Identifying Chiral Centres

To decide whether a molecule is chiral, look for a chiral centre: a carbon atom with four different groups attached. A chiral centre is usually marked with an asterisk (*) on the structure.A quick method:
1. Draw the displayed or skeletal formula so you can see every group.
2. Rule out carbons that cannot be chiral centres:
• any carbon in a C=C or C=O double bond, because it has only three groups attached
• CH₃ and CH₂ carbons, because they have two or more identical hydrogen atoms
3. For each remaining carbon, list the four groups attached. Look at the whole group, not just the first atom: CH₃ and CH₂CH₃ both start with carbon, but they are different groups.
4. If all four groups are different, the carbon is a chiral centre.Worked example 1: 2-bromobutane, CH₃CHBrCH₂CH₃.
Carbon 2 is bonded to H, Br, CH₃ and CH₂CH₃. All four are different, so C2 is a chiral centre: CH₃C*HBrCH₂CH₃.Worked example 2: 3-bromopentane, CH₃CH₂CHBrCH₂CH₃.
Carbon 3 is bonded to H, Br and two CH₂CH₃ (ethyl) groups. Two groups are the same, so there is no chiral centre.Worked example 3: 2-aminopropanoic acid (alanine), CH₃CH(NH₂)COOH.
Carbon 2 carries H, NH₂, CH₃ and COOH, so it is a chiral centre. Most amino acids are chiral for this reason; the exception is glycine, H₂NCH₂COOH, which has two H atoms on that carbon.Some molecules have more than one chiral centre, so check every carbon that could fit the rule.