Difficulty: Intermediate | Prerequisites: R/S configuration, chirality and meso concepts
Once you can assign R/S and determine whether a molecule is chiral, the next skill is comparing two structures and deciding their relationship. This is the core of stereochemistry problem-solving: you are given two drawings and asked whether they represent the same compound, enantiomers, or diastereomers. The ability to do this quickly and accurately is tested relentlessly. It requires you to see past the way a structure happens to be drawn and focus on the configuration at each stereocentre.
Two structures with the same connectivity are either the same compound (identical configuration at every stereocentre), enantiomers (opposite configuration at every stereocentre), or diastereomers (opposite at some, same at others). Assign R/S at each centre in both structures, then compare.
Same compound (identical)
Two drawings that represent the same molecule. Every stereocentre has the same R/S configuration in both. The drawings may look different (rotated, flipped, drawn from a different angle), but the 3D arrangement is identical. In simple terms, if you built both with a model kit, you could superimpose them perfectly.
Enantiomers
Stereoisomers that are non-superimposable mirror images. Every stereocentre in one has the opposite configuration in the other (all R becomes all S, and vice versa). They have identical physical properties (boiling point, solubility, etc.) except for the direction they rotate plane-polarised light.
Diastereomers
Stereoisomers that are not mirror images. They differ at some stereocentres but not all. Unlike enantiomers, diastereomers have different physical properties (different melting points, solubilities, reactivities). Think of it as the "everything else" category: not the same compound, not enantiomers, but still stereoisomers.
Epimers
A specific type of diastereomer that differs at exactly one stereocentre out of two or more. Common in sugar chemistry. In simple terms, if two molecules are diastereomers and only one centre is flipped, they are epimers.
Constitutional isomers (structural isomers)
Molecules with the same molecular formula but different connectivity (different bonds between atoms). These are not stereoisomers at all. Before comparing R/S, confirm that the two structures have the same connectivity; if they do not, the comparison stops there.
When given two structures, work through this sequence:
Check connectivity first. Do both structures have the same atoms connected in the same order? If not, they are constitutional isomers (not stereoisomers), and the analysis ends.
Assign R/S at every stereocentre in both structures. This is the core work. Be careful with structures drawn differently; you may need to mentally rotate or redraw one to assign correctly.
Compare the configurations centre by centre.
All centres match (same R/S at every position) = same compound.
All centres are opposite (every R in one is S in the other) = enantiomers.
Some centres match and some are opposite = diastereomers.
When two wedge-dash structures look different:
Remember that rotating a structure 180 degrees in the plane does not change the configuration. The molecule is the same; it is just drawn from a different vantage point.
Flipping a structure (reflecting it through a plane) inverts every stereocentre. If you flip and get a match, the originals are enantiomers.
The safest approach is to assign R/S independently in each drawing rather than trying to mentally superimpose them.
Fischer projections have specific manipulation rules:
Rotating a Fischer projection 180 degrees in the plane gives the same compound.
Rotating 90 degrees gives the enantiomer.
Swapping any two groups on a single stereocentre inverts that centre. One swap = opposite configuration; two swaps = same configuration.
The fastest method: assign R/S at each centre in both projections and compare.
For cyclic molecules (especially substituted cyclohexanes):
Draw the two structures in a comparable format (both as chairs, or both as flat rings with wedges and dashes).
Identify the stereocentres and assign R/S in each.
Watch for ring flips: a chair can flip to another chair where axial becomes equatorial and vice versa, but the cis/trans relationship (and therefore R/S) does not change.
For disubstituted cyclohexanes, cis and trans isomers are diastereomers of each other.
If one of the two structures is meso, its mirror image is the same compound (not an enantiomer). This is a frequent exam trap. When you assign R/S at each centre in both structures and find that all centres are opposite, check whether the molecule is meso. If it is, the two drawings are the same compound, not enantiomers.
For a molecule with n stereocentres, the maximum number of stereoisomers is 2^n. Meso compounds reduce this count. For example, a molecule with 2 stereocentres has a maximum of 4 stereoisomers (RR, SS, RS, SR), but if RS and SR are meso (the same molecule), the actual count is 3.
"The drawings look different, so they must be different compounds." A molecule can be drawn from many angles. Two very different-looking wedge-dash structures can represent the same compound. Always assign R/S rather than trying to eyeball it.
