Difficulty: Intermediate | Prerequisites: Lewis structures, IUPAC nomenclature, functional groups, 3D molecular geometry
Stereochemistry is about the 3D arrangement of atoms in molecules. Two molecules can have the same atoms and bonds but differ in how those atoms sit in space, and that difference can change everything from biological activity to physical properties. This set of notes covers R/S and E/Z naming, how to classify pairs of structures (enantiomers, diastereomers, conformers, constitutional isomers) and how to count stereoisomers using the 2ⁿ rule.
Stereocentre (stereogenic centre, asymmetric centre, chiral centre)
A carbon bonded to four different groups. Swapping any two of those groups produces a different stereoisomer. In simple terms, it is the atom where "handedness" lives in the molecule.
Chiral
A molecule that is not superimposable on its mirror image. Think of your left and right hands: same parts, same connectivity, but you cannot stack one perfectly on top of the other.
Achiral
A molecule that is superimposable on its mirror image. Usually has an internal plane of symmetry.
R and S configuration
Labels assigned to a stereocentre using Cahn-Ingold-Prelog priority rules. R (rectus) = clockwise rotation of priorities 1→2→3 when the lowest-priority group points away. S (sinister) = anticlockwise.
E/Z nomenclature
Labels for alkene geometry. Z (zusammen, "together") = the two higher-priority groups are on the same side of the double bond. E (entgegen, "opposite") = they are on opposite sides.
Enantiomers
Stereoisomers that are non-superimposable mirror images. They have opposite R/S configuration at every stereocentre. Same physical properties except for the direction they rotate plane-polarised light.
Diastereomers
Stereoisomers that are not mirror images. They differ at some, but not all, stereocentres (or differ in E/Z geometry). Different physical properties, different reactivity.
Constitutional isomers (structural isomers)
Molecules with the same molecular formula but different connectivity of atoms. They are not stereoisomers.
Conformers (conformational isomers)
Different spatial arrangements of the same molecule achieved by rotation around single bonds. They are not different compounds; they interconvert rapidly at room temperature.
Meso compound
A molecule that has stereocentres but is achiral because it possesses an internal plane of symmetry. The stereocentres cancel each other out. In simple terms, one half of the molecule is the mirror image of the other half.
Identify the stereocentre: a carbon with four different substituents.
Assign priorities 1–4 to the four groups using Cahn-Ingold-Prelog rules:
Higher atomic number = higher priority.
If two atoms are the same, move outward along each chain until a point of difference is found.
Double/triple bonds are treated as if each bonded atom is duplicated/triplicated (phantom atoms).
Orient the molecule so priority 4 (lowest) points away from you.
Trace a path from 1 → 2 → 3.
Clockwise = R.
Anticlockwise = S.
Shortcut: if priority 4 is on a wedge (pointing towards you), assign R/S as above and then reverse the answer.
Applies to alkenes where the two groups on each carbon of the double bond are different.
On each carbon of the C=C, assign priorities to the two substituents using CIP rules (same as R/S).
Z (zusammen): the two higher-priority groups are on the same side of the double bond.
E (entgegen): the two higher-priority groups are on opposite sides.
Example from the practice set: (Z)-2-cyclohexyl-2-buten-1-ol. The cyclohexyl group and the methyl group are on C2; the CH₂OH and H are on C3. Determine which has higher priority on each carbon and check whether the high-priority groups sit on the same side (Z) or opposite sides (E).
This is a common exam task. Work through it in stages:
Draw the parent chain with the correct number of carbons and the correct functional groups at the correct positions.
Add substituents at the numbered positions.
Assign stereochemistry last: use wedges and dashes to show R or S at each stereocentre, or Z/E at each double bond.
Example: (2S,3R)-2-bromo-3-hexanol
Parent chain: hexane (6 carbons).
Functional groups: Br at C2, OH at C3.
Stereochemistry: C2 is S, C3 is R. Draw the molecule in any orientation, then assign priorities at each centre and adjust wedges/dashes until the configuration matches.
Example: (Z)-2-cyclohexyl-2-buten-1-ol
Parent chain: 2-buten-1-ol (four carbons, double bond at C2, OH at C1).
Substituent: cyclohexyl group on C2.
Stereochemistry: Z means the higher-priority groups on each carbon of the double bond are on the same side.
Given two structures, you need to determine their relationship. Work through this decision tree:
Same molecular formula? If no, they are not isomers at all.
Same connectivity (same bonds)? If no, they are constitutional isomers (I).
Same connectivity but different spatial arrangement? They are stereoisomers (S), which means they are also isomers (I). Now determine the subtype:
Mirror images? If yes, and non-superimposable, they are enantiomers (E). Write I, S, E.
Not mirror images? They are diastereomers (D). Write I, S, D.
Same structure, just rotated around a single bond? They are conformers (C). They are not isomers because they are the same compound.
Identical in every way? They are the same compound. Write "none."
Results from the practice set:
A1/A2: none (same compound or not isomers at all)
B1/B2: I, S, E (enantiomers)
C1/C2: none (same compound viewed differently)
D1/D2: I (constitutional isomers, different connectivity due to the CF₃ and CN placement)
E1/E2: none (meso compound considerations, same compound)
F1/F2: I, S, D (diastereomers)
Scan each carbon in the molecule. A carbon is an asymmetric centre if it bears four different groups.
