A carbon atom is a chiral centre when it is tetrahedral and bonded to four different substituents. This arrangement produces two mirror-image structures that cannot be superimposed on each other.
The Reasoning
A typical chiral carbon has four single bonds and approximately tetrahedral geometry. Because its four substituents are different, their three-dimensional arrangement can occur in two distinct forms.
These forms are enantiomers: stereoisomers that are non-superimposable mirror images. They have the same atom connectivity but differ in the spatial arrangement of atoms, linking this concept to Structure 3.2.7 in IB Chemistry HL.
Since all four substituents are different, carbon 2 is a chiral centre.
Carbon environment | Chiral centre? | Reason |
|---|---|---|
Four different substituents | Yes | Two non-superimposable mirror-image arrangements are possible |
Two or more identical substituents | No | Interchanging identical groups does not create a distinct stereoisomer |
Carbon in a double bond | No | The carbon is trigonal planar, not tetrahedral |
Carbon with three bonds and one lone pair | Not a normal carbon centre | Carbon does not form the required stable tetrahedral arrangement |
A common misconception is that any carbon bonded to four atoms is chiral. This is incorrect: the carbon must be bonded to four different substituent groups, not merely four atoms. For example, the carbon in ethanol, (\text{CH}_3\text{CH}_2\text{OH}), is not chiral because the (\text{CH}_2) carbon is bonded to two identical hydrogen atoms.