Difficulty: Intermediate | Prerequisites: Chirality and R/S Configuration notes (Doc 1 of this set).
Once you can assign R/S to individual stereocentres, the next skill is classifying the relationship between two molecules. Organic chemistry uses precise vocabulary for this: enantiomers, diastereomers, constitutional isomers, conformers, and identical compounds each describe a specific structural relationship with specific consequences for physical and chemical properties. Exam questions regularly present two structures and ask you to name the relationship. This requires you to compare connectivity first (are the atoms joined in the same sequence?), then configuration (are the stereocentres the same or different?), then conformation (is it just a bond rotation?).
Two molecules can be related as constitutional isomers (same formula, different connectivity), enantiomers (non-superimposable mirror images, all stereocentres inverted), diastereomers (stereoisomers that are not mirror images, at least one but not all stereocentres inverted), conformers (same molecule in different rotational arrangements), or identical compounds (same in every respect). Enantiomers share all physical properties except the direction of optical rotation; diastereomers differ in most physical properties.
Enantiomers (mirror-image isomers, optical isomers)
Stereoisomers that are non-superimposable mirror images of each other. Every stereocentre in one molecule has the opposite configuration in the other. Think of it as left hand vs right hand: identical in every measurement except the spatial arrangement.
Diastereomers
Stereoisomers that are not mirror images of each other. At least one stereocentre has a different configuration between the two molecules, but not all of them are inverted. In simple terms, they are stereoisomers that fail the mirror-image test.
Constitutional isomers (structural isomers)
Molecules with the same molecular formula but different connectivity: the atoms are bonded in a different sequence. Think of it as different skeletons built from the same set of atoms.
Conformers (conformational isomers, rotamers)
Different spatial arrangements of the same molecule produced by rotation around single bonds. They are not different compounds; they are the same molecule caught in different poses. For exam purposes in this course, conformers are not considered identical.
Identical compounds (same compound)
Two drawings that represent the exact same molecule with the same connectivity and the same configuration at every stereocentre. They may look different on paper because of rotation or a different drawing angle.
Racemic mixture (racemate, racemic modification, d,l-mixture)
A 50:50 mixture of two enantiomers. It shows zero net optical rotation because the (+) rotation of one enantiomer is exactly cancelled by the (–) rotation of the other.
Optical rotation (specific rotation, [alpha])
The ability of a chiral compound to rotate the plane of plane-polarised light. Measured with a polarimeter. (+) or dextrorotatory means clockwise; (–) or levorotatory means anticlockwise. No correlation with R/S.
Resolution
The process of separating a racemic mixture into its individual enantiomers. In simple terms, pulling apart the left-handed molecules from the right-handed ones.
Check connectivity first. Are the atoms bonded in the same order? If not, they are constitutional isomers, and stereochemistry is irrelevant.
If connectivity is the same, check all stereocentres. Assign R/S at every stereocentre in both molecules.
All stereocentres inverted (every R becomes S and every S becomes R): enantiomers.
Some but not all stereocentres inverted: diastereomers.
All stereocentres identical: either identical compounds or conformers.
If all stereocentres match, check whether one drawing can be superimposed on the other. If yes, identical. If the only difference is rotation around a single bond, they are conformers.
Enantiomers have identical melting points, boiling points, solubilities, IR spectra, NMR spectra, and refractive indices. The only measurable difference in an achiral environment is the sign of optical rotation: one is (+), the other is (–), with equal magnitude.
Diastereomers differ in all physical properties: different melting points, different boiling points, different solubilities, different spectral data, and different magnitudes (not just sign) of optical rotation. This is why diastereomers can be separated by ordinary techniques (distillation, chromatography), while enantiomers require special chiral methods.
Enzymes are themselves chiral (built from L-amino acids). Their active sites are three-dimensional pockets that bind substrates through a specific arrangement of interactions: hydrogen bonds, electrostatic contacts, and hydrophobic surfaces. When (+)-glucose enters an enzyme's active site, the –OH groups align properly with the complementary residues. (–)-glucose has the opposite spatial arrangement: its –OH groups point the wrong way, so it cannot bind effectively. This is the three-point attachment model: a chiral receptor distinguishes enantiomers because at least three points of interaction are needed to tell mirror images apart.
When two cyclohexane chairs are drawn and you need to determine the relationship, first flatten both to their planar forms and assign R/S. Chair flips do not change configuration; they only change whether substituents are axial or equatorial. Two chairs of the same molecule are conformers.
