Difficulty: Intermediate | Prerequisites: Lewis structures, 3D molecular geometry, IHD, basic nomenclature
This is the single highest-tested topic area on the CHEM 225 Mock Exam 2, covering roughly half the questions.
Stereochemistry asks: given two molecules with the same atoms, how are they related in three-dimensional space? The core skill is classifying pairs of structures as identical, enantiomers, diastereomers, constitutional isomers or not isomeric at all. This requires fluency with chiral centres, R/S assignments, Fischer projections and the ability to mentally rotate or flip structures.
Enantiomers
Stereoisomers that are non-superimposable mirror images of each other. Every chiral centre has the opposite configuration (R becomes S, S becomes R).
Think of it as your left and right hands: same connectivity, mirror images, but you cannot stack one on top of the other.
Diastereomers
Stereoisomers that are not mirror images of each other. At least one chiral centre has the same configuration between the two molecules, and at least one differs.
In simple terms, this means they are stereoisomers where some, but not all, stereocentres are flipped.
Constitutional isomers (structural isomers)
Molecules with the same molecular formula but different connectivity, meaning atoms are bonded to different partners.
Think of it as: same Lego pieces, assembled in a different order.
Identical molecules
Two drawings that represent exactly the same molecule. All stereocentres match once you account for rotation or redrawing. If you can rotate one to get the other, they are identical.
Meso compound
A molecule that has chiral centres but is achiral overall because it possesses an internal plane of symmetry. The stereocentres cancel each other out.
In simple terms, this means the molecule has mirror-image halves built into itself, so it is not optically active despite having stereocentres.
Optical rotation
The ability of a chiral substance to rotate the plane of polarised light. Enantiomers rotate light by equal amounts in opposite directions. A racemic mixture (equal parts of both enantiomers) shows zero net rotation.
Fischer projection
A two-dimensional representation of a three-dimensional molecule, used especially for sugars and amino acids. Horizontal bonds point toward the viewer; vertical bonds point away. The rules for manipulating Fischer projections are strict and heavily tested.
R and S configuration
The Cahn-Ingold-Prelog system for assigning absolute configuration at a chiral centre. Assign priorities 1-4 to substituents by atomic number, orient the lowest-priority group away from you, and trace 1 to 2 to 3: clockwise = R, anticlockwise = S.
When you are given two structures and asked how they are related, work through these steps in order:
Same molecular formula? If no, they are not isomeric.
Same connectivity (same bonds between the same atoms)? If no, they are constitutional isomers.
Superimposable (identical after rotation/translation)? If yes, they are identical.
Mirror images of each other? If yes, they are enantiomers. If no, they are diastereomers.
That is the entire framework. Every "I and II are:" question on this exam uses it.
Many exam questions give you dash-wedge drawings on rings (cyclohexanes, cyclopentanes) or open-chain molecules and ask you to compare them.
Wedge = bond coming toward you (out of the page)
Dash = bond going away from you (into the page)
Before comparing two structures, try to orient them the same way. Mentally rotate one molecule. If all substituents match after rotation, they are identical.
If the molecular formula and connectivity are the same but the 3D arrangement differs, you are dealing with stereoisomers. Then ask: mirror images or not?
Fischer projections are tested in questions like Q12 ("which structure represents the same compound as I?"). The manipulation rules:
Rotation by 180 degrees in the plane of the paper: gives the same molecule. This is allowed.
Rotation by 90 degrees: gives the enantiomer. This swaps the configuration.
Swapping any two groups: swaps the configuration (R becomes S, S becomes R). Two swaps return you to the original.
Holding the top group fixed and rotating the other three: gives the same molecule (equivalent to two swaps).
To check whether two Fischer projections are the same compound, count how many swaps it takes to get from one to the other. An even number of swaps = same molecule. An odd number = enantiomer.
This is tested in Q8 ("the compounds would have...").
Enantiomers have identical physical properties (same melting point, same boiling point, same solubility) except for the direction of optical rotation, which is equal in magnitude but opposite in sign.
