Chirality

Some molecules are chiral: they are not superimposable on their mirror image. Your hands are a good model. Your left hand is the mirror image of your right, but however you turn them, you cannot place one exactly on top of the other.In organic chemistry, chirality usually comes from a chiral centre (also called an asymmetric carbon atom): a carbon atom bonded to four different atoms or groups. The four groups are arranged tetrahedrally, and there are two different ways to arrange four different groups around a tetrahedral carbon. These two arrangements are mirror images that cannot be superimposed.If any two of the four groups are the same, the molecule is not chiral: its mirror image can be rotated to fit exactly on top of it.Example: butan-2-ol, CH₃CH(OH)CH₂CH₃. Carbon 2 is bonded to H, OH, CH₃ and C₂H₅. These are four different groups, so carbon 2 is a chiral centre and butan-2-ol is chiral. A chiral centre is often marked with an asterisk: CH₃C*H(OH)CH₂CH₃.Counter-example: propan-2-ol, CH₃CH(OH)CH₃. Carbon 2 is bonded to H, OH and two CH₃ groups. Two groups are the same, so there is no chiral centre.Worked example: 2-chlorobutane, CH₃CHClCH₂CH₃, has carbon 2 bonded to H, Cl, CH₃ and C₂H₅, so it is chiral. 1-chlorobutane has no carbon with four different groups, so it is not chiral.