Optical isomerism

Optical isomers are hard to conceptualise.

They are molecules with the same molecular formula and structural formula. They look exactly the same both on paper and in the real world.

However, they only exist in pairs, it is only possible to get two optical isomers of the same molecule which makes things a bit easier to deal with (it should be notes that a molecule such as C₅H₁₂ has three isomers – not optical isomers but structural isomers, namely pentane, 2 methylbutane & 2,2 dimethylpropane).

Like all types of isomer covered in the IB chemistry course they are made of organic molecules.

Optical isomers are different to each other only in the fact that they are mirror images of each other. Molecules that are mirror images of each other are referred to as enantiomers.

These mirror images may look the same have the same chemical and physical properties but there are not the same molecule as the mirror images are not superimposable. The enantiomers are asymmetric – they are lacking in symmetry.

What makes an optical isomer?

An optical isomer will have a carbon atom that is joined to four different ‘groups’. The carbon that is attached to the four different groups is referred to as a ‘chiral’ carbon. Chiral means ‘asymmetric in such as way that the structure and its mirror image are not superimposable.

By group, we don’t necessarily mean different atoms, we mean different groups of atoms, so a carbon joined to four different alkyl groups such as 4-ethyl-4-methyloctane (shown below) is considered to a have a chiral carbon:The chiral carbon is carbon atom number 4 as it is joined to four different alkyl groups

How do we tell optical isomers apart?

The only ‘easy’ way to do this is to take a solution and shine polarised light on it. Polarised light contains light that is only travelling in one direction. If it is passed though a solution of one optical isomer it will be rotated, either clockwise or anticlockwise. The other optical isomer will rote the light an equal amount but in the opposite direction.

The terms + and – or ‘D’ and ‘L’ are used to distinguish between the pairs of optical isomers.

The solution is said to be optically active.

If a solution of optical isomers does not rotate polarised light the solution is said to be racemic – it has equal proportions of each enantiomer and is therefore optically inactive.

Do you have any questions on optical isomers? If so, please feel free to post them below.