The big idea: Isomers are different compounds that share the same molecular formula.
Because the same atoms can be put together in more than one way, one formula can describe several real, distinct substances — often with different properties.
There are two big families:
- Structural isomers — same molecular formula, but a different connectivity (which atom is bonded to which). - Stereoisomers — same molecular formula and the same connectivity, but a different arrangement in space.
Key terms: - Molecular formula — how many of each atom (e.g. C5H12); says nothing about the arrangement. - Connectivity — the bonding pattern: which atom is joined to which. - Structural isomers — differ in connectivity. - Stereoisomers — same connectivity, differ only in spatial arrangement. - Chiral carbon — a carbon bonded to four different groups (introduced in §4).
The whole topic is a single decision tree: do the isomers differ in which atoms are bonded together? If yes they are structural isomers; if no (same bonds, different shape in space) they are stereoisomers.
| Class of isomer | What is the same | What is different |
|---|---|---|
| Structural isomers | molecular formula | connectivity — which atom is joined to which |
| Stereoisomers | molecular formula and connectivity | the spatial arrangement of the atoms |
| – cis/trans (E/Z) | connectivity, around a C=C | the geometry across the double bond |
| – optical (enantiomers) | connectivity, around a chiral C | left/right-handed mirror images |
Structural (constitutional) isomers have the same molecular formula but a different connectivity. There are three kinds you must recognise.
The three structural types: - Chain isomerism — the carbon skeleton differs (a straight chain vs a branched chain). - Position isomerism — the functional group sits in a different place on the same chain. - Functional-group isomerism — the atoms form a different functional group entirely (a different family of compound).
A five-carbon straight chain. Same molecular formula C5H12 as the branched isomers below, but a different arrangement of carbons.
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A four-carbon chain with a CH3 branch on carbon 2 — same formula C5H12, different connectivity. A chain (skeletal) isomer of pentane.
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Chain isomers — C_{5}H_{12}: Pentane (a straight five-carbon chain) and 2-methylbutane (a four-carbon chain with a CH3 branch) have the same formula C5H12 but different skeletons — they are chain isomers. There is a third: 2,2-dimethylpropane.
The –OH is on the end carbon. Same formula C3H8O, but the functional group is in a different position from propan-2-ol.
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The –OH is on the middle carbon. A position isomer of propan-1-ol (same formula, group in a different place).
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Position isomers — C_{3}H_{8}O: Propan-1-ol (–OH on the end carbon) and propan-2-ol (–OH on the middle carbon) are position isomers: the same chain and the same –OH group, just in a different position.
| Type of structural isomer | What changes | Example pair (same formula) |
|---|---|---|
| Chain isomerism | the carbon skeleton (straight vs branched) | pentane and 2-methylbutane (both C5H12) |
| Position isomerism | the position of the functional group on the same chain | propan-1-ol and propan-2-ol (both C3H8O) |
| Functional-group isomerism | the functional group itself (a different family) | ethanol (an alcohol) and methoxymethane (an ether), both C2H6O |
Functional-group isomers: Ethanol (CH3CH2OH, an alcohol) and methoxymethane (CH3OCH3, an ether) are both C2H6O — but the atoms form a different functional group, so they belong to different families with very different chemistry. That is functional-group isomerism.
Worked example — name the structural isomerism
Butan-1-ol and butan-2-ol both have the molecular formula C4H10O. What type of structural isomerism do they show?
Solution
- Check the formula: both are C4H10O — so they are isomers.
- Both are alcohols (–OH) on the same four-carbon chain, so the family and skeleton are the same.
- The only difference is where the –OH sits: carbon 1 vs carbon 2. That is a change of position.
Final answer
Position isomerism (same chain and same –OH group, different position).
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Stereoisomers have the same connectivity — every atom is bonded to the same neighbours — but a different arrangement in space. At HL there are two kinds: cis/trans (E/Z) isomerism and optical isomerism (§4).
cis/trans isomerism: cis/trans isomerism happens around a C=C double bond.
The two atoms in a C=C cannot rotate about the bond — the π bond locks them. So groups attached to the two carbons are fixed on one side or the other of the double bond, giving two distinct molecules.
- cis — the two like (higher-priority) groups are on the same side of the double bond. - trans — they are on opposite sides.
