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 (lactic acid) is shown in the image below. 2-hydroxypropanenitrile contains one chiral carbon atom which is highlighted by an asterisk (*). This carbon atom has four different groups attached, meaning it will exist then as a pair of optically active enantiomers. Enantiomers are mirror image forms of optically active molecules; they are non-superimposable mirror images of each other.

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. Now depending on which type of formula is shown for a particular molecule can determine how easy it is to determine if the molecule is optically active or not, for example the condensed formula can make it difficult to determine if there are any chiral carbon atoms present, while the structural formula can make it easier to identify the presence of chiral carbon atoms. However if the 3d structure of the molecule is drawn out it can make it relatively straight forward to identify any chiral carbon atoms present, as shown in the image below:

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 substituents. I have given one example below, out of the three molecules shown below only the last molecule, 1,2-dimethylhexane is the only one which contains a chiral carbon atom.

• Cyclohexane is an achiral molecule; it is not optically active since none of the carbon atoms in the molecule contain four different groups. All the -CH₂ carbon atoms have two identical hydrogen atoms, so will be achiral.


• methylcyclohexane is also achiral. To identify any chiral carbon atoms in this molecule we can discard all the -CH₂ groups since each carbon atom here has two identical hydrogen atoms. Carbon atom number 1 has a methyl substituent (-CH₃), a hydrogen substituent. We can also say that carbon atom C2 is the same as C3 and C4, while C6, C5 and C4 are also all equivalent. So we can say the the C6, C5, C4 and the C2, C3, C4 substituent are identical. So carbon atom C1 has a methyl group (-CH₃), a H atom and two other substituents namely C2, C3, C4 and C6, C5, C4 which are identical groups, so this molecule is also achiral and not optically active.


• However with 1,2-dimethylcyclohexane, the substituent C2, C3, C4 is NOT the same as the substituent C6, C5, C4, since there is a methyl group attached to C2 and there is only hydrogen atoms on C6. This means that this molecule is optically active with a chiral carbon atom, that is carbon atom C1. Using a similar argument carbon atom C2 is also chiral, since it contains 4 different “groups” attached to it.

Key Points

• Optical isomers or enantiomers are non-superimposable mirror images of each other.
• Using the structural formula or 3d formula can make it much easier to find chiral carbon atoms as the condensed formula can make it more difficult to locate chiral carbon atoms within a molecule.
• To identify chiral carbon atoms in rings or cyclic molecules consider what is bonded to adjacent carbon atoms either side of a suspected chiral carbon atom in the ring.

Check your understanding - Questions on identifying optically active molecules.

Next