A structural formula tells us which atoms are connected, but it does not always tell the whole chemical story. Molecules are three-dimensional. Bonds rotate, rings fold, substituents may lie on different sides of a rigid framework, and a tetrahedral atom can arrange four different groups in two mirror-image ways.
At H3 level, the aim is therefore not simply to recognise that stereoisomers exist. You should be able to visualise them, assign their configurations systematically and explain why apparently small changes in spatial arrangement can produce different stabilities, physical properties or chemical behaviour.
Try these five questions before reading the chapter. They test the essential ideas behind conformations, geometric isomerism and chirality.
- 5 diagnostic questions
- 3D reasoning where useful
- Instant explanation for every answer option
Conformational isomerism and Newman projections
Begin with the simplest kind of three-dimensional change. A single σ bond is cylindrically symmetrical, so rotation can occur about its internuclear axis. As one end of a molecule rotates relative to the other, the connectivity remains unchanged but its conformation changes.
These different arrangements are called conformations or conformers. They normally interconvert rapidly at room temperature because no σ bond needs to be broken.
Conformation changes by rotation about bonds. Configuration cannot normally be changed without breaking and reforming bonds.
Looking directly along a bond: the Newman projection
A Newman projection is drawn as though you were looking directly down a particular carbon–carbon bond. The carbon nearest your eye is the front carbon; the other is the back carbon.
The front carbon is represented by a point at the centre. Its three bonds radiate from that point. The back carbon is represented by a circle, and its three bonds begin at the circumference of the circle.
In the staggered conformation, the bonds on the back carbon lie between those on the front carbon. The nearest front–back bonds have a dihedral angle of 60°. In the eclipsed conformation, corresponding bonds line up and the dihedral angle is 0°.
Why staggered conformations are more stable
Rotation about a C–C bond is therefore not completely energetically free. An eclipsed arrangement is higher in energy because electron density in neighbouring σ bonds is brought into an unfavourable arrangement. This contribution is commonly described as torsional strain.
Staggered: lower torsional strain and therefore lower energy.
Eclipsed: greater torsional strain and therefore higher energy.
Butane introduces steric effects
Ethane only requires us to compare hydrogen atoms. Butane is more interesting because we can look along the central C₂–C₃ bond and follow the relative positions of its two CH₃ groups.
Several important conformations appear during a complete 360° rotation.
| Conformation | CH₃–CH₃ dihedral angle | Relative stability |
|---|---|---|
| Anti | 180° | Lowest energy; the two CH₃ groups are furthest apart |
| Gauche | 60° or 300° | Local minimum; staggered but the CH₃ groups are closer |
| Eclipsed CH₃/H | 120° or 240° | Higher energy because of eclipsing interactions |
| Fully eclipsed CH₃/CH₃ | 0° or 360° | Highest energy |
Staggered does not automatically mean equally stable. Anti-butane and gauche-butane are both staggered, but anti-butane is more stable because the two CH₃ groups are further apart.
Cyclohexane: conformational isomerism in a ring
Rotation becomes more constrained when the carbon atoms form a ring. Cyclohexane is especially important because it does not normally exist as a flat hexagon. A planar cyclohexane ring would contain unfavourable eclipsing interactions and would not maintain ideal tetrahedral bond angles.
Instead, cyclohexane puckers. Its most stable conformation is the chair conformation, in which bond angles remain close to the tetrahedral value and neighbouring C–H bonds are largely staggered.
Axial and equatorial positions
Each carbon in a cyclohexane chair possesses one axial bond and one equatorial bond. The six axial bonds are approximately parallel to one another and alternate up, down, up, down around the ring. Equatorial bonds point outward around the perimeter of the ring.
Cyclohexane can undergo a ring flip in which one chair converts into another. During this process:
Axial becomes equatorial.
Equatorial becomes axial.
Up remains up and down remains down.
The last point is crucial. A ring flip changes axial/equatorial orientation but does not change the configuration of a substituent from one face of the ring to the other.
