Difficulty: Intermediate | Prerequisites: IUPAC nomenclature, Lewis structures, orbital hybridisation
Stereochemistry is about the 3D arrangement of atoms in a molecule. Two molecules can have identical connectivity but different spatial orientations, giving them different physical and biological properties. The key skills here are assigning R/S configuration to chiral centres, assigning E/Z to double bonds, drawing and analysing Newman projections, and understanding chair conformations of cyclohexane.
Chirality / chiral centre (stereogenic centre, stereocentre)
A carbon bonded to four different groups. The molecule is non-superimposable on its mirror image. Think of it as your left and right hands: same parts, different spatial arrangement.
Enantiomers
Stereoisomers that are non-superimposable mirror images of each other. They have identical physical properties (boiling point, melting point, solubility) except for the direction they rotate plane-polarised light.
Diastereomers
Stereoisomers that are not mirror images. They have different physical properties. Cis/trans isomers and molecules with multiple chiral centres where some (but not all) configurations differ are diastereomers.
R and S configuration
The absolute configuration at a chiral centre, assigned using Cahn-Ingold-Prelog (CIP) priority rules. R (rectus) = clockwise; S (sinister) = anticlockwise, when viewed with the lowest-priority group pointing away.
E/Z nomenclature
Describes the geometry of substituents around a double bond. Z (zusammen, "together") = higher-priority groups on the same side. E (entgegen, "opposite") = higher-priority groups on opposite sides. This replaces cis/trans when substituents are not simple.
Meso compound
A molecule with chiral centres but an internal plane of symmetry, making it achiral overall. It has stereocentres but is optically inactive because one half cancels the other.
Optical activity
The ability of a chiral compound to rotate plane-polarised light. Measured by a polarimeter. Dextrorotatory (+) rotates right; levorotatory (-) rotates left. The sign cannot be predicted from R/S configuration.
Newman projection
A way of viewing a molecule along a specific C-C bond axis. The front carbon is a dot; the back carbon is a circle. Used to analyse torsional and steric strain in different conformations.
Conformational isomers (conformers, rotamers)
Different spatial arrangements of the same molecule produced by rotation around single bonds. They are not different molecules; they interconvert freely at room temperature.
Gauche interaction
A steric interaction between two large groups separated by a 60-degree dihedral angle in a Newman projection. Causes modest strain.
Anti conformation
The lowest-energy staggered conformation, where the two largest groups are 180 degrees apart. Least steric strain.
Axial and equatorial positions (cyclohexane)
In a chair conformation, each carbon has one axial bond (pointing straight up or down) and one equatorial bond (pointing roughly sideways). Large substituents prefer equatorial positions to avoid 1,3-diaxial interactions.
Rank the four groups attached to the chiral centre by atomic number of the atom directly bonded to the stereocentre. Higher atomic number = higher priority.
If two atoms are the same, move outward along each chain until you find a point of difference.
Double bonds are treated as if each atom is bonded to the other atom twice (phantom atoms). A C=O is treated as C bonded to O and O bonded to C.
Isotopes: higher mass = higher priority (deuterium > hydrogen).
Orient the molecule so that the lowest-priority group (usually H) points away from you.
Trace a path from highest to lowest priority among the remaining three groups.
Clockwise = R. Anticlockwise = S.
If the lowest-priority group is not pointing away in the drawing, assign R or S as if it were, then reverse the answer.
From the review (Q18): (2S, 4S)-4-chloropentan-2-ol has two stereocentres, both S.
Applied to alkenes (double bonds) where each carbon of the double bond bears two different substituents.
Use CIP rules to rank the two groups on each carbon of the double bond.
Higher-priority groups on the same side = Z. Higher-priority groups on opposite sides = E.
From the review (Q22): I = E, II = E. Both isomers shown had their higher-priority groups on opposite sides of the double bond.
Eclipsed conformations: bonds on the front and back carbon are aligned (0-degree dihedral). Maximum torsional strain. These are energy maxima.
Staggered conformations: bonds are offset by 60 degrees. Lower energy.
Anti: the two largest groups are 180 degrees apart. Lowest energy staggered conformation.
Gauche: the two largest groups are 60 degrees apart. Slightly higher energy than anti due to steric strain.
For butane, the energy ordering from most stable to least: anti > gauche > eclipsed (methyl-H) > eclipsed (methyl-methyl).
Cyclohexane adopts a chair conformation to minimise angle strain, torsional strain, and steric strain.
Each carbon has one axial and one equatorial position. A ring flip converts all axial positions to equatorial and vice versa.
