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.

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.

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.