Why bulky substituents favour equatorial positions
When a large group occupies an axial position, it approaches other axial groups on the same side of the ring. These unfavourable interactions are called 1,3-diaxial interactions.
Moving the substituent to the equatorial position reduces these steric interactions. Consequently, the more sterically demanding a substituent is, the more strongly it usually favours the equatorial position.
Example: In methylcyclohexane, the chair containing equatorial CH₃ is more stable than the chair containing axial CH₃. The two structures are conformers, not different configurational isomers.
Geometric isomerism: cis–trans and E/Z
A C=C double bond behaves very differently from a C–C single bond. Rotation about the double bond would require disruption of the π bond, so free rotation does not occur under ordinary conditions.
If each alkene carbon is attached to two different groups, two distinct arrangements may therefore exist. These are geometric stereoisomers.
Why E/Z is more general than cis–trans
The words cis and trans work conveniently when there is an obvious pair of comparable groups. However, many alkenes have four different substituents, making simple cis/trans descriptions ambiguous.
The E/Z system avoids this problem by assigning a priority to the two substituents attached to each alkene carbon.
The Cahn–Ingold–Prelog priority rules
- Compare the atoms directly attached to the alkene carbon.
- The atom with the higher atomic number receives the higher priority.
- If the directly attached atoms are identical, move one bond further outward and compare the attached atoms in decreasing atomic-number order.
- Continue until the first point of difference is found.
- Multiple bonds are handled by treating an atom as though it were bonded to the appropriate number of duplicate atoms for priority comparison.
- For isotopes of the same element, the isotope of greater mass number receives higher priority.
Z — zusammen: the higher-priority group on each alkene carbon lies on the same side of the double bond.
E — entgegen: the higher-priority groups lie on opposite sides.
Suppose one alkene carbon is attached to H and CH₃, while the other is attached to Cl and Br. CH₃ has higher priority than H because C has a higher atomic number than H. Br has higher priority than Cl because Br has a higher atomic number than Cl. The relative positions of CH₃ and Br therefore determine whether the alkene is E or Z.
A useful CIP tie-breaker
Consider comparing –CHO with –CH₂OH. In both groups the atom directly attached to the stereochemical centre is carbon, so the first comparison is tied.
For CIP purposes, the carbonyl carbon of –CHO is treated as being attached to O, O and H, whereas the carbon of –CH₂OH is attached to O, H and H. The first point of difference is therefore O versus H, so –CHO has higher priority than –CH₂OH.
Geometric isomerism in coordination compounds
Restricted spatial arrangements are not confined to organic molecules. Coordination compounds can also display geometric isomerism because ligands occupy fixed positions around a metal centre.
Square-planar complexes
A square-planar complex of the form MA₂B₂ can often exist as cis and trans forms. A classic example is [Pt(NH₃)₂Cl₂].
In the cis isomer, identical ligands occupy neighbouring positions and are separated by 90°. In the trans isomer, the identical ligands lie opposite one another and are separated by 180°.
Octahedral complexes
The same idea occurs in suitable octahedral complexes. For example, [Co(NH₃)₄Cl₂]⁺ can contain its two Cl ligands either 90° apart (cis) or 180° apart (trans).
Cis/trans is about relative position, not whether a ligand is drawn literally to the left or right on the page.
Chirality and enantiomerism
We now move from geometric isomerism to chirality. An object is chiral if it cannot be superimposed on its mirror image. Your left and right hands provide the familiar macroscopic analogy.
One common source of molecular chirality is a tetrahedral carbon atom attached to four different groups. Such an atom is commonly called a chiral centre or stereogenic centre.
The two structures have the same formula, the same sequence of bonded atoms and the same ordinary functional groups. Their difference is purely three-dimensional: they are non-superimposable mirror images and are therefore enantiomers.
Assigning R and S configurations
The R/S system again uses the Cahn–Ingold–Prelog rules.
- Assign priorities 1, 2, 3 and 4 to the four substituents.
