Recognising chirality
Chiral centre: an asymmetric carbon atom attached to four different atoms or groups of atoms.
Optical isomerism is a type of stereoisomerism associated with chirality. When four different groups are attached to a carbon atom, the groups have a tetrahedral arrangement around that carbon.
Chiral centres may be shown in either 2D structures or 3D representations. In either form, inspect each possible carbon and compare the four groups attached to it.
- A carbon attached to four different atoms or groups is chiral.
- If two attached atoms or groups are identical, that carbon is not a chiral centre.
- A chiral carbon can be identified on a structure using an asterisk, .
If the bonding is not immediately clear from the representation given, drawing a structural or displayed formula can make the groups around each carbon easier to compare.
In the example above, the marked carbon is attached to , , and . Since all four groups are different, it is a chiral centre.
Worked example: A carbon atom is attached to , , and . Determine whether it is chiral.
All four groups attached to the carbon are different, so the carbon is an asymmetric carbon atom and therefore a chiral centre.
Exam Tip: When identifying a chiral centre on a structure, check all four attached groups and mark the required carbon clearly with an asterisk.