Difficulty: Intermediate | Prerequisites: CHEM 237 Chapters 1–4 (functional groups, bonding, molecular geometry)
Stereochemistry is central to organic chemistry because molecules with the same connectivity can behave very differently depending on their three-dimensional arrangement. This set of notes covers how to rank substituents using CIP (Cahn–Ingold–Prelog) priority rules, how to count the number of possible stereoisomers for a given structure, and how to classify the relationship between two molecules (enantiomers, diastereomers, conformers, constitutional isomers, or identical). These skills appear repeatedly in CHEM 237 and carry forward into reaction mechanisms, spectroscopy, and biochemistry. If you are comfortable drawing Lewis structures and identifying functional groups, you have the background to work through this material.
CIP priority rules rank substituents around a stereocentre by atomic number, working outward until a difference is found. The maximum number of stereoisomers is 2^n (where n = number of stereocentres), but internal symmetry (meso compounds) can reduce that count. Two structures with the same molecular formula can be enantiomers, diastereomers, conformers, constitutional isomers, or identical, and correctly classifying the relationship is one of the most common exam tasks in CHEM 237.
Stereocentre (stereogenic centre, chiral centre)
A carbon (or other atom) bonded to four different substituents, making it non-superimposable on its mirror image. Think of it as the atom where "handedness" originates.
CIP priority (Cahn–Ingold–Prelog priority)
The ranking system used to assign R or S configuration at a stereocentre. Substituents are ranked by atomic number of the atom directly attached, then you move outward along the chain until a point of difference is found. In simple terms, heavier atoms win.
R and S configuration
Labels describing the three-dimensional arrangement at a stereocentre. With the lowest-priority group pointing away, trace a path from highest to lowest priority: clockwise = R (Latin rectus, "right"), anticlockwise = S (Latin sinister, "left").
Enantiomers
Stereoisomers that are non-superimposable mirror images. Every stereocentre is inverted (R becomes S and vice versa). Think of it as left hand vs right hand.
Diastereomers
Stereoisomers that are not mirror images. At least one, but not all, stereocentres differ in configuration. They have different physical properties (melting point, boiling point, solubility).
Conformers (conformational isomers)
Different spatial arrangements of the same molecule produced by rotation around single bonds. No bonds are broken. In simple terms, they are the same molecule caught in different poses.
Constitutional isomers (structural isomers)
Molecules with the same molecular formula but different connectivity of atoms. The bonds are joined up differently.
Meso compound
A molecule that contains stereocentres but is achiral overall because it has an internal plane of symmetry. Its mirror image is superimposable on itself, so it does not rotate plane-polarised light.
Stereoisomers
Isomers with the same connectivity but different spatial arrangement. The two subcategories are enantiomers and diastereomers.
Step 1: Look at the atoms directly attached to the stereocentre
Rank by atomic number. Higher atomic number = higher priority.
Common ranking: I > Br > Cl > S > F > O > N > C > H.
If two substituents start with the same atom, move outward to the next set of atoms ("first point of difference").
Step 2: Handle ties by expanding outward
Compare the next atoms along the chain for each tied substituent.
A carbon bonded to (O, O, H) outranks a carbon bonded to (C, C, H) because oxygen's atomic number beats carbon's.
Step 3: Double and triple bonds are treated as "phantom atoms"
A C=O double bond is treated as if the carbon is bonded to two oxygens (one real, one phantom) and the oxygen is bonded to two carbons (one real, one phantom).
A C≡C triple bond is treated as if each carbon is bonded to three of the other carbon.
This convention means an aldehyde group (–CHO) outranks a simple –CH₂OH group at the first point of difference.
Step 4: Assign R or S
Orient the molecule so the lowest-priority group (usually H) points away from you.
Trace the path from highest (#1) to second (#2) to third (#3) priority.
Clockwise path = R. Anticlockwise path = S.
The 2^n rule
For a molecule with n stereocentres, the maximum number of stereoisomers is 2^n.
Two stereocentres: up to 4 stereoisomers. Three stereocentres: up to 8.
When 2^n overcounts: meso compounds
If a molecule has an internal plane of symmetry, some of those 2^n "isomers" are identical (the meso form).
Example: 2,3-dibromobutane has 2 stereocentres, so 2^2 = 4 in theory. One pair turns out to be the same meso compound, leaving only 3 distinct stereoisomers.
Spotting internal symmetry
Look for a mirror plane that cuts the molecule in half, making one half a reflection of the other.
Cyclic compounds can have meso forms too. A cyclohexane with two identical substituents in a trans-1,2 arrangement on a symmetric ring can be meso.
Geometric (cis/trans) isomers also count
A double bond (C=C) or a ring that prevents rotation can create geometric isomers.
Geometric isomers are a subset of diastereomers. They are not mirror images; they have different physical properties.
Each rigid element that can have a cis or trans arrangement adds a factor of 2 to the stereoisomer count, just as a stereocentre does.
Decision flowchart for classifying two structures
Do the two structures have the same molecular formula? If no: they are not isomers at all.
Do they have the same connectivity (same bonds between the same atoms)? If no: constitutional isomers.
