AQA A-Level Chemistry: Optical isomerism (7405) – Revision Notes

These notes condense section 3.3.7 Optical isomerism of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards. The whole section is A-level only. At A-level it is assessed on Paper 2 (organic chemistry, section 3.3), and Paper 3 can test any content. For full explanations and worked examples, read the optical isomerism study guide.

Test yourself afterwards with the practice questions. The course hub is AQA A-Level Chemistry, the printable checklist lists every specification point, and the free 10-minute diagnostics show where to start.

Key definitions

Term	Definition to learn
Stereoisomers	Same structural formula, different arrangement of atoms in space (from 3.3.1.3)
Optical isomerism	A form of stereoisomerism caused by chirality; at A-level, limited to molecules with a single chiral centre
Asymmetric carbon / chiral centre	A carbon atom bonded to four different atoms or groups
Enantiomers (optical isomers)	Non-superimposable mirror images that differ in their effect on plane-polarised light
Plane-polarised light	Light vibrating in one plane only
Optically active	Rotates the plane of plane-polarised light
Racemic mixture (racemate)	A mixture of equal amounts of the two enantiomers; optically inactive

The two types of stereoisomerism at A-level

	E–Z isomerism (3.3.1.3)	Optical isomerism (3.3.7)
Cause	Restricted rotation about a planar C=C bond	A carbon with four different groups (chiral centre)
What to look for	Each C of the C=C has two different groups	A saturated carbon with four different groups
How the isomers differ	Groups on the same or opposite sides of the C=C	Non-superimposable mirror images
Physical properties	Often differ	Identical (except the direction plane-polarised light is rotated)
How you show them	Draw the C=C with the groups in place	3D drawing: wedge, hashed and in-plane bonds

A molecule can show both. Pent-3-en-2-ol, CH₃CH=CHCH(OH)CH₃, has a C=C with two different groups on each carbon (E–Z) and a chiral C2 carrying H, OH, CH₃ and CH=CHCH₃.

Method in steps

Finding a chiral centre

1. Skip carbons in C=C, C=O or C≡N bonds.
2. Skip CH₃ and CH₂ carbons.
3. For each remaining carbon, follow every group to its end and compare whole groups.
4. Four different groups: mark the carbon with *.

Drawing a pair of enantiomers

1. Find and mark the chiral centre.
2. Draw the chiral carbon with two bonds in the plane, one wedge (towards you) and one hashed bond (away).
3. Put one group on each bond.
4. Draw a mirror line and reflect: left-hand groups move to the right; bond types stay the same.
5. For a displayed formula, keep the same 3D centre and draw every bond in every group.

Explaining why a racemate is optically inactive

1. Each enantiomer rotates the plane of plane-polarised light by the same angle.
2. The rotations are in opposite directions.
3. There are equal amounts of each enantiomer, so the effects cancel.

Explaining how a racemate forms

1. Name the planar group (trigonal planar C=O, or a planar carbocation).
2. The attacking species (for example CN⁻ or Br⁻) can attack from either side of the plane.
3. Attack from each side is equally likely.
4. So equal amounts of the two enantiomers form: a racemate.

Worked reminders

2-chlorobutane in 3D. C2 carries H, Cl, CH₃ and C₂H₅. In this layout CH₃ is in the plane at the top, C₂H₅ in the plane at lower left, Cl on a wedge at lower right and H on a hashed bond behind:

            CH3           mirror           CH3
             |              :               |
             C              :               C
           / ⋮ ▼            :             ▼ ⋮ \
       C2H5  H  Cl          :          Cl  H  C2H5

(▼ = wedge towards you, ⋮ = hashed bond away from you, plain lines in the plane.)

Butan-2-ol from but-1-ene. Hydration of but-1-ene gives butan-2-ol as the major product, through a planar secondary carbocation. Water attacks either face equally, so the butan-2-ol is a racemate. It contains a chiral centre but does not rotate plane-polarised light.

KCN with ethanal. Ethanal + KCN, then dilute acid, gives 2-hydroxypropanenitrile, CH₃CH(OH)CN. C2 carries H, OH, CH₃ and CN, so the product is chiral. CN⁻ attacks either face of the planar C=O, so a racemate forms.

Swap test. One swap of two groups on a chiral carbon gives the other enantiomer. Two swaps give the same molecule back.

