First-year chemistry · Interactive guide
STEREOCHEMISTRY
Same atoms. Different arrangements. Different possibilities.
Discover how the three-dimensional arrangement of molecules influences their stability, properties and biological activity.
What is stereochemistry?
Stereochemistry is the study of how atoms are arranged in three-dimensional space. Two molecules can share the same formula and the same bonds yet differ in shape — and that difference can change how stable they are, how they melt and boil, and how they interact with the molecules of life.
Conformation
Rotation about single bonds — staggered vs eclipsed ethane.
Open module 02Cis–Trans Isomerism
Locked double bonds create distinct isomers of 2-butene.
Open module 03Chirality
Mirror-image molecules and the R/S naming system.
Open module 04Biological Activity
How enzymes and receptors read molecular shape.
Open moduleModule 01 · Conformation Lab
Rotation around a single bond
C–H bonds alternate: minimum torsional strain and lowest energy.
Relative energy vs dihedral angle
Staggered = 0 kJ/mol; eclipsed ≈ 12 kJ/mol higher.
Approximate ethane model: E(θ) = 6[1 + cos(3θ)] kJ/mol. The curve repeats every 120°; zero is the staggered minimum, not absolute energy.
Conformation
A 3D arrangement produced only by rotation about single bonds. Conformations interconvert rapidly.
Why rotation is possible
The C–C σ bond maintains overlap during rotation: no bond needs to break. Rotation is rapid at room temperature, but not energy-free.
Dihedral angle
The angle between H–C1–C2 and C1–C2–H planes, seen in a Newman projection.
Staggered (60°)
Front and rear C–H bonds alternate. Minima at 60°, 180° and 300°.
Eclipsed (0°)
Rear C–H bonds line up behind front bonds. Maxima at 0°, 120°, 240° and 360°.
Torsional strain
The energy penalty from eclipsing bonds on adjacent atoms. Ethane rapidly crosses its small ≈ 12 kJ/mol barrier at room temperature.
Two views of the same ethane molecule
Staggered · 60°
Alternating bonds · minimum energy · 0 kJ/mol
Eclipsed · 0°
Aligned bonds · maximum energy · ≈ 12 kJ/mol
Viewed along C1→C2: the centre dot is the front carbon; the circle is the rear carbon. Red H atoms define θ. Eclipsed rear bonds are offset 6° only to keep them visible.
Module 02 · Cis–Trans Explorer
When rotation is locked
Both CH₃ groups lie on the same side of the double bond.
Comparison
| cis | trans | |
|---|---|---|
| Formula | C₄H₈ | C₄H₈ |
| Arrangement | CH₃ same side | CH₃ opposite sides |
| Relative geometry | U-shaped, CH₃ groups crowded | Extended, more symmetric |
| Net dipole | ≈ 0.3 D | ≈ 0 D (cancels) |
| Intermolecular forces | Dipole–dipole + London | Mainly London |
| Boiling point* | ≈ 3.7 °C | ≈ 0.9 °C |
| Melting point* | ≈ −139 °C | ≈ −106 °C |
| Heat of hydrogenation* | ≈ 120 kJ/mol released | ≈ 116 kJ/mol released |
| Relative stability | Less stable (steric strain) | More stable by ≈ 4 kJ/mol |
*Approximate literature values (CRC Handbook). The higher melting point of trans reflects better packing in the solid.
Geometric isomerism
Stereoisomers with the same connectivity that differ in the spatial arrangement around a rigid unit (C=C or a ring).
Restricted rotation
The π bond needs parallel p orbitals. Twisting 90° destroys overlap (≈ 260 kJ/mol), far above thermal energy.
Different properties
Different shapes give different dipoles and packing, so boiling point, melting point and reactivity differ.
Rings too
Rings cannot rotate freely: cis- and trans-1,2-dimethylcyclohexane are distinct compounds.
When does cis–trans isomerism occur?
Both conditions must be met:
- Rotation is restricted (a C=C double bond or a ring).
- Each carbon of the double bond carries two different groups.
Yes: but-2-ene (each C has H and CH₃).
No: but-1-ene and 2-methylpropene — one alkene carbon has two identical groups.
The trans isomer is more stable because its CH₃ groups are farther apart; it releases less heat on hydrogenation to butane.
Viva questions
Why around C=C but not a C–C single bond?
A single bond is only a σ bond, so the ends rotate freely and every arrangement interconverts. A double bond also has a π bond from sideways p-orbital overlap; rotating would break that overlap, so the groups stay locked cis or trans.
