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Optical isomerism occurs when a molecule has a chiral centre, a carbon atom bonded to four different groups. Free AQA notes and 11 questions answered.
Introduction to Optical Isomerism is part of Optical isomerism (A-level only) in A Level AQA Chemistry.
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Optical isomerism occurs when a molecule has a chiral centre, a carbon atom bonded to four different groups. Free AQA notes and 11 questions answered.
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Use this lesson to stay inside Optical isomerism (A-level only) while connecting the detail back to the wider Organic chemistry topic.
Read the core explanations from Introduction to Optical Isomerism before testing yourself from memory.
Two molecules. Identical formula. Identical bonds. Yet one can be a life-saving drug and the other biologically inert — or even harmful. That is the real-world consequence of optical isomerism, and it is why this topic appears in pharmaceutical chemistry, biochemistry, and AQA exam papers alike.
Optical isomerism is a type of stereoisomerism — meaning the isomers share the same structural formula but differ in how their atoms are arranged in three-dimensional space. Unlike structural isomers, you cannot tell enantiomers apart by looking at a displayed formula on paper; you need to think in 3D.
You may already know that stereoisomers come in two broad families: E/Z isomerism (arising from restricted rotation around a C=C double bond) and optical isomerism (arising from chirality). Both involve the same connectivity of atoms; the difference is purely spatial.
At A-level, optical isomerism is limited to molecules containing a single chiral centre. Once you can spot that centre, everything else follows logically.
Picture a carbon atom sitting at the centre of a tetrahedron. If all four groups attached to it are different from one another, there is no plane of symmetry through that carbon — it is asymmetric. That carbon is called a chiral centre, and its presence is the sole requirement for optical isomerism at A-level.
A classic example is the amino acid alanine ($\text{CH}_3\text{CH(NH}_2\text{)COOH}$). The central carbon carries four different groups: $-\text{CH}_3$, $-\text{NH}_2$, $-\text{COOH}$, and $-\text{H}$. No two are the same, so that carbon is chiral.
To check for a chiral centre systematically, go through every carbon in the molecule and ask: are all four substituents different? If yes, you have found a chiral centre. If any two substituents are the same, that carbon is not chiral and cannot give rise to optical isomers.
When a molecule contains a chiral centre, it can exist as two distinct three-dimensional arrangements. These two forms are mirror images of each other — like your left and right hands. You can rotate them any way you like, but you cannot superimpose one onto the other perfectly. That is the defining feature of enantiomers.
The hand analogy is worth dwelling on. Hold your left hand up to a mirror: the reflection looks like your right hand. Yet no matter how you rotate your left hand, it will never match your right hand exactly — the thumb is always on the opposite side. Enantiomers behave in exactly the same way at the molecular level.
A key point for the exam: the two enantiomers of a compound have identical physical properties — the same melting point, boiling point, and solubility — and identical chemical properties in achiral environments. They differ in the direction in which they rotate plane-polarised light, and they can also interact differently with other chiral molecules such as enzymes or chiral reagents.
Ordinary light oscillates in all planes simultaneously. When it passes through a polarising filter, only the waves oscillating in one specific plane get through — this is plane-polarised light. When plane-polarised light passes through a solution of a chiral compound, the plane of polarisation is rotated.
One enantiomer rotates the plane clockwise (to the right) — described as dextrorotatory — and the other rotates it anticlockwise (to the left) — described as laevorotatory. The magnitudes of rotation are equal; only the direction differs. This is the one measurable property that distinguishes the two enantiomers.
An equal mixture of both enantiomers is called a racemic mixture (or racemate). Because the rotations cancel out, a racemic mixture produces no net rotation of plane-polarised light. This is directly relevant to synthesis: many laboratory reactions that produce a chiral product do so as a racemic mixture, because the reagents attack both faces of the molecule with equal probability.
| Property | Enantiomer 1 (dextrorotatory) | Enantiomer 2 (laevorotatory) | Racemic mixture |
|---|---|---|---|
| Melting point | Identical | Identical | May differ slightly |
| Boiling point | Identical | Identical | Identical |
| Solubility | Identical | Identical | Identical |
| Rotation of plane-polarised light | Clockwise (+) | Anticlockwise (−) | No net rotation (0) |
| Reactivity in achiral environment | Identical | Identical | Identical |
| Reactivity with chiral molecules (e.g. enzymes) | Different | Different | Both present |
Exam questions often ask you to identify whether a molecule displays optical isomerism, or to draw the two enantiomers. The method below gives you a reliable, mark-winning approach every time.
