Last updated: 21 August 2026
A molecular formula tells us which atoms are present and how many of each there are, but it does not necessarily tell us how those atoms are connected or arranged in space. Two compounds can therefore have exactly the same molecular formula and yet be different substances.
This is the basis of isomerism. Sometimes the difference lies in connectivity: different atoms are bonded to one another. In other cases, the connectivity remains unchanged but the atoms occupy different fixed positions in three-dimensional space.
Try these five questions before reading the chapter. Focus on molecular formulae, connectivity and what the structures actually show.
- 5 diagnostic questions
- Structures and stereochemistry where useful
- Instant explanation for every answer option
What Is Isomerism?
Isomers are compounds with the same molecular formula but different arrangements of atoms.
The requirement for the same molecular formula is fundamental. If two compounds contain different numbers of any type of atom, they cannot be isomers.
Once the formula has been confirmed to be the same, the next question is whether the difference lies in connectivity or only in the three-dimensional arrangement.
| Type of isomerism | What remains the same? | What differs? |
|---|---|---|
| Constitutional (structural) isomerism | Molecular formula | Connectivity |
| Stereoisomerism | Molecular formula and connectivity | Three-dimensional arrangement |
Always compare connectivity before considering stereochemistry.
Constitutional or Structural Isomerism
Constitutional isomers, also known as structural isomers, have the same molecular formula but their atoms are connected differently.
Three useful forms are chain isomerism, positional isomerism and functional group isomerism.
Chain isomerism
Chain isomerism occurs when compounds have the same molecular formula but different carbon skeletons.
Both compounds have molecular formula C₄H₁₀. Butane contains an unbranched carbon skeleton, whereas 2-methylpropane contains a branched skeleton.
Positional isomerism
Positional isomers have the same carbon skeleton and the same type of functional group, substituent or multiple bond, but that feature occurs at a different position.
Both compounds have molecular formula C₃H₈O and contain the alcohol functional group. Only the position of –OH has changed.
Functional group isomerism
Functional group isomers have the same molecular formula but contain different functional groups.
Both compounds have molecular formula C₂H₆O. Ethanol is an alcohol whereas methoxymethane is an ether.
Positional isomerism: the functional group remains the same but occurs at a different position.
Functional group isomerism: the functional group itself is different.
Stereoisomerism
Stereoisomers have the same molecular formula and the same connectivity but differ in the arrangement of atoms in three-dimensional space.
Two forms important here are:
- cis-trans isomerism;
- enantiomerism.
A different-looking drawing does not necessarily represent a different stereoisomer.
Rotation about an ordinary C–C single bond may change the appearance of a structural drawing without producing a different compound. A genuine stereoisomeric difference must remain after ordinary bond rotations are considered.
Cis-Trans Isomerism
Cis-trans isomerism commonly occurs in alkenes because rotation about a C=C double bond is restricted.
A carbon-carbon double bond contains one σ bond and one π bond. The π bond is formed by sideways overlap of parallel p orbitals.
Rotating one carbon atom relative to the other would destroy this sideways overlap. Rotation about C=C is therefore restricted.
The π bond prevents free rotation about C=C.
Conditions required for cis-trans isomerism
The presence of C=C alone is not sufficient. Two requirements must be met:
- rotation must be restricted, commonly by a C=C bond;
- each carbon atom of C=C must be bonded to two different groups.
But-2-ene
Each carbon of the C=C bond in but-2-ene is bonded to H and CH₃. Both alkene carbons therefore satisfy the two-different-groups condition.
cis-but-2-ene: the two CH₃ groups lie on the same side of C=C.
trans-but-2-ene: the two CH₃ groups lie on opposite sides of C=C.
Why propene does not show cis-trans isomerism
Propene has the condensed structural formula CH₃CH=CH₂.
One carbon of C=C is bonded to H and CH₃, but the terminal carbon is bonded to two H atoms.
Exam reasoning: propene cannot show cis-trans isomerism because one carbon of the C=C bond is bonded to two identical groups.
Enantiomerism and Chirality
Enantiomerism is a form of stereoisomerism in which two structures are non-superimposable mirror images.
A pair of hands provides a useful analogy. Your left and right hands are mirror images, but rotating one hand cannot make every part coincide exactly with the other.
Chiral carbon centres
A common source of enantiomerism in the molecules considered here is a tetrahedral carbon bonded to four different groups.
Such a carbon is described as a chiral carbon centre.
Compare complete groups, not merely the first atom attached.
For example, CH₃ and CH₂CH₃ are different groups even though both begin with a carbon atom.
Example: 2-bromobutane
Carbon-2 of 2-bromobutane is bonded to:
- H;
- Br;
- CH₃;
- CH₂CH₃.
These four groups are different, so carbon-2 is a chiral carbon centre.
Example: propan-2-ol is achiral
The central carbon atom of propan-2-ol is bonded to H, OH, CH₃ and CH₃.
Two attached groups are identical, so this carbon is not a chiral carbon centre.
Alkene carbons are not the usual chiral carbon centres
A carbon atom involved in C=C is trigonal planar rather than tetrahedral. It therefore does not satisfy the usual tetrahedral-carbon test for a chiral carbon centre.