- Orient the structure so that the lowest-priority group, 4, points away from you.
- Follow the sequence 1 → 2 → 3.
- A clockwise sequence gives R.
- An anticlockwise sequence gives S.
Important: if priority 4 points towards you rather than away from you, the clockwise/anticlockwise result must be reversed.
What properties do enantiomers share?
In an achiral environment, two pure enantiomers have the same ordinary melting point, boiling point and many other bulk physical properties. They differ, however, in their interaction with other chiral systems.
One directly measurable difference is their effect on plane-polarised light. Under identical conditions, the two enantiomers rotate the plane of polarisation by equal magnitudes but in opposite directions.
This difference can be especially important in biological systems because enzymes, receptors and other biomolecules are themselves chiral.
Racemic mixtures and optical purity
A 50:50 mixture of two enantiomers is called a racemic mixture. Because the rotations produced by the two enantiomers cancel, an ideal racemic mixture has zero net optical rotation.
The rotations must be compared under the same experimental conditions, including temperature, wavelength, concentration and path length where relevant.
Example: A pure enantiomer gives a rotation of +100°, while a sample measured under identical conditions gives +80°.
Optical purity = 80 / 100 × 100% = 80%.
If the sample contains only the two enantiomers, this corresponds to an enantiomeric excess of 80%. The composition is therefore 90% of the major enantiomer and 10% of the minor enantiomer.
Diastereomerism
Not every pair of stereoisomers is related as mirror images. Diastereomers are stereoisomers that are not mirror images of one another.
This category includes many molecules containing two or more stereogenic centres. Geometric E/Z or cis/trans pairs are also diastereomeric relationships because the two structures are stereoisomers but are not mirror images.
Enantiomers: non-superimposable mirror images.
Diastereomers: stereoisomers that are not mirror images.
Multiple stereocentres
For a molecule containing n independent stereogenic centres, the maximum possible number of configurational stereoisomers is often:
The word maximum matters. Molecular symmetry can reduce the actual number.
Meso compounds
A meso compound contains stereogenic centres but is nevertheless achiral because the molecule possesses internal symmetry that makes it superimposable on its mirror image.
2,3-dichlorobutane provides a useful example. Its stereochemical possibilities can be summarised as follows.
| Configuration | Relationship | Chirality |
|---|---|---|
| (2R,3R) | Enantiomer of (2S,3S) | Chiral |
| (2S,3S) | Enantiomer of (2R,3R) | Chiral |
| (2R,3S) | Meso form | Achiral |
The presence of stereogenic centres does not by itself prove that the whole molecule is chiral. Always consider the symmetry of the complete molecule.
Why diastereomers often have different physical properties
Enantiomers interact identically with an achiral environment, but diastereomers do not possess this mirror-image relationship. Their energies and intermolecular interactions can therefore differ.
Diastereomers may consequently have different melting points, boiling points, solubilities, chromatographic behaviour and reactivities.
Enantiomerism in coordination compounds
Chirality does not require a carbon stereocentre. Certain coordination compounds are chiral because of the three-dimensional arrangement of ligands around the metal.
A classic example is an octahedral complex containing three bidentate ligands, such as [M(en)₃]ⁿ⁺. The chelate rings can wind around the metal centre in two opposite helical senses.
Δ form: one helical arrangement of the three chelate rings.
Λ form: the opposite helical arrangement.
The Δ and Λ forms are non-superimposable mirror images and are therefore enantiomers.