Are they superimposable (identical in 3D)? If yes: identical (or, if reached by bond rotation only, conformers).
Are they non-superimposable mirror images (every stereocentre inverted)? If yes: enantiomers.
Are they non-superimposable and not mirror images (some but not all stereocentres inverted, or cis/trans differs)? If yes: diastereomers.
Can one be converted to the other purely by rotating around single bonds, with no bond breaking? If yes: conformers.
How to tell enantiomers from diastereomers in practice
Assign R/S at every stereocentre in both structures.
If every stereocentre is flipped (all R become S and all S become R): enantiomers.
If only some stereocentres are flipped: diastereomers.
Conformers vs identical
In this course, conformers are treated as a distinct category from "identical," even though no bonds break. This is a convention specific to certain exam questions.
Newman projections that show different rotational positions of the same molecule are conformers.
Two drawings of the exact same conformation (perhaps drawn from different angles) are identical.
Real-world application
Pharmaceutical companies must distinguish enantiomers because they can have dramatically different biological activity. Thalidomide is the classic example: one enantiomer treated morning sickness, while the other caused birth defects. Identifying stereochemical relationships is the foundation for understanding why.
Students often think that a larger substituent always gets higher CIP priority. Size does not matter; atomic number at the first point of difference does. A tert-butyl group (–C(CH₃)₃) is bulky, but it still starts with carbon, so it ranks below a fluorine (–F) at the attached atom.
Students often think that any molecule with stereocentres must be chiral. Meso compounds contain stereocentres yet are achiral because of an internal mirror plane.
Students often confuse diastereomers with constitutional isomers. If the connectivity is the same and only the 3D arrangement differs, the relationship is stereoisomeric (enantiomers or diastereomers), not constitutional.
Students often forget to check whether two drawings are simply the same molecule viewed from a different angle. Before assigning a relationship, try mentally rotating one structure to see whether it superimposes on the other.
⚠️ CIP priority with phantom atoms for double/triple bonds is a perennial exam question. Be comfortable expanding C=O and C≡C into their phantom-atom equivalents.
⚠️ The exam asks you to fill in exactly two boxes (highest and lowest, most and fewest). Read the prompt carefully, because partial answers score zero.
⚠️ When counting stereoisomers, always check for meso compounds before defaulting to 2^n. The fewest-stereoisomers answer is often the structure with a meso form or the one with no stereocentres at all.
⚠️ Relationship-identification questions test whether you can distinguish enantiomers from diastereomers. Assign R/S at each stereocentre in both structures as a routine step.
True or False: A bromine atom always has higher CIP priority than a carbon atom.
Fill in the blank: The maximum number of stereoisomers for a molecule with 3 stereocentres is ___.
True or False: Meso compounds are optically active.
Fill in the blank: Two non-superimposable mirror-image molecules are called ___.
True or False: Conformers involve breaking and re-forming bonds.
Answers: 1. True (Br = 35, C = 6). 2. 8 (2^3). 3. False (meso compounds have an internal mirror plane and are achiral). 4. Enantiomers. 5. False (conformers arise from rotation around single bonds, no bond breaking).
Q: A stereocentre has four substituents: –OH, –CH₃, –H, and –Br. Rank them from highest to lowest CIP priority.
A: Br (35) > OH (8) > CH₃ (6) > H (1). Rank by atomic number of the directly attached atom.
Q: A molecule has two stereocentres and exists as three distinct stereoisomers rather than four. What accounts for the reduced count?
A: One combination of configurations produces a meso compound (an internal mirror plane makes two of the four theoretical stereoisomers identical), so the total drops from four to three.
Q: Two molecules have the same molecular formula and the same connectivity. At stereocentre 1, both are R. At stereocentre 2, one is R and the other is S. What is their relationship?
A: Diastereomers. They are stereoisomers (same connectivity, different 3D arrangement), but they are not mirror images because only one stereocentre is inverted.
Q: You draw two Newman projections of butane, one showing the anti conformation and one showing a gauche conformation. What is the relationship between them?
A: Conformers. They are the same molecule in different rotational positions around the C2–C3 bond. No bonds are broken.
Q: Two structures have the formula C₄H₁₀O. One is butan-1-ol and the other is diethyl ether. What is their relationship?
A: Constitutional isomers. Same formula, different connectivity (one has an –OH group, the other has a C–O–C linkage).
This material connects directly to addition reactions (next set of notes) because predicting reaction stereochemistry requires you to assign and track R/S configurations through a mechanism. It also connects to optical activity: enantiomers rotate plane-polarised light in equal and opposite directions, while meso compounds and racemic mixtures show no net rotation. In biochemistry, enzyme specificity depends on the precise 3D arrangement at stereocentres, which is why CIP assignments matter beyond the exam.
Cahn-Ingold-Prelog, CIP rules, R and S, R/S assignment, stereocentre, stereogenic centre, chiral centre, chirality, asymmetric carbon, enantiomer, diastereomer, meso compound, constitutional isomer, structural isomer, conformer, conformational isomer, Newman projection, optical isomer, stereoisomer count, 2^n rule, mirror image, superimposable, non-superimposable, CHEM 237, organic chemistry stereochemistry, University of Washington