Identifying Chiral Centres

To find a chiral centre, look for a carbon atom bonded to four different atoms or groups. Mark it with an asterisk (*).A quick method:
1. Ignore any carbon in a C=C or C=O double bond (it has only three groups).
2. Ignore CH₃ and CH₂ carbons (they have two or more identical H atoms).
3. For each remaining carbon, list the four groups in full. Compare the whole group, not just the first atom: CH₃ and CH₂CH₃ both start with C but are different groups.Examples:
• 2-hydroxypropanoic acid, CH₃C*H(OH)COOH: carbon 2 carries H, OH, CH₃ and COOH, so it is a chiral centre.
• 2-aminopropanoic acid, CH₃C*H(NH₂)COOH: carbon 2 carries H, NH₂, CH₃ and COOH, so it is chiral.
• 3-methylhexane: carbon 3 carries H, CH₃, CH₂CH₃ and CH₂CH₂CH₃, so it is a chiral centre.
• 3-methylpentane: carbon 3 carries H, CH₃ and two CH₂CH₃ groups, so there is no chiral centre.A molecule can have more than one chiral centre.Worked example: 2,3-dichloropentane, CH₃CHClCHClCH₂CH₃. Carbon 2 carries H, Cl, CH₃ and CHClCH₂CH₃: four different groups. Carbon 3 carries H, Cl, CH₂CH₃ and CHClCH₃: four different groups. So the molecule has two chiral centres: CH₃C*HClC*HClCH₂CH₃.