Difficulty: Intermediate | Prerequisites: General chemistry (Lewis structures, bonding, molecular geometry, sp3 hybridisation).
Stereochemistry is the branch of organic chemistry concerned with how atoms are arranged in three-dimensional space. This topic builds directly on your understanding of molecular geometry and hybridisation from earlier chapters. It matters because molecules with the same formula and even the same connectivity can behave completely differently in biological systems, in drug design, and in laboratory reactions, all because of their 3D shape. If you are comfortable drawing Lewis structures and understand what sp3 hybridisation looks like, you are ready for this material.
Isomers are different compounds that share the same molecular formula. They split into two families: constitutional isomers (different atom connectivity) and stereoisomers (same connectivity, different 3D arrangement). Stereoisomers further divide into enantiomers (non-superimposable mirror images) and diastereomers (stereoisomers that are not mirror images of each other).
Isomers
Different compounds that share the same molecular formula. In simple terms, same atoms, different molecules.
Constitutional isomers (aka structural isomers)
Isomers with the same molecular formula but different connectivity between atoms. Think of it as: the atoms are wired together in a different order. Dimethyl ether (H3C-O-CH3) and ethanol (CH3CH2OH) both have the formula C2H6O, but one is an ether and the other an alcohol.
Stereoisomers
Isomers with the same connectivity but a different spatial arrangement of atoms. Think of it as: same wiring diagram, different 3D shape.
Enantiomers
Stereoisomers that are non-superimposable mirror images of each other. Think of it as: your left hand and right hand. They are mirror images but you cannot stack one perfectly on top of the other.
Diastereomers
Stereoisomers that are not mirror images of each other. In simple terms, they differ in 3D arrangement but are not related as mirror images.
Chiral
A molecule (or object) that is non-superimposable with its mirror image. A chiral molecule has an enantiomer.
Achiral
A molecule (or object) that is superimposable with its mirror image. Its mirror image is the same molecule, so no enantiomer is possible.
Chirality centre (aka stereocenter, chiral centre, asymmetric centre)
A tetrahedral (sp3) atom bonded to four different groups. One chirality centre guarantees a molecule is chiral.
Plane of symmetry (aka mirror plane)
An imaginary plane that divides an object into two halves that are mirror images of each other. Any molecule with a plane of symmetry is achiral.
All isomers share the same molecular formula. From there, the classification branches:
Constitutional isomers differ in how the atoms are connected (different connectivity).
Stereoisomers have the same connectivity but differ in how atoms are arranged in space.
Enantiomers are stereoisomers that are non-superimposable mirror images.
Diastereomers are stereoisomers that are not mirror images of each other.
Constitutional isomers share a molecular formula but their atoms are bonded in a fundamentally different sequence.
C2H6O: dimethyl ether (H3C-O-CH3) vs ethanol (CH3CH2OH). Same atoms, completely different functional groups.
C5H12: n-pentane, isopentane, and 2,2-dimethylpropane. All are C5H12, but the carbon skeleton is branched differently in each.
Because their connectivity differs, constitutional isomers have different physical properties (boiling point, melting point, density) and different chemical behaviour. This is the broadest type of isomerism.
Stereoisomers have identical connectivity but differ in the spatial arrangement of their atoms.
Enantiomers (non-superimposable mirror images)
An enantiomer pair is like your left and right hands: mirror images, but no amount of rotation lets you lay one on top of the other perfectly.
To draw an enantiomer, reflect the molecule in a mirror placed behind, beneath, or next to it. The key is to swap the wedge and dash bonds at each chirality centre.
Enantiomers share identical physical properties (boiling point, melting point, density, refractive index) in an achiral environment. They differ only when they interact with something else that is chiral, such as biological receptors or polarised light.
Diastereomers (not mirror images)
Diastereomers are stereoisomers whose relationship is not that of mirror images.
They have different physical properties (different melting points, boiling points, solubilities) even in achiral environments.
Common sources of diastereomerism: cis/trans isomers around a double bond or a ring, and compounds with multiple chirality centres where only some centres are inverted.
Chirality and achirality
A chirality centre is a tetrahedral (sp3) carbon bonded to four different groups. If any two of the four groups are the same, that carbon is not a chirality centre.
A single chirality centre guarantees the molecule is chiral.
A plane of symmetry makes a molecule achiral, regardless of whether it contains chirality centres. A chair, for example, has a plane of symmetry and is achiral. Your hand does not, and it is chiral.
Planes of symmetry can cut through bonds or atoms, not only between them.
Students often think "same molecular formula" automatically means the compounds are isomers. They must also be different compounds. Two identical molecules with the same formula are not isomers.
Students frequently confuse constitutional isomers with stereoisomers. The test: do the atoms connect in the same sequence? If yes, they are stereoisomers. If no, they are constitutional isomers.
A common error is assuming that any molecule with a chirality centre is automatically different from its mirror image. That is true for a single chirality centre, but molecules with multiple chirality centres can be meso (achiral despite having chirality centres).
Students sometimes think enantiomers have different physical properties. They do not, except in how they interact with other chiral things (polarised light, enzymes, receptors).
⚠️ Classification questions are almost guaranteed on the exam: given two structures, identify whether they are constitutional isomers, enantiomers, diastereomers, or the same molecule.
⚠️ You will be expected to identify chirality centres in a structure and determine whether the molecule is chiral or achiral.
⚠️ The plane of symmetry test is the fastest way to identify an achiral molecule. Practise spotting planes of symmetry in line drawings and Fischer projections.
Real-world application: the thalidomide disaster illustrates why stereochemistry matters in drug design. (R)-thalidomide is a safe sedative; (S)-thalidomide is a teratogen that caused thousands of birth defects. The two enantiomers interconvert in the body, so even a pure sample of the safe form becomes dangerous.
True or false: enantiomers have different boiling points.
Fill in the blank: a tetrahedral atom bonded to four ______ groups is a chirality centre.
True or false: all stereoisomers are enantiomers.
Fill in the blank: a molecule with a plane of symmetry is ______.
True or false: constitutional isomers have the same connectivity but different spatial arrangement.
Answers: 1. False (identical in achiral environments). 2. different. 3. False (diastereomers are also stereoisomers). 4. achiral. 5. False (that describes stereoisomers; constitutional isomers have different connectivity).
Q: Dimethyl ether (H3C-O-CH3) and ethanol (CH3CH2OH) have the same molecular formula, C2H6O. What is their isomeric relationship?
A: They are constitutional isomers. The atoms are connected in a different order (the oxygen sits between two carbons in dimethyl ether, but at the end of the chain in ethanol).
Q: Two molecules have the same connectivity but are non-superimposable mirror images. What type of isomers are they?
A: They are enantiomers.
Q: How do you quickly determine whether a molecule is achiral?
A: Look for a plane of symmetry. If one exists, the molecule is achiral. You can also check whether the mirror image is superimposable on the original.
Q: A molecule has one chirality centre. Is it chiral or achiral?
A: It is chiral. A single chirality centre always produces a chiral molecule.
Q: Two stereoisomers of a dibrominated alkene are not mirror images of each other. What are they called?
A: Diastereomers.
This material connects directly to reaction mechanisms later in the course: many organic reactions create new chirality centres, and understanding whether the product is a single enantiomer or a racemic mixture depends on the concepts here. It also ties into biochemistry, where enzyme-substrate interactions are stereospecific (enzymes recognise only one enantiomer). The E/Z and cis/trans nomenclature covered in Part 2 of these notes builds on the diastereomer concept introduced here.
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