Optical Isomerism
A concise Cambridge International AS Level Chemistry revision guide to optical isomerism (13.4.2 and 13.4.4).
It covers stereoisomerism and its two divisions, chiral centres and enantiomers, chiral centres in chain and cyclic molecules (13.4.5), and how to deduce all the isomers of a molecular formula (13.4.6).
- 13.4.2-a
- 13.4.2-c
- 13.4.4-a
- 13.4.4-b
- 13.4.5-a
- 13.4.6
What these spec points say
- 13.4.2-a describe stereoisomerism
- 13.4.2-c describe optical isomerism as a division of stereoisomerism
- 13.4.4-a explain what is meant by a chiral centre
- 13.4.4-b explain that a chiral centre gives rise to two optical isomers (enantiomers)
- 13.4.5-a identify chiral centres in a molecule of given structural formula including cyclic compounds
- 13.4.6 deduce the possible isomers for an organic molecule of known molecular formula
Mirror-Image Molecules
Choose a step to watch butan-2-ol meet its mirror image, fail to superimpose, and become the other enantiomer when two groups swap. Drag to rotate.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Before You Start
This page builds on structural isomers and on geometrical isomerism in alkenes.
- Isomers have the same molecular formula.
- A carbon with four single bonds is tetrahedral, with angles of 109.5°.
- Cis/trans isomers come from restricted rotation about a C=C bond.
- Your left and right hands are mirror images that cannot be stacked exactly on top of each other.
GCSE Recap: Isomers and Mirror Images
Four quick questions on molecular formulae, isomers and mirror images before you meet stereoisomers.
Stereoisomerism and Its Two Divisions
Stereoisomers have the same molecular formula and the same structural formula, but their atoms are arranged differently in space. Cambridge 13.4.2 divides stereoisomerism into two types.
| Type | Cause | What differs | Example |
|---|---|---|---|
| Geometrical (cis/trans) isomerism | Restricted rotation about a C=C bond or a ring | Which side of the double bond or ring the groups lie on | cis- and trans-but-2-ene (see the Geometric Isomerism page) |
| Optical isomerism | A chiral centre: a carbon atom bonded to four different groups | The two isomers are non-superimposable mirror images of each other | The two enantiomers of butan-2-ol |
Key idea: Structural isomers differ in how the atoms are bonded together; stereoisomers are bonded in the same order but arranged differently in three dimensions.
Stereoisomers divide into geometrical isomers, from restricted rotation, and optical isomers, from a chiral centre.
Quick Check: Which Kind of Stereoisomerism?
Five molecules, one decision each: does it show geometrical isomerism, optical isomerism, both or neither?
Chiral Centres and Enantiomers
A chiral centre is a carbon atom bonded to four different atoms or groups.
Because the four groups are arranged tetrahedrally, a molecule with a chiral centre and its mirror image cannot be superimposed, in the same way that a left hand cannot be superimposed on a right hand.
The two mirror-image forms are called enantiomers (optical isomers).
Take butan-2-ol, CH3CH(OH)CH2CH3. Carbon 2 is bonded to H, OH, CH3 and CH2CH3: four different groups, so it is a chiral centre and butan-2-ol exists as two enantiomers.
Butan-1-ol has no carbon with four different groups (carbon 1 carries two hydrogens), so it has no optical isomers.
The enantiomers of butan-2-ol are mirror images that cannot be superimposed; the chiral centre is marked with an asterisk.
Enantiomers have identical physical properties except for one: they rotate the plane of plane-polarised light in opposite directions, which is why the phenomenon is called optical isomerism.
Their chemical reactions are identical unless they react with another chiral molecule, which is why enantiomers of a drug can behave differently in the body.
Exam sentence: A chiral centre is a carbon atom attached to four different groups; it gives rise to two optical isomers, called enantiomers, which are non-superimposable mirror images of each other.
Enantiomers are identical except in rotating plane-polarised light and in reactions with other chiral molecules.
Quick Check: Explain the Two Carvones
Write a short explanation, then compare it with the mark points and the model answer.
