Optical Isomers

Optical Isomers

  • Optical isomers are a type of stereoisomer that can exist as two non-superimposable mirror image forms, just like left and right hands. These are also known as chiral molecules.
  • This property of being non-superimposable on its mirror image derives from the presence of a chiral centre in a molecule, usually a carbon atom with four different groups attached to it.
  • Each unique form of an optical isomer is termed as an ‘enantiomer’; referring to the two forms as a pair is to refer to their ‘enantiomeric relationship’.
  • Enantiomers rotate plane-polarized light in opposite directions to an equal extent. One form rotates light to the right, or in a clockwise direction, and is known as dextrorotatory (d- or +). The other form rotates light to the left, or in a counter-clockwise direction, and is known as levorotatory (l- or -).

Properties and RaCemates

  • Despite having identical physical and chemical properties in an achiral environment, enantiomers can exhibit different properties when interacting with other chiral molecules or in a chiral environment. This is known as enantiomer-specific interaction.
  • Remarkably, each enantiomer can have completely different smells, tastes, and biological activities. This feature is crucial in the pharmaceutical industry as one enantiomer of a drug may have therapeutic effects whilst the other may be harmful.
  • A racemic mixture is a 50:50 mixture of two enantiomers. It has no effect on plane-polarized light as the rotations of the two enantiomers cancel each other out.
  • Racemates are said to be optically inactive as their overall rotation on plane-polarized light is zero.

Naming and Identifying Optical Isomers

  • The spatial arrangement of different groups around a chiral centre can be presented using wedge and dash notation, which represents bonds projecting out of or into the paper plane, respectively.
  • Another convention used to distinguish between enantiomers is the R/S system. In this system, the different groups around the chiral centre are assigned priority based on atomic numbers, and the arrangement is assessed to be either clockwise (R - Rectus) or counterclockwise (S - Sinister).
  • Key to mastering optical isomers is understanding how to identify potential chiral centres within a molecule, and predicting the optical activity or the lack thereof accurately.

Separation of Enantiomers

  • Separating a racemic mixture into individual enantiomers is a complex process called chiral resolution. Various methods can be employed for chiral resolutions including crystallisation, the use of chiral solvents, or enzymatic resolution.
  • It is often ideal to synthesise chiral molecules in a manner that favours the production of one enantiomer over the other, in a process known as enantioselective synthesis. This approach is more practical, economical, and environmentally friendly than synthesising a racemate and separating it later.