Large substituents strongly prefer equatorial positions to avoid 1,3-diaxial interactions (which are gauche-like steric interactions with axial hydrogens two carbons away).
For disubstituted cyclohexanes, the more stable chair places the larger substituent equatorial.
The review (Q11) asks students to rank conformational stability: IV < II < III < I, reflecting increasing preference for equatorial placement of large groups.
Same molecule (identical): superimposable, same name.
Enantiomers: non-superimposable mirror images. All stereocentres inverted.
Diastereomers: stereoisomers that are not mirror images. Some (but not all) stereocentres differ.
Meso: contains stereocentres but is achiral due to an internal mirror plane.
Constitutional isomers: different connectivity entirely (not stereoisomers).
Students often assume that R configuration means dextrorotatory (+) and S means levorotatory (-). There is no predictable relationship between R/S and the sign of optical rotation. The only way to know is to measure it experimentally.
Confusing E/Z with cis/trans. Cis/trans works only when each carbon of the double bond has one hydrogen. E/Z (based on CIP priorities) works universally and is the system you should default to.
Thinking that all molecules with stereocentres are chiral. Meso compounds have stereocentres but are achiral. Always check for internal mirror planes.
Drawing Newman projections with the wrong group on the front vs. back carbon. Label which carbon you are looking at from the front before you start placing groups.
⚠️ Assigning R/S to molecules with two or more stereocentres is a near-certain exam question. The review key includes (2S, 4S)-4-chloropentan-2-ol, requiring correct assignment at both centres.
⚠️ E/Z assignment on substituted alkenes is tested regularly. Be ready for cases where CIP priorities are less obvious (e.g., when both substituents are carbon chains of different lengths or branching).
⚠️ Ranking conformational stability of different chair conformations is explicitly tested (Q11 in the review). Know how to evaluate 1,3-diaxial strain for common substituents.
⚠️ Identifying the relationship between two structures (enantiomers, diastereomers, constitutional isomers, identical, or meso) is a recurring question type. Multiple review questions (Q6, Q14, Q19) test this.
True or False: R configuration always corresponds to clockwise rotation of plane-polarised light.
False. R/S describes spatial arrangement; (+)/(-) describes optical rotation. No predictable link.
Fill in the blank: In E/Z nomenclature, Z means the higher-priority groups are on the ___ side of the double bond.
Same side (zusammen).
True or False: A meso compound is optically active.
False. A meso compound is achiral (and therefore optically inactive) despite having stereocentres.
Fill in the blank: In a Newman projection, the most stable staggered conformation of butane is the ___ conformation.
Anti.
True or False: In a chair conformation of cyclohexane, a ring flip converts all axial substituents to equatorial.
True.
Q: Assign R or S configuration to a stereocentre bonded to -OH, -CH3, -H, and -Cl. Priorities: Cl > OH > CH3 > H.
A: Orient with H (lowest priority) pointing away. Trace Cl to OH to CH3. If that path is clockwise, the centre is R. If anticlockwise, S.
Q: A molecule has two stereocentres. One is R, the other is S. Its mirror image has both centres inverted (S and R). What is the relationship between these two molecules?
A: They are enantiomers (non-superimposable mirror images, with all stereocentres inverted).
Q: A molecule has two stereocentres, one R and one S. It is superimposable on its mirror image. What is this compound?
A: A meso compound. The internal mirror plane makes it achiral despite having stereocentres.
Q: Draw the most stable Newman projection of 2-methylbutane looking down the C2-C3 bond. What conformation is this?
A: The anti conformation, with the two largest groups (the methyl on C2 and the methyl on C3's other side) at 180 degrees.
Q: For a disubstituted cyclohexane with one large group and one small group, which chair conformation is more stable?
A: The one that places the larger substituent in the equatorial position. The energy cost of a 1,3-diaxial interaction is greater for large groups.
Stereochemistry feeds directly into reaction mechanisms. SN2 reactions invert the stereocentre (Walden inversion). SN1 reactions racemise it. E2 eliminations require an anti-periplanar geometry, which ties back to Newman projections and conformational analysis.
Chirality matters enormously in biochemistry and pharmacology. Many drug molecules are chiral, and the two enantiomers can have completely different biological effects.
Stereochemistry, chirality, chiral centre, stereocentre, enantiomers, diastereomers, R and S configuration, CIP priority rules, Cahn-Ingold-Prelog, E/Z nomenclature, cis trans, meso compound, optical activity, polarimeter, specific rotation, Newman projection, conformational isomers, gauche anti eclipsed, chair conformation, axial equatorial, 1 3 diaxial interaction, ring flip, cyclohexane chair, Organic Chemistry I final review