Identifying Chiral Centres
Cambridge 13.4.5 asks you to identify chiral centres in a given structural formula, including cyclic compounds.
Check every carbon that carries four single bonds and ask whether all four groups are different. A carbon with two hydrogens, two methyl groups, or any repeated group is not chiral.
| Molecule | Carbon checked | Four groups | Chiral? |
|---|---|---|---|
| 2-Bromobutane | C2 | H, Br, CH3, CH2CH3 | Yes |
| 2-Bromopropane | C2 | H, Br, CH3, CH3 | No: two methyl groups |
| 2-Hydroxypropanoic acid (lactic acid) | C2 | H, OH, CH3, COOH | Yes |
| 3-Methylcyclohexene | C3 | H, CH3, and the two ring arms which differ (one leads to C=C, the other to CH2) | Yes |
| Methylcyclohexane | C1 | H, CH3, and two identical ring arms | No |
In a ring, the two “arms” of the ring count as two of the four groups. They are different only if going round the ring in each direction meets different atoms, as in 3-methylcyclohexene where one direction reaches the double bond first.
Checking a ring carbon: the two ring arms are different groups only if the ring is different in each direction.
Exam focus: Mark chiral centres with an asterisk on the structure. Molecules with one chiral centre have two optical isomers; you are not expected to assign R and S labels at AS Level.
Quick Check: Find the Chiral Centres
Click every formula that contains a carbon bonded to four different groups.
Quick Check: Chiral Centres in Rings
Trace each ring in both directions before you choose.
Deducing All the Isomers of a Formula
Cambridge 13.4.6 asks you to deduce the possible isomers for a molecule of known molecular formula. Work through the types in order so that none is missed.
- Chain isomers: draw every carbon skeleton, straight then branched.
- Positional isomers: move the functional group or multiple bond to each distinct position.
- Functional group isomers: check whether a different class fits the formula (for example an alkene and a cycloalkane, or an aldehyde and a ketone).
- Geometrical isomers: for each C=C with two different groups on each carbon, draw cis and trans.
- Optical isomers: mark any chiral centre; each gives a pair of enantiomers.
For C4H9Br: the chain isomers give 1-bromobutane, 2-bromobutane, 1-bromo-2-methylpropane and 2-bromo-2-methylpropane; there are no C=C bonds, so no geometrical isomers; 2-bromobutane has a chiral centre, so it exists as two enantiomers. That makes five isomers in total.
Exam sentence: To find every isomer, list the structural isomers (chain, positional, functional group) first, then check each one for geometrical and optical isomerism.
Quick Check: Count the Isomers
Work each count out on paper, then flip the card to check your working.
Quick Check: Pick the Accurate Statement
In each round, choose the one statement that is accurate.
Common Exam Mistakes
- Calling any carbon with four bonds a chiral centre. The four groups must all be different.
- Treating the two arms of a ring as automatically different, or automatically the same. Trace the ring in both directions.
- Saying enantiomers have different boiling points or reactivity. They differ only in the direction they rotate plane-polarised light and in reactions with other chiral molecules.
- Forgetting the optical isomers when counting the isomers of a formula, or counting them for a molecule with no chiral centre.
Exam sentence: Optical isomers arise from a chiral centre, a carbon bonded to four different groups, and exist as two non-superimposable mirror images called enantiomers.
FAQs
These questions cover the optical isomerism points students most often get wrong.
Is every carbon with four single bonds a chiral centre?
No. All four groups must be different. 2-bromopropane has two methyl groups on carbon 2, so it is not chiral.
How do enantiomers differ?
They rotate plane-polarised light in opposite directions and can react differently with other chiral molecules. Their other physical and chemical properties are the same.
Can a ring carbon be a chiral centre?
Yes, if the two ring arms differ. Trace the ring both ways from that carbon: if you meet different atoms or bonds, the arms are different groups.
How many optical isomers does one chiral centre give?
Two. They are non-superimposable mirror images called enantiomers.
How many isomers does C4H9Br have?
Five. There are four structural isomers, and 2-bromobutane has a chiral centre, so it exists as two enantiomers.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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