13.4 Structural isomerism and stereoisomerism
- Syllabus
- 9701–2028–2029
- Topic
- 13.4
- Level
- AS
Learning objectives
13.4.1Structural isomerism and its division• Describe structural isomerism and its division into chain, positional and functional group isomerism13.4.2Stereoisomerism and its division• Describe stereoisomerism and its division into geometrical (cis/trans) and optical isomerism (use of E/Z nomenclature is acceptable but is not required)13.4.3Geometrical (cis/trans) isomerism in alkenes,• Describe geometrical (cis/trans) isomerism in alkenes, and explain its origin in terms of restricted rotation due to the presence of π bonds13.4.4Chiral centres and enantiomers• Explain what is meant by a chiral centre and that such a centre gives rise to two optical isomers (enantiomers) (Candidates should appreciate that compounds can contain more than one chiral centre, but knowledge of meso compounds, or nomenclature such as diastereoisomers is not required.)13.4.5Chiral centres and geometrical (cis/trans)• Identify chiral centres and geometrical (cis/trans) isomerism in a molecule of given structural formula, including cyclic compounds13.4.6The possible isomers for an organic molecule• Deduce the possible isomers for an organic molecule of known molecular formula
Structural isomers share a molecular formula but differ in connectivity
Structural isomers have the same molecular formula but different structural formulae: their atoms are connected differently.
| Type | What changes | Example pair |
|---|---|---|
| chain | carbon skeleton | butane and 2-methylpropane |
| positional | position of a functional group or multiple bond on the same skeleton | propan-1-ol and propan-2-ol |
| functional group | atoms connect to give different functional groups | propan-1-ol and methoxyethane |
First confirm identical molecular formulae, then compare connectivity and classify the first structural feature that differs. Different names alone do not prove the isomer type.
Rotation around a single bond changes conformation without changing connectivity, so redrawing or rotating one structure does not create a structural isomer.
Stereoisomers keep connectivity but differ in three-dimensional arrangement
Stereoisomers have the same molecular formula and the same atom connectivity but a different arrangement of those atoms in space.
| Stereoisomerism | Structural origin | Relationship |
|---|---|---|
| geometrical (cis/trans) | restricted rotation in a suitable alkene or cyclic structure | substituents occupy different sides |
| optical | one or more chiral centres | non-superimposable mirror-image enantiomers arise from a chiral centre |
Always compare connectivity first. If it differs, the pair is structural rather than stereo; only then test for restricted rotation or a chiral centre. E/Z nomenclature is acceptable but not required here.
The same molecular formula alone is insufficient for stereoisomerism. Same connectivity is the defining extra condition.
Alkene cis/trans isomerism comes from restricted rotation about C=C
A C=C contains a pi bond formed by sideways overlap of p orbitals. Rotation about the double bond would destroy that overlap, so rotation is restricted and different fixed spatial arrangements can exist.
Each alkene carbon must be attached to two different groups. When a corresponding pair lies on the same side the isomer is cis; when it lies on opposite sides it is trans. If cis/trans labels are ambiguous, E/Z may be used but is not required.
| Alkene | Cis/trans possible? | Reason |
|---|---|---|
| but-2-ene | yes | each C of C=C bears H and CH3 |
| but-1-ene | no | terminal C of C=C bears two H atoms |
Restricted rotation alone is not enough: identical substituents on either double-bond carbon make the two apparent arrangements identical.
A chiral carbon gives a pair of non-superimposable mirror-image enantiomers
A chiral centre is a tetrahedral carbon attached to four different atoms or groups. Its two mirror-image arrangements cannot be superimposed and form a pair of optical isomers called enantiomers.
In butan-2-ol, the carbon bearing OH is attached to H, OH, CH3 and CH2CH3, so it is chiral and gives two enantiomers.
A compound can contain more than one chiral centre. Check each tetrahedral carbon independently by tracing all four attached groups until their first point of difference; this section does not require meso or diastereoisomer nomenclature.
A wedge or dash drawing does not itself prove chirality, and a carbon bearing two identical groups is not a chiral centre.
Identify chiral centres and cis/trans possibilities in open-chain and cyclic structures
Chirality test: inspect every tetrahedral carbon and compare its four groups, tracing different paths when rings are present. Geometrical test: find a suitable C=C or ring that restricts relative positions, then check that the substituent pattern can give distinct same-side and opposite-side forms.
| Feature in given structure | Required evidence |
|---|---|
| possible chiral carbon | four different attached groups/paths |
| possible alkene cis/trans | two different groups on each C of C=C |
| possible cyclic cis/trans | substituents fixed on same or opposite faces of the ring |
Do not count every wedge/dash atom as chiral or every double bond as geometrically isomeric. In a ring, two paths that eventually become identical do not create four different groups.
Deduce possible isomers systematically, then remove duplicates
For a known molecular formula: 1) satisfy valency and identify possible unsaturation/rings; 2) enumerate distinct carbon skeletons; 3) place multiple bonds and allowed functional groups at non-equivalent positions; 4) include functional-group alternatives; 5) test every structural formula for cis/trans and chiral-centre stereoisomers; 6) remove structures related only by renumbering, rotation or redrawing.
For C4H8, structural possibilities include straight/branched alkenes and cyclic structures. But-2-ene then contributes cis and trans stereoisomers, whereas but-1-ene does not because one alkene carbon has two H atoms.
For every candidate, recount every element, complete four bonds around carbon and compare connectivity with structures already listed. Only after unique structural isomers are fixed should separate stereoisomers be counted.
Do not count different orientations of the same connectivity, and do not stop after one functional-group family when the molecular formula permits another.