Difficulty: Intermediate | Prerequisites: Lewis structures, IHD, basic 3D molecular geometry.
Stereochemistry is the study of molecules that have the same connectivity (same atoms bonded to the same atoms) but differ in spatial arrangement. This sits at the heart of organic chemistry because two molecules with identical molecular formulae and identical bonding sequences can behave completely differently in biological systems if their three-dimensional shapes differ. You need to know IHD, Lewis structures and basic 3D molecular geometry before this section makes sense.
Molecules with the same connectivity but different 3D arrangements are stereoisomers. Enantiomers are non-superimposable mirror images with identical physical properties in achiral environments. Diastereomers are stereoisomers that are not mirror images, and they have different physical properties. Meso compounds contain chiral centres but are achiral overall because of an internal plane of symmetry.
Stereoisomers
Molecules with the same molecular formula and the same connectivity but a different arrangement of atoms in space.
Think of it as: same recipe, same ingredients, different 3D shape.
Enantiomers
Stereoisomers that are non-superimposable mirror images of each other. They have identical physical properties (boiling point, melting point, density, solubility) in achiral environments, but rotate plane-polarised light in opposite directions.
In simple terms: your left and right hands are enantiomers.
Diastereomers
Stereoisomers that are not mirror images of each other. They have different physical properties (different boiling points, melting points, solubilities) and can be separated by standard techniques like distillation or chromatography.
Think of it as: cis and trans isomers of a disubstituted cyclohexane.
Meso compound
A molecule that contains chiral centres but is achiral overall because it possesses an internal plane of symmetry. Its mirror image is superimposable on itself.
In simple terms: the molecule has stereocentres, but they cancel each other out.
Chiral centre (stereocentre)
A carbon atom bonded to four different groups. The presence of a chiral centre is necessary (but not sufficient) for a molecule to be chiral.
Racemic mixture
A 50:50 mixture of both enantiomers of a chiral molecule. It shows zero net optical rotation because the rotations cancel.
Optical rotation
The rotation of plane-polarised light by a chiral molecule. Measured in a polarimeter. The (R) enantiomer and (S) enantiomer rotate light by equal amounts in opposite directions.
R/S configuration
The Cahn-Ingold-Prelog system for naming the absolute configuration at a stereocentre. Groups are ranked by atomic number, and the arrangement (clockwise = R, anticlockwise = S) is determined with the lowest-priority group pointing away.
The first question is always: do the two molecules have the same connectivity?
Same formula, different connectivity → constitutional isomers (not stereoisomers).
Same formula, same connectivity, different spatial arrangement → stereoisomers.
Mirror images and non-superimposable → enantiomers.
Not mirror images → diastereomers.
cis/trans isomers of disubstituted cyclohexanes are diastereomers. They have the same connectivity (both are 1,2-dimethylcyclohexane, for instance) but differ in whether the substituents are on the same side or opposite sides of the ring.
Enantiomers share all scalar physical properties in achiral environments: identical boiling points, melting points, density, solubility and refractive index. The only differences show up in interactions with polarised light (they rotate it in opposite directions) and with other chiral molecules (e.g. chiral solvents, chiral reagents, biological receptors).
A racemic mixture (equal parts of both enantiomers) has zero net optical rotation. If pure (R)-2-butanol rotates light by +13.5°, then a racemic mixture of 2-butanol rotates light by 0°.
Diastereomers have different physical properties: different boiling points, different melting points, different solubilities. This is why they can be separated by ordinary techniques such as distillation, recrystallisation and standard column chromatography (achiral stationary phase).
Enantiomers cannot be separated by these methods because their properties are identical in achiral environments. Separating enantiomers requires a chiral environment, such as a chiral stationary phase in HPLC or a chiral resolving agent.
A meso compound has chiral centres but is achiral overall. The defining feature is an internal plane of symmetry that makes the molecule superimposable on its mirror image.
Example: (2R,3S)-tartaric acid is meso. It has two stereocentres, but the internal mirror plane means the R configuration at one centre is exactly cancelled by the S configuration at the other.
