Organic Chemistry Key Concepts: Chirality
-A chiral object is any natural object which has no plane of symmetry. An achiral object is one with a
plane of symmetry. All chiral objects are NOT superimposable on their mirror images. Conversely, all
achiral objects are superimposable on their mirror images.
-The most common source of chirality in a molecule is a stereocentre, usually an atom such as carbon
bonded to 4 different groups or substituents. All achiral molecules contain a plane of symmetry
(generally true). The two mirror image forms of a chiral molecule are stereoisomers known as
enantiomers. Enantiomers (stereoisomers) are different compounds (they do not interconvert at room
temperatures) and their chemistry is exactly the same. They only differ in how they affect plane-
polarized light and how they react with other chiral compounds. Stereocentres are commonly indicated
with a * in a 3D molecular drawing.
-The R, S convention is utilized through the application of the same rules used for E, Z isomers. In order
to assign a configuration label to an enantiomer: i) draw the molecule with the lowest priority group (#4)
behind the plane. ii) draw an arrow from the highest to next highest priority atoms. If the rotation is
clockwise, the absolute configuration is R. If the rotation is counter-clockwise, the absolute rotation is S.
-Non-cyclic molecules with > 1 chiral centre are often drawn in Fischer projections.
-Diastereomers are stereoisomers of a chiral compound that ar