H3 molecular stereochemistry

Work through stereochemical projections, conformations, configurations, optical activity and transition-metal stereochemistry, with focused practice.

GCE A-Level H3 Chemistry 2027
8 lessons

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Before you begin

Stereochemistry is about the three-dimensional shapes of molecules. This topic starts with reading and drawing projections, then covers conformations and their energies, cis/trans and E/Z isomers, R and S configurations, optical activity and optical purity, and finishes with the stereoisomers of complex ions.

Be comfortable with: isomerism and stereochemistry, complex ions and ligands, and tetrahedral, square-planar and octahedral shapes.

Learning goals

Use Newman and other stereochemical projections
Interpret stereochemical projections
Explain conformational isomerism and rotational barriers
Assign cis/trans and E/Z configurations
Apply R/S, optical activity and optical-purity reasoning
Identify cis/trans isomerism in square-planar and octahedral complexes
Identify enantiomerism in octahedral complexes

Syllabus statements covered

use stereochemical projections, including Newman projections, to represent molecules
interpret stereochemical projections of molecules [knowledge of Fischer projections is not required]
apply their understanding of the following types of isomerism to explain the stereochemistry of molecules, including saturated ring systems: — conformational isomerism, including energy barriers to rotation and interconversion
apply their understanding of the following types of isomerism to explain the stereochemistry of molecules, including saturated ring systems: — cis-trans isomerism, including E, Z nomenclature
apply their understanding of the following types of isomerism to explain the stereochemistry of molecules, including saturated ring systems: — enantiomerism and diastereomerism - R, S configuration - optical activity optical purity as the excess of one enantiomer, including calculation of optical purity by the equation: [α ]obs optical purity = × 100% [α ]pure material
recognise that transition element complexes can also exhibit stereoisomerism: — cis-trans isomerism, e.g. square planar complexes such as [Pt(NH3)2Cl2] and octahedral complexes such as [Co(NH3)4(H2O)2]2+
recognise that transition element complexes can also exhibit stereoisomerism: — enantiomerism, e.g. [Ni(H2NCH2CH2NH2)3]2+ and [Ni(H2NCH2CH2NH2)2(H2O)2]2+ [identification of fac-mer isomerism is not required]

Lessons

Work through them in order.

2.1 Molecular Stereochemistry

Practise and check

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Topic reference

Quick reference

Question	First move	What to show
Draw a Newman projection	state the bond you look along; front carbon is the point, rear carbon the circle	each group on its own carbon; the dihedral angle
Rank conformers	staggered or eclipsed; then how close the large groups are	anti < gauche < eclipsed
Cyclohexane ring flip	swap axial and equatorial	up stays up; large groups prefer equatorial
Assign E/Z	rank the two groups on each alkene carbon by CIP	winners together: Z; opposite: E
Assign R/S	rank all four groups; point group 4 away	1 → 2 → 3 clockwise: R; anticlockwise: S
Optical purity	[α]_obs/[α]ₚᵤᵣₑ × 100%	the excess; major = (100 + e)/2 %
Cis/trans complex	identify the shape; compare one pair of identical ligands	90°: cis; 180°: trans
Chiral complex?	draw the mirror image, keeping each bidentate ligand intact	can it be superimposed by rotation?

Core knowledge to remember

Conformers interconvert by rotation about single bonds; configurations change only if bonds are broken.
In a Newman projection, staggered bonds are 60° apart and eclipsed bonds 0°; anti is a dihedral angle of 180° and gauche 60°.
A cyclohexane ring flip swaps axial and equatorial positions but keeps up and down.
E/Z and R/S use Cahn–Ingold–Prelog priorities: higher atomic number at the first point of difference wins.
Enantiomers are non-superimposable mirror images; diastereomers are stereoisomers that are not mirror images. A meso compound has stereocentres but is achiral.
R and S do not predict the sign of optical rotation. A racemic mixture has no optical rotation.
Square-planar [Pt(NH₃)₂Cl₂] and octahedral [Co(NH₃)₄(H₂O)₂]²⁺ have cis/trans isomers; [Ni(en)₃]²⁺ and cis-[Ni(en)₂(H₂O)₂]²⁺ have enantiomers.

Common mistakes

Moving a group from one carbon to the other when redrawing a projection.
Saying a ring flip moves a substituent to the other face of the ring.
Ranking groups by size or chain length instead of CIP priority.
Reading R/S without first pointing the lowest-priority group away.
Assuming that R means (+) rotation.
Taking the optical purity as the percentage of the major enantiomer.
Calling a complex chiral without testing its whole mirror image, with the bidentate ligands kept intact.