Diastereomers have different physical properties across the board: different melting points, different boiling points, different solubilities, and different magnitudes of optical rotation.
So if two molecules are enantiomers, they share the same melting point and show equal but opposite optical rotations. Both A and B are true for enantiomers, meaning the answer involves more than one property being shared.
Q19 tests whether two Newman projections of the same molecule are conformational isomers or something else. Conformational isomers (conformers) are the same molecule viewed in different rotational states about a single bond. They are not different compounds.
A meso compound has stereocentres but an internal mirror plane. It is achiral and optically inactive. When two molecules look like enantiomers but both turn out to be meso, they are identical (the same achiral compound).
Students often confuse enantiomers and diastereomers. The test is simple: if every stereocentre is inverted, they are enantiomers. If only some are inverted, they are diastereomers.
Rotating a Fischer projection 90 degrees does not preserve the molecule. It gives the enantiomer. Only 180-degree rotations are safe.
Students sometimes call conformational isomers (different rotations about a single bond) "diastereomers" or "different compounds." They are the same molecule in a different conformation.
A meso compound is not chiral, even though it contains stereocentres. Students who see stereocentres and immediately call the molecule chiral will get meso questions wrong.
"Same molecular formula" does not mean "isomers." Two molecules could have the same formula and be identical. You must check further.
"I and II are:" classification questions made up roughly half this mock exam. Expect multiple questions of this type.
Fischer projection manipulation (Q12) is a reliable exam question. Practise converting between Fischer projections and identifying when two projections are the same, enantiomers or diastereomers.
Questions about physical properties of enantiomers vs diastereomers (Q8) test conceptual understanding rather than calculation.
Exam questions often use cyclohexane or bicyclic ring systems drawn in 3D to test whether you can identify stereorelationships on rings.
True or false: Enantiomers have different melting points. (False, enantiomers have identical melting points.)
Fill in the blank: Rotating a Fischer projection by 90 degrees gives the ____. (Enantiomer.)
True or false: If two molecules are stereoisomers and mirror images, they are diastereomers. (False, they are enantiomers.)
Fill in the blank: A meso compound is optically ____ despite having chiral centres. (Inactive.)
True or false: Conformational isomers are different compounds. (False, they are the same compound in different conformations.)
Q: Two molecules have the same molecular formula and the same connectivity, but at one of three stereocentres the configuration is R in molecule A and S in molecule B. The other two centres are the same. What is the relationship?
A: Diastereomers. They are stereoisomers (same connectivity) but not mirror images (only one of three centres is inverted, not all).
Q: You rotate a Fischer projection 180 degrees in the plane of the paper. Is the result the same molecule or its enantiomer?
A: The same molecule. A 180-degree rotation preserves all configurations.
Q: Two structures are non-superimposable mirror images. They are placed in a polarimeter. What do you observe about their optical rotations?
A: Equal magnitude, opposite sign. One rotates plane-polarised light clockwise (+) and the other anticlockwise (-) by the same number of degrees.
Q: A molecule has two stereocentres, one R and one S, and an internal mirror plane. Is this molecule chiral?
A: No. It is a meso compound. The internal mirror plane makes it achiral despite having stereocentres.
Q: Two cyclohexane-based molecules have the same substituents on the same carbons, but in one drawing the OH is on a wedge and in the other it is on a dash, while all other substituents are the same. What is the relationship?
A: Diastereomers (assuming there is more than one stereocentre and not all are flipped). If the molecule has only one stereocentre and it is inverted, they are enantiomers.
Stereochemistry connects to reaction mechanisms throughout the rest of CHEM 225. SN2 reactions invert configuration at the stereocentre (Walden inversion), while SN1 reactions produce racemic mixtures. Addition reactions to alkenes can be syn or anti, producing specific stereochemical outcomes. Understanding enantiomers and diastereomers is also essential for biochemistry, where enzymes are stereospecific.
Related Terms / Search Tags
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