Why does cis boil higher if trans is more stable?
In cis the C–CH₃ bond dipoles add to a small net dipole, adding dipole–dipole attractions to London forces; in trans they cancel. Stronger intermolecular forces mean a higher boiling point. Stability is separate: trans is lower in energy because its methyl groups are farther apart.
Module 03 · Chirality & R/S Studio
Left hand, right hand
Is there a chiral centre?
| Molecule | Groups on C | Chiral? |
|---|---|---|
| CHFClBr | H, F, Cl, Br | Yes |
| CH₂ClBr | H, H, Cl, Br | No — two H |
| Butan-2-ol (C2) | H, OH, CH₃, CH₂CH₃ | Yes |
| Lactic acid (C2) | H, OH, CH₃, COOH | Yes |
Quick test: a molecule with a plane of symmetry is achiral. Enantiomers share melting point, boiling point, solubility and spectra in an achiral environment; they differ in the sign of optical rotation and in how they react with other chiral molecules such as enzymes. A 1:1 mixture (a racemic mixture) shows no net rotation.
The four CIP ranking rules
- Atomic number: I > Br > Cl > S > F > O > N > C > H.
- Tie? Go outward: compare sets in order, not sums. –CH₂OH {O,H,H} > –CH₂CH₃ {C,H,H} > –CH₃ {H,H,H}.
- Multiple bonds count twice: –CHO is {O,O,H}, so it beats –CH₂OH {O,H,H}.
- Isotopes: heavier wins, so D beats H.
I > Br > Cl > SH > F > OH > NH₂ > COOH > CHO > CH₂OH > C₆H₅ > CH=CH₂ > CH₂CH₃ > CH₃ > H
R/S vs (+)/(−): same label, opposite sign
R/S is read from the structure. (+)/(−) is measured in a polarimeter, where specific rotation is [α] = α / (l × c) with l in dm and c in g/mL. Enantiomers always have equal and opposite [α].
(S)-2-methylbutan-1-ol
HOCH₂–CH(CH₃)–CH₂CH₃
rotation: (−)
(S)-2-methylbutanoic acid
HOOC–CH(CH₃)–CH₂CH₃
rotation: (+)
Also: (R)-butan-2-ol is (−). D/L is a separate system (compared with glyceraldehyde, used for sugars and amino acids) — don’t mix it up with R/S.
Viva questions
A compound is (R). Does it rotate light to the right?
Not necessarily. R comes from the CIP rules; the sign of rotation must be measured. (R)-butan-2-ol is (−). The sign alone can’t give R/S either unless the absolute configuration is known another way, e.g. X-ray crystallography.
–CHO or –CH₂OH: which ranks higher?
–CHO. The C=O counts twice, giving {O,O,H} against {O,H,H}. The first atoms tie; at the second position O beats H.
Practice mode
1 / 4 · Bromochlorofluoromethane
H is on a dashed bond (pointing away).
Step 1 — assign priorities (H = 4)
Module 04 · Biological activity
Why does molecular shape matter to life?
Simplified “three-point contact” model. Real binding depends on many interactions.
Specific 3D structures
Enzymes, receptors and transporters have precisely shaped, chiral binding pockets.
Recognition
A pocket that makes three or more contacts can tell enantiomers apart.
Different binding
Stereoisomers may bind with different strength — or one may not bind at all.
Drugs
Stereochemistry can affect a drug’s effectiveness, metabolism and safety. Many modern drugs are sold as single enantiomers.
Case study
L-DOPA
levodopa · (S)-configuration
Parkinson’s disease involves loss of dopamine-producing neurons. Dopamine itself cannot cross the blood–brain barrier, but L-DOPA is carried across by amino-acid transporters and converted to dopamine in the brain by aromatic L-amino acid decarboxylase.
The drug is given as the single L (S) enantiomer — the form the body’s transporters and enzymes process. It is usually combined with carbidopa, which blocks conversion outside the brain.
- 1
L-DOPA taken orally
- 2
Crosses the blood–brain barrier
- 3
Converted into dopamine in the brain
- 4
Helps improve movement symptoms
Simplified educational pathway — not every process involved is shown.
Effect on drug activity
One stereoisomer may fit the receptor and give the desired response; another may be less active or act differently.
Effect on metabolism
Enzymes can process stereoisomers at different rates, changing absorption, breakdown and elimination.
Effect on safety
Differences in binding and metabolism can change side effects, so a drug’s stereochemical make-up is checked during development.