A meso compound does not rotate plane-polarised light.
A stereocentre is a carbon bonded to four different groups. In 2,3-dibromobutane, carbons 2 and 3 are each bonded to H, Br, CH3 and the rest of the chain, making two stereocentres.
With n stereocentres, the maximum number of stereoisomers is 2^n, though the actual number may be lower if meso forms exist.
Standard silica gel chromatography is achiral. (R,R)-tartaric acid and (S,S)-tartaric acid are enantiomers, so a standard silica column will not separate them. Both interact identically with the achiral stationary phase.
To separate enantiomers, you need a chiral column (chiral stationary phase) or must convert them to diastereomers first (using a chiral resolving agent), separate the diastereomers, then remove the resolving agent.
Students often think enantiomers have different melting or boiling points. They do not, in achiral environments. The only differences are in optical rotation direction and behaviour with chiral partners.
"Meso compounds have no chiral centres" is wrong. Meso compounds must have chiral centres; they just happen to be arranged so an internal symmetry plane makes the overall molecule achiral.
Confusing constitutional isomers with diastereomers. Constitutional isomers have different connectivity (different bonding sequence). Diastereomers have the same connectivity but different spatial arrangement, and they are not mirror images.
Assuming a racemic mixture rotates light by the sum of the two rotations. The rotations are equal and opposite, so they cancel to zero.
⚠️ "What is the relationship between X and Y?" is one of the most common exam question types. Be prepared to classify any pair as constitutional isomers, enantiomers, diastereomers, identical, or unrelated.
⚠️ Optical rotation of racemic mixtures is a frequent exam question. The answer is always 0°.
⚠️ Questions about separating enantiomers test whether you understand that achiral methods fail. Know that chiral chromatography or a resolving agent is needed.
⚠️ Meso compound identification requires you to look for the internal plane of symmetry, not just count stereocentres.
True or false: enantiomers have different boiling points. (False. Identical in achiral environments.)
A meso compound must have ____ and ____. (Chiral centres and an internal plane of symmetry.)
The optical rotation of a racemic mixture is ____. (0°.)
True or false: cis- and trans-1,2-dimethylcyclohexane are enantiomers. (False. They are diastereomers.)
How many stereocentres does 2,3-dibromobutane have? (2.)
Q: Which physical properties are identical for a pair of enantiomers?
A: Boiling point and melting point (and density, solubility, refractive index) are identical in achiral environments. Direction of optical rotation differs.
Q: (R)-2-butanol has an observed rotation of +13.5°. What is the observed rotation for a racemic mixture of 2-butanol?
A: 0°. The equal and opposite rotations of the two enantiomers cancel.
Q: What is the relationship between (S)-2-bromobutane and (R)-2-bromobutane?
A: They are enantiomers. Same connectivity, single stereocentre with opposite R/S configurations, non-superimposable mirror images.
Q: A chemist tries to separate (R,R)-tartaric acid from (S,S)-tartaric acid on a standard silica gel column. Will they succeed?
A: No. These are enantiomers and have identical interactions with the achiral silica gel stationary phase. A chiral stationary phase or a chiral resolving agent is needed.
Q: Explain why diastereomers can be separated by distillation but enantiomers cannot.
A: Diastereomers have different physical properties (different boiling points, solubilities) because their spatial arrangements are not mirror images. Enantiomers have identical physical properties in achiral environments, so any method relying on differences in those properties will fail.
Q: How many stereocentres are in 2,3-dibromobutane?
A: Two. Carbons 2 and 3 each bear four different groups: H, Br, CH3, and the remainder of the chain.
Stereochemistry connects directly to reaction mechanisms: SN1 reactions produce racemic mixtures (because the carbocation intermediate is planar), while SN2 reactions produce inversion of configuration. In biochemistry, enzyme active sites are chiral environments, which is why one enantiomer of a drug can be therapeutic while the other is inactive or harmful.
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