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7.1Advanced Organic Chemistry

Optical Isomerism — AQA A Level Chemistry Revision Notes & Topical Questions

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Optical Isomerism: Chiral Centres and Racemic Mixtures — AQA A LEVEL CHEMISTRY — COMPLETE NOTES notes: Identify chiral centres, draw enantiomers and explain the effects of racemic mixtures.Stereoisomers are molecules that have the same structural formula but differ in the arrange…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Plane-polarised light
  • Saying any carbon with four bonds is chiral - the four groups must all be different

Part 1 of 6Stereoisomers and optical isomers

Stereoisomers are molecules that have the same structural formula but differ in the arrangement of atoms in space. There are two types: E/Z (geometric) isomers, and optical isomers. Optical isomers (enantiomers) are non-superimposable mirror images of each other, like left and right hands.

Part 2 of 6Identifying chiral centres

A chiral centre (asymmetric carbon) is a carbon atom bonded to four different atoms or groups.

1
Look for sp3 carbons and check whether all four substituents differ.
2
For example, in 2-hydroxypropanoic acid (lactic acid), CH3CH(OH)COOH, the central carbon carries H, OH, CH3 and COOH, so it is chiral.
3
In propanoic acid no carbon has four different groups, so it is not chiral.
4
Molecules with a chiral centre can exist as a pair of enantiomers.
5
To draw them, show the chiral carbon with two bonds in the plane of the paper, one wedge and one dashed bond,
  • and draw the second enantiomer as its mirror image.

Part 3 of 6Properties of enantiomers

Enantiomers have identical physical properties (melting point, boiling point, solubility) and identical chemical reactions with achiral reagents.

Properties of enantiomers
They differ in two ways: they rotate the plane of plane-polarised light in opposite directions by equal amounts (one clockwise, +, the other anticlockwise, -),
  • and they can react differently in chiral environments such as enzymes.
This is why one enantiomer of a drug can be active while the other is inactive or harmful.

Part 4 of 6Racemic mixtures

A racemic mixture (racemate) contains equal amounts of the two enantiomers.

1
It is optically inactive, because the rotations cancel.
2
Racemic mixtures form when a planar reactant is attacked from either side with equal probability.
3
For example, nucleophilic addition of cyanide to an aldehyde or unsymmetrical ketone attacks the planar carbonyl carbon from above or below with equal chance,
  • giving a racemic hydroxynitrile.
4
In contrast, SN1 substitution through a planar carbocation also produces a racemate, while SN2 gives inversion of one enantiomer.
5
Making a single enantiomer is costly (it needs chiral catalysts or chiral starting materials), but important for pharmaceuticals.

Part 5 of 6Naming and drawing enantiomers

Enantiomers are often labelled R and S using Cahn–Ingold–Prelog priority rules, or D and L for amino acids and sugars, but at this level you mainly need to show the two three-dimensional forms clearly.

Naming and drawing enantiomers
Draw the chiral carbon with two bonds in the plane of the paper, one bond as a solid wedge towards you and one as a dashed wedge away from you; the second enantiomer is the reflection of the first in a vertical mirror line.
Checking is simple: if you cannot rotate one structure to lie exactly on top of the other, they are enantiomers.
Remember that a molecule with two or more chiral centres can have more than two stereoisomers,
  • and some such molecules are achiral overall if they contain an internal mirror plane.

In an exam, always circle or label the chiral carbon and list its four different groups, because the mark is usually for naming the groups as well as identifying the atom.

Part 6 of 6Worked examples

  1. Identify the chiral centre in butan-2-ol. [2]2 marks

  2. Explain why the reaction of propanal with HCN gives a racemic mixture. [3]3 marks

  3. State how two enantiomers differ in a polarimeter. [2]2 marks

  4. Does 2-methylbutane have optical isomers? [1]1 mark

Key terms

TermMeaning
EnantiomersNon-superimposable mirror image isomers.
Chiral centreCarbon atom bonded to four different groups.
Racemic mixtureEqual mixture of two enantiomers, optically inactive.
Plane-polarised lightLight whose oscillations lie in a single plane.
StereoisomersSame structural formula, different spatial arrangement.

Avoid these mistakes

1
Saying any carbon with four bonds is chiral - the four groups must all be different.
2
Claiming enantiomers have different boiling points.
3
Saying a racemic mixture is a single compound that does not rotate light - it is a 50:50 mixture of two compounds.
4
Drawing the "mirror image" as a rotated molecule.

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