Second example: ibuprofen
(S)-(+)-ibuprofen is the main form that inhibits cyclooxygenase and gives the anti-inflammatory effect. Some of the (R)-(−) form is converted into the (S) form in the body — a case where the difference between enantiomers is partly reduced by metabolism.
L-DOPA facts: 3,4-dihydroxy-L-phenylalanine, C₉H₁₁NO₄. Its chiral carbon carries NH₂, COOH, H and CH₂–C₆H₃(OH)₂.
Viva questions
Why is L-DOPA an example of stereochemistry?
It has one chiral carbon, so it exists as two enantiomers (L and D). The body’s transporters and enzymes recognise the L form, which is the one used as a medicine.
Why is L-DOPA used in Parkinson’s disease?
Unlike dopamine, it crosses the blood–brain barrier and is then converted into dopamine in the brain, easing tremor, rigidity and slowness of movement.
Module 05 · Compare & revise
Three ideas, side by side
| Conformational isomerism | Cis–trans isomerism | Chirality & enantiomers | |
|---|---|---|---|
| Defining feature | Different shapes from rotation about σ bonds | Groups fixed on same/opposite sides of a rigid unit | Non-superimposable mirror images |
| Cause | Free rotation about C–C single bonds | Restricted rotation (π bond or ring) | sp³ centre with four different groups |
| Interconversion | Rapid at room temperature | Requires breaking a bond | Requires breaking a bond |
| Example | Staggered vs eclipsed ethane | cis- vs trans-2-butene | (R)- vs (S)-CHBrClF; L- vs D-DOPA |
| Importance | Stability, reactivity, molecular shape | Physical properties (bp, mp, dipole) | Biological recognition and drug action |
Glossary
- Conformation
- A 3D arrangement of a molecule obtained only by rotation about single bonds.
- Dihedral angle
- The angle between two bonds on adjacent atoms, viewed along the bond joining them.
- Torsional strain
- Increase in energy caused by eclipsing bonds on adjacent atoms.
- Geometric isomerism
- Stereoisomerism caused by restricted rotation about a double bond or within a ring.
- Cis
- Two reference groups on the same side of a double bond or ring.
- Trans
- Two reference groups on opposite sides of a double bond or ring.
- Chiral centre
- An atom (usually sp³ carbon) bonded to four different groups.
- Enantiomer
- One of a pair of stereoisomers that are non-superimposable mirror images.
- CIP priority
- Ranking of substituents by atomic number at the first point of difference (Cahn–Ingold–Prelog).
- R configuration
- 1 → 2 → 3 runs clockwise when priority 4 points away from the viewer.
- S configuration
- 1 → 2 → 3 runs anticlockwise when priority 4 points away from the viewer.
- Biological activity
- The effect a molecule has on a living system, which can depend on how it fits biological targets.
- Stereoisomer
- Molecules with the same formula and bonding sequence but a different 3D arrangement of atoms.
- Absolute configuration
- The actual spatial arrangement at a chiral centre, described as R or S.
- Optical rotation
- The measured rotation of plane-polarised light, (+) or (−); not predictable from R/S alone.
- Stereospecific binding
- Binding of a biological target to one stereoisomer in preference to another.
Check your understanding
Test your knowledge
Which conformation of ethane is the most stable?
Sources
References
- [1]Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed.; Oxford University Press, 2012.Used for: Conformation, cis–trans isomerism, chirality and CIP rules.
- [2]McMurry, J. Organic Chemistry, 10th ed.; Cengage Learning, 2023.Used for: Ethane conformations and ≈12 kJ/mol rotational barrier; R/S assignment.
- [3]Klein, D. Organic Chemistry, 4th ed.; Wiley, 2021.Used for: Newman projections, enantiomers, optical activity.
- [4]IUPAC. Compendium of Chemical Terminology (the “Gold Book”).Used for: Definitions of conformation, torsion angle, chirality centre, enantiomer, CIP priority.
- [5]Haynes, W. M., ed. CRC Handbook of Chemistry and Physics; CRC Press.Used for: Physical constants of cis- and trans-2-butene (approximate values shown).
- [6]Gandhi, K. R.; Saadabadi, A. Levodopa (L-Dopa). StatPearls; NCBI Bookshelf.Used for: L-DOPA crossing the blood–brain barrier, conversion to dopamine, carbidopa co-therapy.
- [7]LibreTexts Chemistry. Organic Chemistry (stereochemistry chapters).Used for: Supplementary open educational explanations.
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