Source: Chemistry 225 Exam 2, University of Michigan
Difficulty: Intermediate | Prerequisites: Organic Chemistry I (CHEM 210 or equivalent), familiarity with Lewis structures, functional groups, and basic bonding.
This material sits at the heart of Organic Chemistry II: how molecules are arranged in three-dimensional space, and why that arrangement matters for reactivity. You need a solid grasp of carbon bonding, functional group identification, and IUPAC naming conventions from Orgo I. Stereochemistry is the bridge between drawing flat structures on paper and understanding how molecules behave in the real world, in reactions, in the body, and in the lab. If you can read a Newman projection and assign R/S configurations confidently, most of the second half of this course becomes far more manageable.
Stereochemistry covers how atoms are arranged in space and how that arrangement affects a molecule's identity and behaviour. This set of notes covers IUPAC naming, Newman and Fischer projections, chair conformations, chirality, enantiomers, diastereomers, meso compounds, and optical activity. Master these and you can predict relationships between structures, identify which stereoisomers exist, and understand why certain reactions flip (or preserve) configuration.
IUPAC Nomenclature
The systematic naming convention maintained by the International Union of Pure and Applied Chemistry for organic compounds. Names are built from the longest carbon chain (parent), with substituents listed alphabetically and numbered to give the lowest set of locants.
In simple terms, this means every organic molecule gets one unambiguous name based on its structure, so chemists worldwide know exactly which compound is being discussed.
Newman Projection
A way of drawing a molecule by looking straight down a carbon-carbon bond. The front carbon is shown as a dot (or the centre of a circle) and the back carbon as a circle, with bonds radiating outward from each.
Think of it as standing behind someone and looking over their shoulder: you see their arms (front carbon's bonds) and the other person's arms poking out behind (back carbon's bonds).
Fischer Projection
A two-dimensional representation of a three-dimensional molecule in which horizontal lines represent bonds coming toward the viewer and vertical lines represent bonds going away. The most oxidised carbon goes at the top.
Think of it as a cross, where horizontal = pointing at you and vertical = pointing away.
Chair Conformation
The most stable three-dimensional shape of a six-membered ring (e.g. cyclohexane), where bond angles closely match the ideal tetrahedral angle (109.5 degrees) and staggering is maximised.
In simple terms, this means the ring puckers into a shape that looks like a chair, minimising strain.
Axial Position
Bonds on a cyclohexane chair that point straight up or straight down, roughly parallel to the axis of the ring.
Think of it as a flagpole sticking up (or down) from the ring.
Equatorial Position
Bonds on a cyclohexane chair that point outward from the ring, roughly along the "equator" of the molecule.
Think of it as a belt around the waist of the ring. Large substituents prefer equatorial positions because they avoid 1,3-diaxial interactions.
Chirality
The property of a molecule that is non-superimposable on its mirror image. A carbon with four different substituents is a chiral centre (also called a stereogenic centre or stereocentre).
In simple terms, this means your left and right hands are chiral: mirror images of each other, but you cannot stack one perfectly on top of the other.
Enantiomers
Stereoisomers that are non-superimposable mirror images of each other. They have identical physical properties (melting point, boiling point, solubility) except for the direction in which they rotate plane-polarised light.
Think of it as a pair of gloves: same shape, opposite handedness.
Diastereomers
Stereoisomers that are not mirror images of each other. They have different physical properties (melting points, boiling points, solubilities) and different specific rotations.
In simple terms, this means molecules that differ at some, but not all, of their chiral centres.
Meso Compound
A molecule with chiral centres but an internal plane of symmetry, making the whole molecule achiral. Meso compounds do not rotate plane-polarised light.
Think of it as a molecule whose "left half" is the mirror image of its "right half," so the optical rotations cancel internally.
Racemic Mixture (Racemate)
A 50:50 mixture of two enantiomers. The equal and opposite optical rotations cancel, so the mixture shows no net rotation of plane-polarised light.
In simple terms, this means equal amounts of the left-handed and right-handed versions mixed together.
R/S Configuration (Cahn-Ingold-Prelog Rules)
A system for assigning absolute configuration at a chiral centre. Substituents are ranked by atomic number; if the priority sequence 1 to 2 to 3 runs clockwise with group 4 pointing away, the centre is R (Latin: rectus). Anticlockwise is S (Latin: sinister).
Think of it as a steering wheel: turn the wheel from highest to lowest priority. Clockwise = R, anticlockwise = S.
Optical Activity
The ability of a chiral substance to rotate the plane of plane-polarised light. Measured as specific rotation. A compound that rotates light clockwise is dextrorotatory (+); anticlockwise is levorotatory (-).
In simple terms, this means chiral molecules twist light, and the direction of the twist is a measurable physical property.
Specific Rotation
The measured rotation of plane-polarised light by a chiral compound under standard conditions (given concentration, path length, wavelength, temperature). It is an intrinsic property of the compound.
In simple terms, this means the "fingerprint" rotation value for a pure enantiomer.
Find the longest carbon chain that includes the -OH group. This is the parent chain, and the compound is named as an "-ol."
