How to identify chiral carbon atoms in molecules

Chiral carbon atoms

Molecules with four different atoms or groups attached to a carbon atom are optically active, they will rotate plane polarised light. Carbon atoms with four different atoms or groups attached are also said to be chiral or asymmetric.

The molecule 2-hydroxypropanenitrile is shown in the image below. 2-hydroxypropanenitrile contains one chiral carbon atom. This carbon atom has four different groups attached. It will exist then as a pair of optically active enantiomers. Enantiomers are mirror image forms of optically active molecules, they are non-superimposable on each other.

2-hydroxypropanenitrile is an optical active molecule

Finding chiral carbon atoms

In small simple molecules such as lactic acid shown above it can be fairly easy and quick to find any chiral carbon atoms present, however with larger molecules and especially in molecules which consist of rings it is not always immediately obvious if a chiral carbon atom is present or not. Consider the example shown below, here the optically active molecule 3-chlorohexane is chosen as a simple example.

example of a molecule which is optically active, 3-chlorhexane

Finding chiral carbon atoms in rings

Finding chiral carbon atoms in ring or cyclic structures needs a bit more care. The trick to finding chiral centres in rings and cyclic molecules is to imagine parts of the ring structure as individual substitents. I have given one example below. In the last molecule, 1,2-dimethylhexane is the only molecule out of the three cyclic structures which contains chiral carbon atoms.

How to identify chiral carbon atoms in rings

Key Points

Practice questions

Check your understanding - Questions on identifying optically active molecules.