How can two molecules with the same structural formula differ in their arrangement in space?
E/Z (geometric) isomerism from restricted rotation about a C=C bond, optical isomerism from a chiral centre, the meaning of enantiomers and optical activity, and the priority rules for naming.
An Eduqas A-Level Chemistry OA1.1 answer on E/Z (geometric) isomerism, optical isomerism and chirality, enantiomers and optical activity, and the priority rules for naming stereoisomers.
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What this topic is asking
Eduqas topic OA1.1 covers stereoisomerism: E/Z (geometric) isomerism arising from restricted rotation about a double bond, optical isomerism arising from a chiral centre, the meaning of enantiomers and optical activity, and the priority rules used to assign E and Z. It builds on the categories introduced in the AS topic C3.1.
E/Z (geometric) isomerism
Priorities are assigned by the Cahn-Ingold-Prelog rules: the atom of higher atomic number bonded directly to the double-bond carbon takes priority. The older cis/trans labels correspond to Z and E in simple cases.
Optical isomerism and chirality
Identifying isomers
Why stereoisomerism matters
Because enantiomers have identical bonds, they react identically with ordinary (non-chiral) reagents and have the same melting point, density and spectra. They differ only in their interaction with plane-polarised light and with other chiral molecules, which is biologically crucial: living systems are built from single enantiomers, so the two forms of a drug can behave very differently.
Examples in context
Example 1. Thalidomide. One enantiomer of thalidomide is an effective sedative while the other causes birth defects; the tragedy made single-enantiomer drug manufacture essential, a direct consequence of optical isomerism.
Example 2. Smell and taste. The two enantiomers of carvone smell completely different (spearmint and caraway) because our chiral smell receptors interact differently with each, showing how enantiomers differ only in a chiral environment.
Try this
Q1. State the two conditions required for a molecule to show E/Z isomerism. [2 marks]
- Cue. There must be restricted rotation (a double bond), and each carbon of the double bond must carry two different groups.
Q2. Explain why a pair of enantiomers cannot be separated by fractional distillation. [1 mark]
- Cue. Enantiomers have identical physical properties (including boiling point), so distillation cannot distinguish them.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marksBut-2-ene shows E/Z isomerism. (a) Explain why E/Z isomerism arises in this molecule. (b) Draw and label the E and Z isomers.Show worked answer →
(a) E/Z isomerism arises because there is restricted rotation about the double bond, and each doubly bonded carbon carries two different groups (a methyl group and a hydrogen) (2).
(b) The Z isomer (cis) has the two methyl groups on the same side of the double bond; the E isomer (trans) has them on opposite sides (2). (Award marks for correct displayed structures labelled E and Z.)
Eduqas 20214 marks(a) Explain what is meant by a chiral centre and optical isomers. (b) State how the two optical isomers of a compound differ in their effect on plane-polarised light, and why they are otherwise hard to distinguish.Show worked answer →
(a) A chiral centre is a carbon atom bonded to four different atoms or groups (1). Optical isomers (enantiomers) are non-superimposable mirror images of each other (1).
(b) The two enantiomers rotate the plane of plane-polarised light by equal amounts in opposite directions (one clockwise, one anticlockwise) (1). They have otherwise identical physical and chemical properties, so they cannot be told apart by melting point or ordinary reactions (1).
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Sources & how we know this
- WJEC Eduqas GCE A Level Chemistry specification (from 2015) — WJEC Eduqas (2015)