Chirality and optical isomerismEdexcel International A Level Chemistry: Revision notes
Section 1
Chiral centres
A chiral centre (asymmetric carbon atom) is a carbon atom bonded to four different groups. A molecule with one chiral centre can exist in two forms that cannot be superimposed on each other. This is optical isomerism, a type of stereoisomerism.
Example: in lactic acid, CH₃CH(OH)COOH, C2 has H, CH₃, OH and COOH attached, so it is chiral. In propan-2-ol, the central carbon has two identical CH₃ groups, so it is not chiral.
To identify a chiral centre, check each carbon bonded to four groups and ask if all four are different.
Calling a carbon chiral because it is bonded to four groups. All four must be different, and a C=O or C=C carbon is never chiral.
Section 2
Enantiomers and 3D diagrams
The two optical isomers are called enantiomers. They are object and non-superimposable mirror images, like left and right hands.
Draw them in 3D: the chiral carbon at the centre, two bonds in the plane of the paper, one bond as a wedge (towards the viewer) and one as a dash (away). The second enantiomer is the mirror image: swap the wedge and dash groups, or reflect across a vertical line.
Enantiomers have the same physical properties (melting point, boiling point, density) and the same chemical reactions with achiral reagents. They differ only in their effect on plane-polarised light and in their reactions with other chiral molecules, such as enzymes.
When drawing the second enantiomer, keep the same four groups in the same bonds-in-the-plane positions and swap only the wedge and dash groups.
Section 3
Optical activity and racemic mixtures
Optical activity is the ability of a single enantiomer to rotate the plane of plane-polarised monochromatic light. Plane-polarised light vibrates in only one plane; it is passed through the sample in a polarimeter.
One enantiomer rotates the plane clockwise (+) and the other by an equal angle anticlockwise (−).
A racemic mixture (racemate) contains equal amounts of both enantiomers. The rotations cancel, so it is optically inactive.
Saying a racemic mixture has no chiral molecules. Both enantiomers are present, but their effects cancel.
Section 4
Evidence for SN1 and SN2 mechanisms
If a halogenoalkane that is a single enantiomer is hydrolysed, the optical activity of the product shows the mechanism.
- SN1 (tertiary halogenoalkanes): the halogen leaves first to give a planar carbocation. OH⁻ attacks from either side with equal probability, giving a racemic mixture, so the product has no optical activity.
- SN2 (primary and secondary halogenoalkanes): OH⁻ attacks from the side opposite the halogen in one step, through a transition state. The configuration is inverted and only one enantiomer forms, so the product is optically active.
Example: 3-bromo-3-methylhexane (tertiary) gives a racemic product via SN1.
Product optically inactive from a pure enantiomer means SN1; product optically active means SN2.
Section 5
Evidence for addition to carbonyl compounds
Aldehydes and most unsymmetrical ketones react with HCN (with KCN present) to form hydroxynitriles. For example, ethanal gives 2-hydroxypropanenitrile, CH₃CH(OH)CN, which has a chiral centre.
Mechanism: the nucleophile CN⁻ attacks the δ+ carbon, the C=O π bond breaks to give an intermediate anion, and this is protonated by HCN or water.
The carbonyl group is planar, so CN⁻ attacks from either side with equal probability, forming equal amounts of both enantiomers. The product is a racemic mixture and has no optical activity, evidence for the planar structure of the carbonyl carbon.
Saying the product is inactive because it has no chiral centre. It is chiral; it is optically inactive because it is a racemic mixture.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Chirality and optical isomerism
- Lactic acid (2-hydroxypropanoic acid), CH₃CH(OH)COOH, is made by muscle cells during hard exercise and by bacteria in yoghurt production.Explain why lactic acid shows optical isomerism and what is meant by its optical isomers.2 marks
- Ibuprofen, (CH₃)₂CHCH₂C₆H₄CH(CH₃)COOH, is a painkiller that is sold as an equal mixture of its two optical isomers, although only one of them is pharmacologically active. Optical activity is measured in a polarimeter using plane-polarised monochromatic light.Explain why the equal mixture of ibuprofen isomers has no effect on plane-polarised light.2 marks
- Two samples of halogenoalkane, each consisting of a single optical isomer, are hydrolysed by heating with aqueous sodium hydroxide. Compound X, 3-bromo-3-methylhexane, CH₃CH₂C(CH₃)(Br)CH₂CH₂CH₃, gives an alcohol product that does not rotate the plane of plane-polarised light. Compound Y, 2-bromobutane, gives butan-2-ol that does rotate the plane of plane-polarised light.Explain what the result for compound X indicates about the mechanism of its hydrolysis.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).