Putting the different forms of isomerism together
Students often find stereochemistry difficult because several different ideas are introduced at once. The most useful approach is to ask a series of questions in a fixed order.
| Question to ask | If yes | What to think about |
|---|---|---|
| Can the structures interconvert simply by bond rotation? | Conformers | Newman projections, dihedral angle and conformational energy |
| Is rotation restricted around a C=C bond or within a rigid geometry? | Geometric isomerism may occur | cis/trans or E/Z |
| Are the structures non-superimposable mirror images? | Enantiomers | Chirality and R/S configuration |
| Are they stereoisomers but not mirror images? | Diastereomers | Multiple stereocentres, E/Z or cis/trans relationships |
| Does a molecule with stereocentres possess internal symmetry? | It may be meso | Check the whole molecule before declaring it chiral |
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Treating staggered and eclipsed ethane as different configurational isomers | They interconvert by rotation about the C–C σ bond. | Classify them as conformers. |
| Assuming every staggered conformation has the same energy | In butane, anti and gauche are both staggered but differ in steric interaction. | Consider both torsional and steric effects. |
| Using the visually largest substituent to assign E/Z priority | CIP priority depends on atomic number and the first point of difference, not apparent size. | Apply the CIP rules systematically. |
| Reading clockwise as R without checking priority 4 | The direct clockwise/anticlockwise rule applies only when the lowest-priority group points away. | Orient priority 4 away first, or reverse the result if it points towards you. |
| Assuming a ring flip converts cis into trans | A ring flip changes axial ↔ equatorial but preserves up/down orientation. | Track the face of the ring separately from axial/equatorial position. |
| Assuming a molecule with two stereocentres must have four stereoisomers | Internal symmetry can produce a meso form and reduce the total. | Treat 2ⁿ as a maximum and check molecular symmetry. |
| Saying a racemic mixture is inactive because each molecule is achiral | Each enantiomer is chiral and optically active. | Explain that equal and opposite rotations cancel macroscopically. |
What you should be able to do
Read Newman projections
Identify staggered, eclipsed, anti and gauche conformations and determine relevant dihedral angles.
Compare conformational stability
Explain conformational energy in terms of torsional strain and steric interactions.
Analyse cyclohexane chairs
Distinguish axial and equatorial positions and predict the effect of a ring flip.
Assign E and Z
Apply CIP priorities correctly, including tie-breaking and multiple bonds.
Assign R and S
Rank four substituents and determine absolute configuration from a three-dimensional representation.
Distinguish stereoisomer relationships
Recognise enantiomers, diastereomers, meso compounds and chiral coordination compounds.
These ten questions test whether you can apply conformational, geometric and optical-isomerism ideas rather than simply recall definitions.
- 10 mastery questions
- Conformations, CIP rules and stereochemical reasoning
- Instant explanation for every answer option
Frequently asked questions
Questions students often ask about stereochemistry
Once the different labels are separated clearly, most stereochemistry questions become a matter of deciding what type of three-dimensional relationship you are looking at.
Are conformers really different isomers if they interconvert so quickly?
They are different conformations of the same molecule and may be called conformational isomers. They usually interconvert by rotation about single bonds without breaking covalent bonds.
Why do we need E/Z notation when cis and trans already exist?
Cis/trans terminology is convenient only when the comparison is unambiguous. E/Z notation works more generally because the substituents on each alkene carbon are ranked using the CIP rules.
Does a carbon attached to four different groups guarantee that the whole molecule is chiral?
Not always. A molecule containing several stereogenic centres can still be achiral if internal symmetry makes it superimposable on its mirror image, as in a meso compound.
Does a cyclohexane ring flip change a cis isomer into a trans isomer?
No. A ring flip changes axial positions into equatorial positions and vice versa, but an up substituent remains up and a down substituent remains down. The cis/trans relationship is preserved.
Is optical purity the same as enantiomeric excess?
For a sample containing only a pair of enantiomers and measured under appropriate identical conditions, optical purity is commonly equated with enantiomeric excess. An enantiomeric excess of 80%, for example, corresponds to a 90:10 mixture of the two enantiomers.
Stereochemistry becomes much easier once you stop treating every three-dimensional drawing as a separate rule. First ask whether the structures differ merely by rotation, then whether restricted geometry creates E/Z or cis/trans forms, and finally whether the structures are mirror images. Newman projections, CIP priorities, R/S notation and enantiomer–diastereomer relationships are simply different tools for describing that same three-dimensional molecular world precisely.