Stereochemistry

The essentials

Some compounds rotate the plane of plane-polarised light; they are optically active. Rotation to the right is dextrorotatory (d or +) and to the left is laevorotatory (l or −). Optical activity arises when a molecule is chiral, i.e. not superimposable on its mirror image. The most common cause is an asymmetric (stereogenic) carbon bonded to four different groups.

Key points

• A polarimeter measures optical rotation.
• Enantiomers are non-superimposable mirror images; they have identical physical properties except the direction of rotation of plane-polarised light.
• Racemic mixture: equal amounts of the two enantiomers; optically inactive because the rotations cancel (external compensation). Converting an enantiomer into a racemic mixture is racemisation.
• Achiral molecules are superimposable on their mirror images, e.g. propan-2-ol.
• Examples of chiral molecules: butan-2-ol and 2-bromobutane (C-2 carries H, CH₃, C₂H₅ and OH or Br).
• Retention: the spatial arrangement around the stereocentre is unchanged. Inversion: the arrangement is flipped, as in SN2 reactions.
• In SN1 reactions of an optically active halide, the planar carbocation can be attacked from either side, giving a largely racemic product.

Exam tips

• To spot a chiral carbon, check that all four attached groups differ; two identical groups means not chiral.

Common mistakes

• Saying a racemic mixture is a single compound.
• Confusing d/l signs of rotation with the D/L configuration used for sugars.