3.3.7 Optical Isomerism
Stereoisomerism and Chirality
Stereoisomers have the same formula, and the atoms are joined in the same order. However, they have
different special arrangements. Optical isomerism is a form of stereoisomerism.
Optical isomers are formed when a molecule (in this case, organic) contains a chiral carbon. A carbon atom
is chiral if it is bonded to four different molecules or groups. In this case, the four atoms could be arranged
in two different ways within the 3D structure, and no rotation of either molecule will make the structures
identical – they are non-superimposable (mirror images).
This can also be described using chirality. A molecule with a plane of symmetry is achiral, and a molecule
with no plane of symmetry is chiral. The central carbon atom that causes this lack of symmetry is the
chiral centre.
Enantiomers
Optical isomers are named because of their effect non plane polarised light. Simple substances which
display optical isomerism exist as two isomers known as enantiomers.
• A solution of one enantiomer rotates the plane of polarisation in the clockwise direction.
• A solution of the other enantiomer rotates the plane of polarisation in the anti-clockwise direction.
Racemic Mixtures
If a solution contains an exact 50/50 split of enantiomers, then the plane will appear not to rotate, as the
two opposites cancel each other out. This solution is known as a racemic mixture or racemate.
Stereoisomerism and Chirality
Stereoisomers have the same formula, and the atoms are joined in the same order. However, they have
different special arrangements. Optical isomerism is a form of stereoisomerism.
Optical isomers are formed when a molecule (in this case, organic) contains a chiral carbon. A carbon atom
is chiral if it is bonded to four different molecules or groups. In this case, the four atoms could be arranged
in two different ways within the 3D structure, and no rotation of either molecule will make the structures
identical – they are non-superimposable (mirror images).
This can also be described using chirality. A molecule with a plane of symmetry is achiral, and a molecule
with no plane of symmetry is chiral. The central carbon atom that causes this lack of symmetry is the
chiral centre.
Enantiomers
Optical isomers are named because of their effect non plane polarised light. Simple substances which
display optical isomerism exist as two isomers known as enantiomers.
• A solution of one enantiomer rotates the plane of polarisation in the clockwise direction.
• A solution of the other enantiomer rotates the plane of polarisation in the anti-clockwise direction.
Racemic Mixtures
If a solution contains an exact 50/50 split of enantiomers, then the plane will appear not to rotate, as the
two opposites cancel each other out. This solution is known as a racemic mixture or racemate.