The pharmaceutical industry spends billions on chiral resolution and asymmetric synthesis precisely because enantiomers interact differently with biological targets. Ibuprofen, for instance, is sold as a racemic mixture, but only the (S)-enantiomer is the active anti-inflammatory agent. The (R)-enantiomer is slowly converted to (S) in the body, which is why the racemate still works, but a pure (S) formulation would be more efficient.
Students often assume that if two molecules look like mirror images in a drawing, they must be enantiomers. They could also be identical (if the molecule is achiral or meso). Always assign R/S to confirm.
Confusing conformers with stereoisomers is common. A ring flip on cyclohexane changes axial/equatorial positions but does not change configuration. The two chairs are conformers, not diastereomers.
Students frequently think diastereomers must have two or more stereocentres. Cis/trans isomers of alkenes are diastereomers and involve zero traditional stereocentres.
Assuming that a racemic mixture is a single compound with zero rotation because "the rotations cancel." A racemate is a mixture of two distinct enantiomers, not a single achiral compound.
⚠️ Expect pairs of structures drawn as chairs, wedge-dash, or Fischer projections, and you will need to classify the relationship. Assign R/S at each stereocentre in both structures: it is the fastest, most reliable method.
⚠️ The question "are these chiral or achiral?" often appears alongside a molecule with a potential plane of symmetry. Do not skip the symmetry check.
⚠️ Know the physical-property consequences. A short-answer question may ask: "What physical properties differ between enantiomers? Between diastereomers?"
⚠️ The enzyme/glucose question (how does the body distinguish (+) and (–) glucose?) has appeared on past exams. The answer hinges on the three-point attachment model and the chirality of the enzyme active site.
True or false: Enantiomers have different melting points.
False. Enantiomers have identical melting points.
Fill in the blank: Two stereoisomers in which all stereocentres are inverted relative to each other are called ______.
Enantiomers.
True or false: Diastereomers can be separated by standard column chromatography.
True. They have different physical properties.
Fill in the blank: A 50:50 mixture of two enantiomers is called a ______ and has a net optical rotation of ______.
Racemic mixture; zero.
True or false: Cis and trans isomers of 2-butene are enantiomers.
False. They are diastereomers (stereoisomers that are not mirror images).
Q: Two molecules have the same molecular formula and the same connectivity. Molecule A has stereocentres with configurations (R,R). Molecule B has (R,S). What is the relationship between A and B?
A: Diastereomers. They are stereoisomers (same connectivity), but not all stereocentres are inverted (only one of two changed), so they are not enantiomers.
Q: Explain the difference between enantiomers and diastereomers in terms of (A) how you identify each and (B) their physical properties.
A: (A) Enantiomers are non-superimposable mirror images; every stereocentre in one molecule has the opposite configuration in the other. Diastereomers are stereoisomers where some but not all stereocentres are inverted, so they are not mirror images. (B) Enantiomers share all physical properties except the sign of optical rotation. Diastereomers differ in melting point, boiling point, solubility, and both the sign and magnitude of optical rotation.
Q: Explain how enzymes in your cells are able to distinguish between (+) and (–) glucose.
A: Enzymes are chiral molecules (composed of L-amino acids) with three-dimensional active sites. The active site makes multiple simultaneous contacts (hydrogen bonds, electrostatic interactions, hydrophobic contacts) with the substrate. (+)-glucose fits these contact points; (–)-glucose, being the mirror image, cannot align correctly at all contact points simultaneously. This is the three-point attachment model: at least three non-coplanar interactions are needed to distinguish mirror images.
Q: Two cyclohexane chair drawings show the same substituents in the same positions, but in one drawing a –Cl is axial and in the other it is equatorial. What is the relationship?
A: Conformers. A chair flip interconverts axial and equatorial positions without breaking any bonds or changing any configurations.
Q: Compound X has a specific rotation of +45 degrees. What is the specific rotation of its enantiomer?
A: –45 degrees. Enantiomers rotate plane-polarised light by equal magnitude but opposite sign.
Classifying stereochemical relationships is essential for predicting reaction outcomes. When you study alkene addition reactions (Doc 3 of this set), you will need to determine whether a product is formed as a single enantiomer, a racemic mixture, or a pair of diastereomers. The concepts here also feed directly into substitution and elimination reactions (SN1, SN2, E1, E2), where stereochemistry of the product depends on the mechanism.
Enantiomers, diastereomers, constitutional isomers, structural isomers, conformers, conformational isomers, identical compounds, stereoisomers, optical isomers, mirror image, non-superimposable, racemic mixture, racemate, optical rotation, specific rotation, polarimeter, dextrorotatory, levorotatory, resolution, three-point attachment, chiral recognition, enzyme specificity, glucose enantiomers, CHEM 237, organic chemistry, UW, University of Washington, chapters 6 and 7