Optical isomerism and chiralityAQA A-Level Chemistry: Revision notes
Section 1
Stereoisomerism and optical isomers
Stereoisomers have the same structural formula but a different arrangement of atoms in space. Optical isomerism is one type of stereoisomerism (E/Z isomerism is the other). It occurs because of chirality: a molecule is chiral if it cannot be superimposed on its mirror image, just as a left hand cannot be superimposed on a right hand.
A pair of optical isomers are enantiomers: non-superimposable mirror images of each other. At A Level this is limited to molecules with a single chiral centre.
Section 2
Finding the chiral centre
A chiral centre (asymmetric carbon atom) is a carbon atom bonded to four different atoms or groups. To find one, look at each sp³ carbon in turn and ask whether all four groups attached to it are different.
Examples:
- Butan-2-ol, CH₃CH(OH)CH₂CH₃: C2 carries H, OH, CH₃ and CH₂CH₃, so it is chiral.
- Propan-2-ol, CH₃CH(OH)CH₃: C2 carries two CH₃ groups, so it is not chiral.
- Lactic acid, CH₃CH(OH)COOH: C2 carries H, OH, CH₃ and COOH, so it is chiral.
A carbon atom with a double bond, or with two identical groups (such as CH₂ or CH₃), can never be a chiral centre.
Do not say a carbon is chiral because it has four groups: they must all be different. A carbon with two CH₃ groups is not chiral.
Mark each candidate carbon and list its four groups. If any two match, it is not chiral.
Section 3
Drawing enantiomers
Enantiomers are drawn in three dimensions around the chiral carbon. Two bonds are drawn as ordinary lines in the plane of the paper, one as a wedge (coming towards you) and one as a dashed line (going away from you). The second enantiomer is the mirror image: swap the groups on the wedge and the dash (or reflect the whole drawing in a vertical line).
For structural formula answers, write the groups around the chiral carbon, for example CH₃CH(OH)CH₂CH₃ for butan-2-ol, then state that the two enantiomers differ only in the arrangement of H, OH, CH₃ and C₂H₅ in space. In a displayed formula show every bond and atom, using wedges and dashes at the chiral carbon.
Draw the first enantiomer, then the second by swapping any two groups. Check that rotating one never makes it identical to the other.
Section 4
Effect on plane-polarised light
Light in which the waves vibrate in a single plane is plane-polarised. Enantiomers have the same melting and boiling points and the same chemical reactions with non-chiral reagents, but they differ in how they affect plane-polarised light.
One enantiomer rotates the plane of polarisation clockwise and the other rotates it anticlockwise by exactly the same angle. A substance that does this is optically active. The rotation is measured with a polarimeter.
Section 5
Racemic mixtures
A racemic mixture (racemate) contains equal amounts of two enantiomers. The clockwise rotation of one enantiomer is exactly cancelled by the equal anticlockwise rotation of the other, so a racemic mixture is optically inactive and shows no rotation.
Racemic mixtures are formed when a chiral product is made from non-chiral starting materials by a mechanism in which either enantiomer is equally likely to form. Both enantiomers form in equal amounts, so the product mixture does not rotate plane-polarised light. Making a single enantiomer needs a more selective route, which is important in the pharmaceutical industry because enantiomers can have different biological effects.
A racemic mixture is optically inactive because the rotations cancel, not because the molecules are not chiral. Each molecule in it is still chiral.
Section 6
Putting it together in an exam answer
A strong answer to an optical isomerism question follows four steps:
- Identify the chiral centre by naming the carbon and its four different groups.
- State that this gives two non-superimposable mirror images (enantiomers).
- Describe the effect on plane-polarised light: opposite directions, equal angle.
- If a mixture is mentioned, decide whether the amounts are equal (racemic, optically inactive) or not.
Worked example: pentan-2-ol, CH₃CH(OH)CH₂CH₂CH₃, has a chiral C2 (H, OH, CH₃, CH₂CH₂CH₃), so it forms two enantiomers. Pentan-3-ol has two identical ethyl groups on C3, so it is not chiral.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Optical isomerism and chirality
- Lactic acid, CH₃CH(OH)COOH, builds up in muscle tissue during hard exercise and is also the acid that makes milk taste sour. Its molecules show optical isomerism.Explain why lactic acid exists as a pair of optical isomers.2 marks
- A chemist has two samples, X and Y, of the same chiral compound. When each is placed separately in a polarimeter, X rotates the plane of plane-polarised light by 12° clockwise and Y rotates it by 12° anticlockwise. When equal masses of X and Y are dissolved together in the same volume of solvent, the solution produces no rotation at all.Explain why the mixture of X and Y produces no rotation of plane-polarised light.2 marks
- The amino acid alanine, CH₃CH(NH₂)COOH, is found in proteins. Glycine, H₂NCH₂COOH, is the simplest amino acid. A student synthesises alanine in the laboratory from starting materials that are not chiral.Alanine is chiral. Identify the chiral centre in alanine and the four groups bonded to it, and state how its two enantiomers differ in their effect on plane-polarised light.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).