Number the chain so the -OH gets the lowest possible locant.
Name and number all substituents alphabetically.
Substituents like "ethyl," "methyl," and "chloro" are listed as prefixes.
Halogens are treated as substituents (e.g. "chloro"), not as part of the parent name.
When a substituent itself has branches (e.g. a chloroethyl group), you may see complex substituent naming. Be careful: "2-chloroethyl" is a substituent name meaning the ethyl group has a chlorine on its second carbon. The IUPAC preference is to name each substituent separately off the parent chain when possible.
Example from the exam: a four-carbon chain with an -OH at one end, plus an ethyl branch with a terminal chlorine. The correct name assigns the lowest locant to -OH, identifies the parent as butanol, and lists substituents (chloro, ethyl) by position.
A Newman projection looks down a specific C-C bond. The front carbon is the intersection of three lines; the back carbon is the circle.
Each carbon has three substituents visible (the fourth is the bond you are looking along).
To identify which compound a Newman projection represents, list all the groups on the front and back carbons, then reconstruct the full molecule.
Worked approach for 2-amino-pentane (CH3-CH(NH2)-CH2-CH2-CH3):
The molecule has an NH2 group on carbon 2.
When sighting down the C2-C3 bond, the front carbon (C2) carries H, CH3, and NH2. The back carbon (C3) carries H, H, and CH2CH3.
When sighting down the C1-C2 bond, the front carbon (C1) carries H, H, H. The back carbon (C2) carries NH2, CH2CH2CH3, and H.
Any valid Newman projection of this molecule must account for all five carbons and the NH2 group in the correct connectivity.
Key check: count the total carbons and heteroatoms visible in the projection. If they do not add up to the molecular formula, the projection does not represent that compound.
In a cyclohexane chair, each carbon has one axial and one equatorial bond.
A ring flip converts all axial positions to equatorial and vice versa.
The most stable chair conformation places the largest substituents in equatorial positions, because equatorial groups avoid 1,3-diaxial interactions (steric clash with other axial groups on the same side of the ring).
How to assign axial/equatorial from a structure:
Draw (or identify) the chair with substituents shown.
Bonds pointing straight up or down from the ring are axial.
Bonds pointing outward at a slight angle (roughly in the plane of the ring's "equator") are equatorial.
For a trisubstituted cyclohexane, the most stable chair will maximise the number of large groups in equatorial positions. If all three substituents can be equatorial simultaneously, that conformation is strongly preferred.
A practical rule: if a chair drawing shows a large group axial, consider whether the ring-flipped chair would put it equatorial. The conformation with fewer (or no) large axial groups wins.
Horizontal bonds point toward the viewer. Vertical bonds point away.
The most oxidised group (e.g. -CHO, -COOH) or the group with the highest oxidation state traditionally goes at the top; for simple alcohols, the -OH bearing the lowest-numbered carbon goes at the top.
To convert a line formula (wedge-dash drawing) to a Fischer projection:
Orient the molecule so the carbon chain runs vertically, with the top carbon being C1 (or the most oxidised end).
At each chiral centre, determine which groups point left and right (toward the viewer in Fischer convention).
Draw the cross at each chiral centre with horizontal groups as written.
Allowed manipulations:
Rotating the entire Fischer projection 180 degrees in the plane of the paper gives the same compound.
Rotating 90 degrees gives the enantiomer (this is a common trap).
Swapping any two groups on a single chiral centre inverts that centre's configuration.
Swapping two groups twice returns you to the original configuration.
To check whether a Fischer projection matches a line formula, assign R/S at each centre in both representations. If all centres match, the structures are identical. If all centres are opposite, they are enantiomers. If some match and some do not, they are diastereomers.
Determining chirality:
A molecule is chiral if it has no internal plane of symmetry and is non-superimposable on its mirror image.
A single chiral centre (four different groups on one carbon) makes the molecule chiral, unless a plane of symmetry elsewhere in the molecule cancels it (meso case).
Molecules with no chiral centres are usually achiral (exceptions exist with axial chirality, but these are beyond this exam's scope).
Relationships between structures:
Same compound: identical connectivity and identical configuration at every stereocentre.
Enantiomers: identical connectivity, opposite configuration at every stereocentre.
Diastereomers: identical connectivity, opposite configuration at some (but not all) stereocentres.
Conformations: same compound, just rotated around single bonds. A ring flip of a cyclohexane gives a different conformation, not a different stereoisomer.
Different compounds: different connectivity (different constitutional isomers).
Meso compounds:
Contain two or more chiral centres but possess an internal mirror plane.
The molecule as a whole is achiral and optically inactive.
Example: (2R,3S)-tartaric acid, where the top half of the molecule is the mirror image of the bottom half.
Optical activity and racemic mixtures:
A pure enantiomer rotates plane-polarised light by a specific amount (+/- degrees).
An equal mixture of two enantiomers (racemic mixture) shows zero net rotation, because the rotations cancel.
A meso compound also shows zero rotation, but for a different reason: internal compensation within each molecule.