"All centres are opposite, so they must be enantiomers." This is usually true, but check for the meso exception first. If the molecule with all-opposite centres is meso, the two structures are actually the same compound.
"Diastereomers are just less important enantiomers." Diastereomers have different physical and chemical properties. Enantiomers share most properties. This is a fundamental distinction, not a matter of degree.
Confusing cis/trans with R/S. cis and trans describe the relative positions of substituents (same side or opposite side). R and S describe absolute configuration. A cis isomer can be RR or SS depending on the substituents; cis/trans alone does not tell you the relationship.
⚠️ "Classify the relationship between these two structures" is one of the most common exam question formats. Expect 10 to 20 pairs to classify under time pressure.
⚠️ The meso trap is a favourite: two structures that look like enantiomers but are actually the same meso compound. If you see identical substituents on two stereocentres with opposite configurations, check for meso before answering "enantiomers."
⚠️ Structures drawn in different representations (one as a Fischer projection, one as wedge-dash, or one as a Newman projection) are common at the harder end. The safest strategy is always to assign R/S independently in each.
⚠️ For cyclic structures, watch for ring flips and remember that cis/trans does not change with a chair flip. The configurational relationship is fixed.
True or false: If two structures have opposite configuration at every stereocentre, they are always enantiomers.
False. If the molecule is meso, the two structures are the same compound.
Fill in the blank: Diastereomers differ at ______ but not all stereocentres.
Some (at least one, but not every one).
True or false: Rotating a Fischer projection 180 degrees in the plane gives the enantiomer.
False. A 180-degree rotation gives the same compound. A 90-degree rotation gives the enantiomer.
Fill in the blank: The maximum number of stereoisomers for a molecule with 3 stereocentres is ______.
8 (2^3), though meso compounds may reduce this.
True or false: Diastereomers have identical physical properties.
False. Diastereomers have different physical properties. Enantiomers are the ones with identical physical properties (except optical rotation direction).
Q: Structure A has stereocentres with configurations (2R, 3S). Structure B has (2S, 3R). Both have identical connectivity. What is their relationship?
A: First check: every centre is opposite (R became S, S became R). This would normally mean enantiomers. But also check whether this molecule is meso. If the substituents at C2 and C3 are identical (same groups, just mirrored), and (2R,3S) has an internal mirror plane, then A and B are the same compound (both meso). If the substituents are not identical, they are enantiomers.
Q: Structure A is (2R, 3R, 4S). Structure B is (2S, 3R, 4R). What is the relationship?
A: Compare centre by centre: C2 differs (R vs S), C3 is the same (R vs R), C4 differs (S vs R). Some centres match, some differ. They are diastereomers.
Q: You are given two Fischer projections of a four-carbon sugar. Both have -OH groups on C2 and C3. In one, both OHs are on the right. In the other, the C2 OH is on the right and the C3 OH is on the left. What is their relationship?
A: They differ at C3 (one has OH right, the other has OH left, meaning opposite configuration at that centre) but agree at C2 (OH right in both). Since they differ at one centre but not the other, they are diastereomers. More specifically, they are epimers (differing at exactly one stereocentre).
Q: Two chair conformations of trans-1,2-dichlorocyclohexane are drawn. In one, both Cl atoms are axial. In the other, both are equatorial. Same, enantiomers, or diastereomers?
A: Same compound. A ring flip interconverts axial and equatorial positions but does not change the cis/trans relationship or the R/S configurations. The two chairs are conformers, not different stereoisomers.
Q: Explain why enantiomers have the same melting point but diastereomers do not.
A: Enantiomers have identical intermolecular forces because their shapes are mirror images, so their crystal packing energies are the same (leading to the same melting point). Diastereomers have genuinely different 3D shapes, which produce different intermolecular interactions and therefore different physical properties.
This topic ties together everything from the previous two sets of notes. R/S assignment is the tool you use, chirality and meso knowledge tells you when the expected answer (enantiomers) is actually wrong (same compound, if meso). Going forward, stereoisomer relationships determine the products of reactions: SN2 gives inversion (enantiomeric product at the reacting centre), SN1 gives racemisation, and addition reactions to alkenes can give syn or anti products (diastereomers of each other).
In biochemistry, enzyme specificity is built on stereoisomer recognition. An enzyme that binds one enantiomer will not bind the other. Diastereomers, having different shapes, interact differently with all environments, chiral or not.
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