomers are stereoisomers that are not mirror images. Diastereoisomers have different spectra and physical properties.

•	Some molecules have more than one stereogenic center. Enantiomers have opposite configuration at all stereogenic centers, whereas diastereoisomers have the same configuration in at least one center but opposite configurations at the other(s).

•	Compounds that contain stereogenic centers but are achiral (due to a symmetry plane) are called meso compounds.

•	Molecules can also display optical activity due to other structural features, including: quadrivalent chiral atoms other than carbon, tervalent chiral atoms, restricted rotation about a particular bond, and helical shape.