Must-know distinctions

• Chiral centre vs chiral molecule: the centre is the carbon; the molecule is chiral because of it.
• Optically active vs racemate: a single enantiomer is optically active; an equal mixture is not, even though every molecule in it is chiral.
• Mirror image vs enantiomer: every molecule has a mirror image; only a chiral one has a mirror image that cannot be superimposed on it.
• Aldehyde or unsymmetrical ketone vs symmetrical ketone (with KCN, section 3.3.8): the first two give a racemate; the third gives a product with two identical groups and no chiral centre.
• Light vs plane of light: enantiomers rotate the plane of plane-polarised light.

Practical skill

The specification suggests passing polarised light through a sucrose solution. With two polarising filters set so that the light is darkest through water, replacing the water with sucrose solution lets light through. The second filter must be turned through an angle to darken it again. This shows the solution is optically active. This is a skills opportunity, not one of the numbered required practicals.

Quick self-test

1. Define the term enantiomers.
2. Define racemic mixture.
3. Does 3-chloropentane, CH₃CH₂CHClCH₂CH₃, have a chiral centre?
4. Which carbon in 2-methylbutanal, CH₃CH₂CH(CH₃)CHO, is chiral? Give its four groups.
5. Is aminoethanoic acid, H₂NCH₂COOH, chiral?
6. State one physical property that is the same for two enantiomers and one way in which they differ.
7. Name the product when butanone reacts with KCN followed by dilute acid, and say whether it is a racemate.
8. A sample contains 2.22 g of one enantiomer of butan-2-ol. What mass of the other enantiomer must be added to make a racemate, and how many moles of butan-2-ol will the racemate contain in total? (Ar: H = 1.0, C = 12.0, O = 16.0)
9. How many structural isomers of C₅H₁₁Cl have a chiral centre?
10. Explain why 3-methylpent-1-ene, CH₂=CHCH(CH₃)CH₂CH₃, shows optical isomerism but not E–Z isomerism.

Answers

1. Stereoisomers that are non-superimposable mirror images of each other (and rotate plane-polarised light in opposite directions).
2. A mixture of equal amounts of two enantiomers.
3. No. C3 carries H, Cl and two identical C₂H₅ groups.
4. C2. Its groups are H, CH₃, C₂H₅ and CHO.
5. No. The only saturated carbon is a CH₂, with two H atoms.
6. Same: melting point (or boiling point, or density). Differ: they rotate the plane of plane-polarised light in opposite directions (and may interact differently with other chiral molecules, such as enzymes).
7. 2-hydroxy-2-methylbutanenitrile, CH₃CH₂C(OH)(CH₃)CN. Yes, a racemate: C2 carries OH, CN, CH₃ and C₂H₅, and CN⁻ attacks the planar C=O from either side equally.
8. 2.22 g. Mr of C₄H₁₀O = 74.0, so 2.22 g is 0.0300 mol of each enantiomer and 0.0600 mol in total.
9. Three: 2-chloropentane, 1-chloro-2-methylbutane and 2-chloro-3-methylbutane.
10. C3 carries H, CH₃, C₂H₅ and CH=CH₂, so it is a chiral centre. C1 of the C=C carries two hydrogen atoms, so there is no E–Z isomerism.

Where marks are usually lost

• Writing “mirror images” without “non-superimposable”.
• Defining a racemate as “a mixture of enantiomers” without “equal amounts”.
• Saying a racemate is inactive because it “has no chiral centre”. Each molecule is chiral; the rotations cancel.
• Missing “equal and opposite” rotation when explaining optical inactivity.
• Treating two ethyl groups as different, or ethyl and propyl as the same, because they start with CH₂.
• Putting an asterisk on a CH₂, CH₃ or C=O carbon.
• In a 3D drawing, using all four bonds in the plane, or drawing the “mirror image” with a group swapped as well, which gives the same enantiomer twice.
• In a racemate-formation answer, leaving out the planar group or the equal chance of attack from each side.
• Calling a sample of a single enantiomer “racemic” because it came from a reaction; only an equal mixture is a racemate.

Related pages

The alkenes and alcohols guides cover the addition reactions that form racemic products here, and the transition metals guide covers optical isomerism in complexes with bidentate ligands (3.2.5.3).

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.3.7 Optical isomerism (A-level only).