On an exam, if asked which pair "will not rotate polarised light when mixed in equal amounts," look for a pair of enantiomers. Their equal mixture is racemic and optically inactive.
The maximum number of stereoisomers for a molecule with n chiral centres is 2^n.
Two chiral centres: up to 4 stereoisomers (2^2 = 4).
One chiral centre: up to 2 stereoisomers (a pair of enantiomers).
When the count is reduced:
If a molecule has two chiral centres but also has an internal plane of symmetry possible (i.e. the two halves can mirror each other), one of the four "expected" stereoisomers is actually a meso compound. In that case the total drops to 3: one pair of enantiomers plus the meso form.
A molecule with no chiral centres has no stereoisomers (ignoring geometric isomerism from double bonds).
A molecule with a double bond and two different groups on each carbon of the double bond has E/Z (cis/trans) isomerism, which contributes a factor of 2 per restricted-rotation site.
For the exam: to determine which compound has exactly four stereoisomers, identify how many chiral centres it has and check whether a meso form is possible. Two distinct chiral centres (not equivalent) with no meso possibility gives four stereoisomers.
Students often confuse conformations with stereoisomers. A ring flip of cyclohexane converts axial to equatorial (and vice versa), but it does not create a new stereoisomer. The two chairs are the same molecule.
Rotating a Fischer projection 90 degrees gives the enantiomer, not the same compound. Only 180-degree rotation in the plane preserves configuration. This is one of the most common exam mistakes.
Students sometimes assume that any molecule with chiral centres must be chiral. A meso compound has chiral centres but is achiral overall because of its internal symmetry plane.
Confusing "optically inactive" with "achiral." A racemic mixture is optically inactive but contains chiral molecules. A meso compound is both achiral and optically inactive, but for a completely different reason (internal compensation, not external cancellation).
⚠️ Fischer projection manipulations are heavily tested. Know which operations preserve configuration and which invert it.
⚠️ Identifying the relationship between two structures (enantiomers, diastereomers, conformations, identical, different compounds) is a near-guaranteed exam question. Assign R/S at every centre in both structures and compare systematically.
⚠️ Meso compounds appear frequently. Whenever you see a molecule with two chiral centres and identical substituent patterns on each, check for an internal mirror plane.
⚠️ Chair conformations with multiple substituents: you must be able to determine axial vs equatorial and predict which chair is more stable.
⚠️ Optical activity questions often test whether you know the difference between a racemic mixture (no rotation because of external cancellation) and a meso compound (no rotation because of internal symmetry).
True or False: Rotating a Fischer projection 90 degrees in the plane of the paper gives the same compound.
Fill in the blank: A molecule with two chiral centres and an internal mirror plane is called a ______ compound.
True or False: Enantiomers have different melting points.
Fill in the blank: The maximum number of stereoisomers for a molecule with 3 chiral centres and no internal symmetry is ______.
True or False: In the most stable chair conformation of a substituted cyclohexane, the largest substituent occupies an axial position.
Answers: 1. False (it gives the enantiomer). 2. Meso. 3. False (enantiomers have identical physical properties except for the direction of optical rotation). 4. Eight (2^3). 5. False (largest substituent prefers equatorial).
Q: A molecule has two stereocentres with configurations 2R,3S. Its mirror image has configurations 2S,3R. What is the relationship between these two molecules?
A: They are enantiomers. Every stereocentre is inverted, so the two molecules are non-superimposable mirror images.
Q: A cyclopentane ring has three substituents. One substituent is trans to the other two. If there are three stereocentres on the ring, how many stereoisomers satisfy the "one trans to the other two" requirement?
A: Four. With three stereocentres and the constraint that one is trans to the other two, there are four possible arrangements (two pairs of enantiomers).
Q: Compound X has the configuration 2S,3R,4S on a five-membered ring. A second structure has configuration 2S,3R,4R. What is the relationship between them?
A: They are diastereomers. They share the same configuration at C2 and C3, but differ at C4.
Q: Two compounds are mixed in equal amounts. The mixture does not rotate plane-polarised light. What can you conclude about the two compounds?
A: They are enantiomers of each other. An equal (50:50) mixture of enantiomers is a racemic mixture, which is optically inactive.
Q: A substituted cyclohexane has three substituents. In its most stable chair, the positions are reported as e, e, e. What does this tell you about the substituents?
A: All three substituents can occupy equatorial positions simultaneously in the same chair conformation, which is the most stable arrangement. This means they are on the correct faces of the ring such that no ring flip is needed to get them all equatorial.
This connects to nucleophilic substitution (SN1/SN2) because stereochemistry determines whether a reaction proceeds with retention or inversion of configuration. If you cannot assign R/S at a stereocentre, you cannot track what happens to configuration during a substitution reaction.
Conformational analysis ties into reaction mechanisms: the geometry of a leaving group (axial vs equatorial on a cyclohexane) affects whether elimination or substitution is favoured.
Stereoisomer relationships also appear in biochemistry, where enzymes are stereospecific. The difference between an R and an S drug can be the difference between a medicine and a toxin (as with thalidomide).
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