Ring carbons count too: trace the ring in both directions from the carbon and compare what you encounter.
Heteroatoms (N, S, P) can also be stereocentres under certain conditions, but carbon centres are the focus at this level.
Counting stereoisomers using the 2ⁿ rule:
n = the number of stereocentres.
Maximum possible stereoisomers = 2ⁿ.
Example from the practice set: a molecule with 3 stereocentres has a maximum of 2³ = 8 stereoisomers.
A molecule with 4 stereocentres has a maximum of 2⁴ = 16 stereoisomers.
The actual number may be lower if the molecule has meso forms (internal planes of symmetry that reduce the count).
Which compounds are chiral?
A compound is chiral if it has no internal plane of symmetry and is not superimposable on its mirror image.
Having stereocentres is necessary but not sufficient: a meso compound has stereocentres but is achiral.
From the practice set: B1, B2, C1, C2, D1, E1, E2 are chiral.
Maximum stereoisomers = 2ⁿ (where n = number of stereocentres)
Enantiomers: opposite configuration at every stereocentre
Diastereomers: same configuration at some stereocentres, opposite at others
Meso: has stereocentres but an internal mirror plane makes it achiral
CIP priority: higher atomic number wins; at a tie, go further along the chain
E/Z: higher-priority groups on opposite sides = E, same side = Z
Stereochemistry matters enormously in pharmacology. Enantiomers of the same drug can have completely different biological effects because enzymes and receptors are chiral, so they interact differently with each mirror-image form. Thalidomide is the classic cautionary example: one enantiomer treated morning sickness, the other caused birth defects. Modern drug design and regulatory frameworks require testing of individual enantiomers.
Students often think that having stereocentres automatically makes a molecule chiral. It does not. Meso compounds have stereocentres but are achiral because of an internal plane of symmetry.
A compound with one R and one S stereocentre is sometimes assumed to be achiral. This is only true if the molecule is a meso compound (the two halves are mirror images). Otherwise, it is chiral.
Conformers are frequently mistaken for different isomers. They are not. Rotating around a single bond does not create a new compound.
Students sometimes confuse E/Z with cis/trans. E/Z uses CIP priority rules and works for all substituted alkenes. Cis/trans only works reliably for disubstituted alkenes.
⚠️ "Classify the relationship between these two structures" questions appear on nearly every stereochemistry exam. Practise the decision tree until it is automatic.
⚠️ Drawing a structure from an IUPAC name that includes R/S and E/Z is a high-value skill. If you can do it quickly and accurately, you pick up easy marks.
⚠️ Expect a meso compound trap: a structure with stereocentres that is actually achiral. Check for internal symmetry.
⚠️ Counting stereoisomers (2ⁿ) is straightforward, but exams sometimes test whether you notice a meso form that reduces the count below 2ⁿ.
True or false: A meso compound is optically active. Answer: False. Meso compounds are achiral and therefore optically inactive.
Fill in the blank: The maximum number of stereoisomers for a compound with 4 stereocentres is ______. Answer: 16 (2⁴).
True or false: Conformers are different compounds. Answer: False. They are the same compound in different rotational states.
Fill in the blank: Two stereoisomers that are non-superimposable mirror images are called ______. Answer: Enantiomers.
True or false: E/Z nomenclature and cis/trans nomenclature always give the same answer. Answer: False. They can diverge for trisubstituted and tetrasubstituted alkenes.
Q: Two compounds have the same molecular formula and the same connectivity, but different R/S configurations at two of their three stereocentres. What is their relationship?
A: They are diastereomers. They are stereoisomers (same connectivity, different 3D arrangement) but not mirror images (because only two of three centres are inverted, not all three).
Q: A molecule has two stereocentres, one R and one S. Is it necessarily achiral?
A: No. It is achiral only if it is a meso compound (i.e. the molecule has an internal plane of symmetry making the two halves mirror images). If the two stereocentres bear different sets of substituents, the molecule is chiral.
Q: You are given the name (2S,3R)-2-bromo-3-hexanol. How many stereocentres does it have, and what is the maximum number of stereoisomers?
A: Two stereocentres (C2 and C3). Maximum stereoisomers = 2² = 4.
Q: Compound A has [alpha] = +40°. Compound B has [alpha] = -40°. Same molecular formula, same connectivity. What is their relationship?
A: They are enantiomers. Equal magnitude but opposite sign of optical rotation is the defining physical property difference between enantiomers.
Stereochemistry connects directly to reaction mechanisms: electrophilic additions, SN1, SN2 and elimination reactions all either create or destroy stereocentres, and understanding syn/anti addition or inversion/retention requires fluency in R/S and E/Z. It also ties into optical activity and enantiomeric excess (covered in the companion study notes on Optical Activity). In biochemistry, nearly every enzyme-substrate interaction depends on stereochemistry.
Stereochemistry, chirality, chiral centre, asymmetric carbon, stereocentre, stereogenic centre, R configuration, S configuration, Cahn-Ingold-Prelog, CIP priority rules, E/Z nomenclature, enantiomers, diastereomers, constitutional isomers, structural isomers, conformers, conformational isomers, meso compound, internal plane of symmetry, superimposable mirror image, optical activity, 2^n rule, stereoisomer count, wedge-dash notation, organic chemistry practice problems, University of Washington