Using symmetry
A molecule whose mirror image can be superimposed on the original molecule is achiral. The presence of an internal plane of symmetry can therefore help identify an achiral structure.
Properties of Enantiomers
The members of an enantiomeric pair have many identical properties when measured in an achiral environment.
| Property | Comparison between pure enantiomers |
|---|---|
| Melting point | Same under identical conditions |
| Boiling point | Same under identical conditions |
| Density | Same under identical conditions |
| Reaction with achiral reagents | Generally the same |
| Effect on plane-polarised light | Equal rotation in opposite directions under identical conditions |
| Interaction with chiral substances | May differ |
Optical activity
A pure enantiomer rotates the plane of plane-polarised light.
Its mirror-image enantiomer rotates the plane by the same angle in the opposite direction when measured under identical conditions.
The direction of optical rotation cannot be predicted simply by looking at the structural drawing.
Why enantiomers can behave differently biologically
Enzymes, receptors and many other biological molecules are themselves chiral. Their binding sites therefore have particular three-dimensional shapes.
One enantiomer may interact more effectively with a particular chiral binding site than its mirror image, even though the two enantiomers have the same ordinary physical properties in an achiral environment.
A Systematic Method for Classifying Isomers
When two structures are given, classify them using a fixed sequence.
Step 1: compare molecular formulae
If the formulae differ, the compounds are not isomers.
Step 2: compare connectivity
If the molecular formula is the same but connectivity differs, the compounds are constitutional isomers.
Step 3: investigate restricted rotation
If connectivity is identical and C=C is present, check each alkene carbon separately for two different groups.
Step 4: investigate chirality
Look for tetrahedral carbon atoms bonded to four different groups and determine whether two structures are non-superimposable mirror images.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Calling two compounds isomers without checking their molecular formulae | Isomers must have exactly the same molecular formula | Count each type of atom first |
| Confusing constitutional isomerism with stereoisomerism | Constitutional isomers have different connectivity; stereoisomers do not | Compare connectivity before considering three-dimensional arrangement |
| Assuming every alkene shows cis-trans isomerism | Each carbon of C=C must be bonded to two different groups | Check both alkene carbons separately |
| Saying propene shows cis-trans isomerism | The terminal CH₂ carbon is bonded to two identical H atoms | Apply the two-different-groups test to both carbons |
| Identifying a chiral centre without comparing all four groups | Two identical groups make the carbon achiral | Trace each complete group attached to the tetrahedral carbon |
| Calling every mirror-image pair enantiomers | Some mirror images are superimposable | Check both mirror-image relationship and non-superimposability |
| Marking a carbon of C=C as the usual chiral carbon centre | An alkene carbon is trigonal planar rather than tetrahedral | Look for a tetrahedral carbon bonded to four different groups |
| Saying enantiomers have different boiling points | Their ordinary physical properties are identical in an achiral environment | Distinguish ordinary physical properties from optical activity and chiral interactions |
What you should be able to do
Recognise isomers
Confirm that two compounds have exactly the same molecular formula before attempting to classify them.
Classify constitutional isomers
Distinguish chain, positional and functional group isomerism from molecular structures.
Recognise stereoisomerism
Identify cases where connectivity remains unchanged but the spatial arrangement differs.
Apply the cis-trans test
Check whether each carbon of C=C is bonded to two different groups.
Identify chiral carbon centres
Recognise tetrahedral carbons bonded to four different complete groups.
Recognise enantiomers
Identify non-superimposable mirror images and describe their optical behaviour.
These ten questions test whether you can classify unfamiliar structures systematically rather than simply recognise memorised examples.
- 10 mastery questions
- Structures and stereochemistry where useful
- Instant explanation for every answer option
Frequently asked questions
Questions Students Often Ask About Isomerism
These questions address some of the distinctions students most often need when interpreting unfamiliar organic structures.
Are constitutional isomerism and structural isomerism the same thing?
Yes. Both terms describe compounds with the same molecular formula but different connectivity.
Does every alkene show cis-trans isomerism?
No. Each carbon of C=C must be bonded to two different groups. If either carbon has two identical groups, cis-trans isomerism is impossible.
Does a chiral carbon have to be bonded to four different elements?
No. It must be bonded to four different groups. For example, CH₃ and CH₂CH₃ are different groups even though both begin with carbon.
Are all mirror-image structures enantiomers?
No. Enantiomers must be mirror images that cannot be superimposed. If a structure and its mirror image are superimposable, they represent the same achiral molecule.
What should I check first when comparing two possible isomers?
First confirm that their molecular formulae are identical. Then compare connectivity. Only if connectivity is the same should you investigate stereoisomerism.
Isomerism becomes much easier when structures are classified systematically. First confirm that the molecular formula is the same, then compare connectivity. Different connectivity gives constitutional isomerism, while unchanged connectivity with a different fixed spatial arrangement gives stereoisomerism. Restricted rotation explains cis-trans isomerism, while non-superimposable mirror-image structures explain enantiomerism.