Both conditions must be met: cis/trans isomers exist only when both are true:
| Requirement for cis/trans | Why it matters |
|---|---|
| A C=C double bond (restricted rotation) | atoms cannot rotate about a C=C, so the arrangement is locked in place |
| Two different groups on each doubly-bonded carbon | if a carbon carries two identical groups, swapping them gives the same molecule — no isomers |
cis (same side)
- the two like groups are on the same side of the C=C
- e.g. cis-but-2-ene: both CH3 groups on one side
- polar molecule overall → usually a higher boiling point
trans (opposite sides)
- the two like groups are on opposite sides of the C=C
- e.g. trans-but-2-ene: the CH3 groups across from each other
- more symmetrical, less polar → usually lower boiling point, packs better as a solid
Why but-2-ene works but but-1-ene does not: But-2-ene, CH3CH=CHCH3, has a CH3 and an H on each doubly-bonded carbon — two different groups on each carbon, so cis and trans both exist.
But-1-ene, CH2=CHCH2CH3, has two identical H atoms on the first carbon. Swapping them gives the same molecule, so there are no cis/trans isomers.
The E/Z system (a quick note): When the groups are not simply 'a like pair', chemists use E/Z instead of cis/trans. On each carbon, give priority to the atom of higher atomic number (Cahn–Ingold–Prelog rules).
- Z (from German zusammen, together) — the two higher-priority groups are on the same side (often = cis). - E (entgegen, opposite) — the higher-priority groups are on opposite sides (often = trans).
IB-style question — explain cis/trans
But-2-ene, CH3CH=CHCH3, shows cis/trans isomerism but butane, CH3CH2CH2CH3, does not. Explain why. [3]
How to score the marks
- But-2-ene has a C=C double bond, and there is restricted rotation about a C=C (the π bond prevents the groups from rotating).
- Each doubly-bonded carbon carries two different groups (a CH3 and an H), so the CH3 groups can be locked on the same side (cis) or opposite sides (trans) — two distinct molecules.
- Butane has only single (C–C) bonds, about which groups can rotate freely, so the atoms are never locked into different fixed arrangements — no cis/trans isomers.
Final answer
But-2-ene has a C=C (restricted rotation) with two different groups on each carbon → cis and trans forms; butane has only freely-rotating single bonds, so no cis/trans isomerism.
Optical isomerism: Optical isomerism is the other kind of stereoisomerism. It arises when a molecule contains a chiral carbon — a carbon atom bonded to four different groups.
Such a molecule and its mirror image are non-superimposable — no matter how you turn one, it never lines up exactly on the other (like your left and right hands). The two mirror-image forms are called enantiomers.
Key terms: - Chiral carbon (stereocentre) — a carbon bonded to four different atoms or groups; often marked with an asterisk, C. - Enantiomers — the two non-superimposable mirror-image forms of a chiral molecule. - Non-superimposable — the two cannot be made to coincide by rotation (hence 'handedness'). - Plane-polarised light* — light whose waves vibrate in a single plane.
Spotting a chiral carbon is the key skill: look for a carbon with four different groups attached. Butan-2-ol, CH3CH(OH)CH2CH3, has a central carbon bonded to H, OH, CH_{3} and CH_{2}CH_{3} — four different groups — so it is chiral and exists as two enantiomers.
Chiral carbon → optical isomers
- carbon has four different groups
- molecule and its mirror image are non-superimposable
- e.g. butan-2-ol: central C has H, OH, CH3, C2H5
Not chiral → no optical isomers
- any carbon carries two (or more) identical groups
- the molecule is superimposable on its mirror image
- e.g. propan-2-ol: central C has two CH_{3} groups
How enantiomers differ — optical activity: Enantiomers have identical physical properties (same melting point, boiling point, density) and the same chemistry with non-chiral reagents.
They differ in just one way: each rotates the plane of plane-polarised light by the same angle but in opposite directions — one clockwise (+, dextrorotatory), the other anticlockwise (−, laevorotatory). This is optical activity — the source of the name.
A 50:50 mixture of the two enantiomers (a racemic mixture) shows no net rotation, because the two effects cancel.
IB-style question — identify the chiral carbon
Explain why 2-chlorobutane, CH3CHClCH2CH3, shows optical isomerism, and describe how its two isomers can be distinguished. [3]
How to score the marks
- The second carbon is bonded to four different groups: H, Cl, CH3 and CH2CH3 — so it is a chiral carbon.
- A chiral carbon makes the molecule and its mirror image non-superimposable, giving two enantiomers (optical isomers).
- The two enantiomers rotate plane-polarised light by the same angle in opposite directions (one +, one −) — that is how they are told apart.
Final answer
Carbon 2 has four different groups (H, Cl, CH3, C2H5) → a chiral carbon → non-superimposable mirror images (enantiomers) that rotate plane-polarised light in opposite directions.