AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Isomerism: Stereoisomerism.
Stereoisomers have the same structural formula, the same atoms joined to the same atoms, but a different arrangement in space.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Stereoisomers have the same structural formula, the same atoms joined to the same atoms, but a different arrangement in space. Two kinds appear on this course. E-Z isomerism arises where rotation is restricted, most often about a C=C double bond, so two groups fixed on the same side differ from two fixed on opposite sides. Optical isomerism arises where a carbon atom carries four different groups, making the molecule and its mirror image non-superimposable. The distinction from structural isomerism is worth being precise about: in structural isomers the connectivity changes, in stereoisomers only the geometry does. Square planar and octahedral transition metal complexes show the same phenomenon as cis-trans isomerism.
What earns the marks
- Defining stereoisomers as having the same structural formula but a different arrangement of atoms in space.
- Naming the sub-type, E-Z or optical, that the example shows.
- Identifying the structural feature responsible, a restricted C=C or a carbon with four different groups.
- A drawing that makes the spatial difference unambiguous.
Common mistakes and how to put them right
Defining stereoisomers as having the same molecular formula only, which also describes structural isomers.
Claiming any alkene shows E-Z isomerism, when two identical groups on one carbon prevent it.
Drawing two supposed stereoisomers that are in fact the same molecule rotated.
Identifying a chiral centre on a carbon atom that carries two identical groups.
Exam tips
- Say same structural formula in the definition, because same molecular formula is not enough.
- Check that each doubly bonded carbon carries two different groups before claiming E-Z isomerism.
- Use clear wedge and dashed bonds when drawing optical isomers.
Try the lesson
The start of the Isomerism: Stereoisomerism. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes stereoisomers?
Listen
Stereoisomers have the same structural formula, the same atoms joined to the same atoms, but a different arrangement in space. Two kinds appear on this course. E-Z isomerism arises where rotation is restricted, most often about a C=C double bond, so two groups fixed on the same side differ from two fixed on opposite sides. Optical isomerism arises where a carbon atom carries four different groups, making the molecule and its mirror image non-superimposable. The distinction from structural isomerism is worth being precise about: in structural isomers the connectivity changes, in stereoisomers only the geometry does. Square planar and octahedral transition metal complexes show the same phenomenon as cis-trans isomerism. A strong answer earns marks for these points. defining stereoisomers as having the same structural formula but a different arrangement of atoms in space. naming the sub-type, E-Z or optical, that the example shows. identifying the structural feature responsible, a restricted C=C or a carbon with four different groups. a drawing that makes the spatial difference unambiguous. Exam tip: Say same structural formula in the definition, because same molecular formula is not enough.
Key words
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stereoisomers
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E-Z isomerism
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optical isomerism
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structural isomers
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See every definition
- stereoisomers
- Molecules with the same structural formula, the same atoms joined to the same atoms, but a different arrangement in space.
- E-Z isomerism
- Arises where rotation is restricted, most often about a C=C double bond, so two groups fixed on the same side differ from two fixed on opposite sides.
- optical isomerism
- Arises where a carbon atom carries four different groups, making the molecule and its mirror image non-superimposable.
- structural isomers
- Isomers in which the connectivity changes, unlike stereoisomers where only the geometry does.
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