Worked example: Identifying Optical Isomerism in a Molecule
Does 2-bromobutane ($\text{CH}_3\text{CHBrCH}_2\text{CH}_3$) display optical isomerism? If so, identify the chiral centre.
Answer: Yes. Carbon 2 of 2-bromobutane is a chiral centre because it is bonded to four different groups: $-\text{Br}$, $-\text{H}$, $-\text{CH}_3$, and $-\text{CH}_2\text{CH}_3$. The molecule therefore displays optical isomerism, existing as two non-superimposable mirror image enantiomers.
Notice that 1-bromobutane ($\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Br}$) does not show optical isomerism: the carbon bearing bromine also carries two hydrogen atoms, so two of its four substituents are identical. No chiral centre, no optical isomers.
Optical isomerism questions are reliably worth marks if you know the exact language the mark scheme expects. The following guidance is drawn directly from the patterns seen in AQA mark schemes and examiner reports.
Optical isomerism, then, rests on two interlocking ideas: the geometry of a tetrahedral chiral centre and the observable consequence of that geometry on plane-polarised light. Master those two ideas and you have covered both specification requirements completely.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-09-05.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Optical isomerism is a type of stereoisomerism that occurs when a molecule contains a chiral centre, a carbon atom bonded to four different groups. The molecule then exists as two non-superimposable mirror images, called enantiomers, which rotate plane-polarised light in opposite directions.
A chiral centre is a carbon atom bonded to four different groups, so there is no plane of symmetry through it. In alanine, CH₃CH(NH₂)COOH, the central carbon carries –CH₃, –NH₂, –COOH and –H, so it is a chiral centre.
Enantiomers have identical physical properties, such as melting point, boiling point and solubility, and identical chemical properties in achiral environments. They differ only in the direction they rotate plane-polarised light and in how they interact with other chiral molecules, such as enzymes.
Enantiomers rotate the plane of plane-polarised light by the same angle in opposite directions: one is dextrorotatory, rotating it clockwise, and the other is laevorotatory, rotating it anticlockwise. An equal mixture of both, a racemic mixture, produces no net rotation.
2-bromobutane, CH₃CHBrCH₂CH₃, shows optical isomerism because carbon 2 is a chiral centre, bonded to four different groups: –Br, –H, –CH₃ and –CH₂CH₃. The molecule therefore exists as two enantiomers, whereas 1-bromobutane does not, because its C–Br carbon carries two hydrogen atoms.
1-bromobutane, CH₃CH₂CH₂CH₂Br, does not show optical isomerism because the carbon bearing the bromine also carries two hydrogen atoms, so two of its four substituents are identical. With no chiral centre, there are no optical isomers.
The chiral centre in alanine, CH₃CH(NH₂)COOH, is the central carbon, which carries four different groups: –CH₃, –NH₂, –COOH and –H. Because no two are the same, alanine exists as two enantiomers.
Dextrorotatory describes an enantiomer that rotates the plane of plane-polarised light clockwise, to the right; laevorotatory describes one that rotates it anticlockwise, to the left. The magnitudes of rotation are equal; only the direction differs.
Plane-polarised light is light oscillating in only one plane, produced by passing ordinary light, which oscillates in all planes, through a polarising filter. A solution of a chiral compound rotates this plane, which is how enantiomers are told apart.
Optical isomerism is a type of stereoisomerism: optical isomers have the same structural formula and sequence of bonds but a different arrangement of atoms in space. The other kind of stereoisomerism is E/Z isomerism, from restricted rotation about a C=C double bond.
A chiral centre is found by going through each carbon in the molecule and checking whether all four groups attached to it are different. If they are, that carbon is a chiral centre; if any two are the same, it is not chiral and cannot give optical isomers.