Topic 15 · page 1 of 28
Chiral centres
A carbon atom with four different groups attached is a chiral centre. Its molecule can exist as two mirror-image forms that cannot be placed on top of each other.
Mirror images that do not match 15.1
Hold your left hand up to a mirror. The reflection looks like a right hand. Now try to put your left glove on your right hand. It does not fit, however you turn it. A left hand and a right hand are non-superimposable mirror images.
Some molecules behave in the same way. Butan-2-ol and its mirror image are two different molecules with the same formula and the same bonds. They differ only in how the groups are arranged in space, so they are stereoisomers.
The two mirror-image forms are called optical isomers or enantiomers. The name "optical" comes from the one property that tells them apart, which page 3 explains.
Finding the chiral centre 15.2
In the molecules you meet at this level, chirality comes from one carbon atom that has four different atoms or groups attached. This atom is the chiral centre (also called an asymmetric carbon atom).
To test a carbon atom, write down what is attached to it and compare the four groups. "Different" means different as a whole group, not just the first atom. A CH₃ group and a CH₂CH₃ group both start with carbon, but they are different groups.
| Molecule | Carbon tested | Groups attached | Chiral? |
|---|---|---|---|
| butan-2-ol | C2 | H, OH, CH₃, CH₂CH₃ | yes |
| propan-2-ol | C2 | H, OH, CH₃, CH₃ | no, two CH₃ |
| 2-chlorobutane | C2 | H, Cl, CH₃, CH₂CH₃ | yes |
| pentan-3-ol | C3 | H, OH, CH₂CH₃, CH₂CH₃ | no, two ethyl groups |
| 2-hydroxypropanoic acid | C2 | H, OH, CH₃, COOH | yes |
A carbon in a CH₃ group or a CH₂ group can never be a chiral centre, because it has at least two hydrogen atoms attached.
The rotation test 15.1
How do you know that two drawings really are different molecules and not the same molecule turned round? Turn one of them. If it can be rotated until every group sits on top of the matching group, the two are the same molecule. If no rotation ever works, they are non-superimposable.
With one chiral centre you can match two of the groups by rotating, but the other two are then swapped. This is why a single chiral centre gives exactly two forms, one the mirror image of the other.
Check yourself
- Define the term chiral centre.
- State whether each molecule has a chiral centre, and if so which carbon atom it is: (a) pentan-2-ol, (b) propan-1-ol, (c) 2-methylbutane, (d) 3-methylhexane.
- Explain why propan-2-ol does not show optical isomerism but butan-2-ol does.
- State what is meant by "non-superimposable mirror images".
- 2-hydroxypropanoic acid, CH₃CH(OH)COOH, is chiral. Name the four groups on its chiral carbon atom.
Answers cover these until you have tried the questions
- A carbon atom bonded to four different atoms or groups.
- (a) Yes: C2 has H, OH, CH₃ and CH₂CH₂CH₃. (b) No: C1 has two H, and C2 and C3 have two H each, so no carbon has four different groups. (c) No: C2 has H, CH₃, CH₃ and CH₂CH₃, with two CH₃ groups the same. (d) Yes: C3 has H, CH₃, CH₂CH₃ and CH₂CH₂CH₃.
- In propan-2-ol the central carbon has H, OH and two CH₃ groups, so it has only three different groups and is not a chiral centre. In butan-2-ol the central carbon has H, OH, CH₃ and CH₂CH₃, four different groups, so it is a chiral centre and the molecule has two enantiomers.
- Two molecules that are mirror images of each other and that cannot be placed on top of each other so that all the atoms match, however they are turned.
- H, OH, CH₃ and COOH.