AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
E–Z isomerism is a form of stereoisomerism and occurs as a result of…
Full specification wording
Isomerism: E–Z isomerism is a form of stereoisomerism and occurs as a result of restricted rotation about the planar carbon– carbon double bond.
A C=C double bond consists of a σ bond and a π bond, the π bond formed by sideways overlap of p orbitals above and below the axis.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A C=C double bond consists of a σ bond and a π bond, the π bond formed by sideways overlap of p orbitals above and below the axis. Rotating one end of the bond would break that overlap, so rotation is restricted and the attached groups stay where they are. The two carbon atoms and the four atoms attached to them all lie in one plane. That fixes two distinct arrangements whenever each doubly bonded carbon carries two different groups: but-2-ene exists as Z-but-2-ene, with the methyl groups on the same side, and E-but-2-ene, with them on opposite sides. A mark scheme for exactly this question rewards naming both isomers and then stating that there is no free rotation about the C=C.
What earns the marks
- Stating that there is restricted or no free rotation about the C=C double bond.
- Naming both isomers, such as Z-but-2-ene and E-but-2-ene.
- Stating that each carbon of the double bond must carry two different groups.
- Referring to the π bond or the planar arrangement as the reason rotation is restricted.
Common mistakes and how to put them right
Saying the double bond is too strong to rotate rather than that rotation would break the π overlap.
Claiming but-1-ene shows E-Z isomerism, when one of its double bond carbons carries two hydrogen atoms.
Swapping the labels so that groups on the same side are called E.
Describing the two isomers as structural isomers rather than stereoisomers.
Exam tips
- Write the phrase restricted rotation about the C=C, because that is the marking point.
- Check both ends of the double bond for two different groups before you answer.
- Give both isomer names when a question asks you to deduce names, not just one.
Try the lesson
The start of the Isomerism: E–Z isomerism is a form of stereoisomerism and occurs as a result of restricted rotation about the planar carbon– carbon double bond. lesson, made from checked teaching notes for this specification point.
Have a go first
E–Z isomerism is a form of stereoisomerism that arises because rotation about which bond is restricted?
Listen
A C=C double bond consists of a σ bond and a π bond, the π bond formed by sideways overlap of p orbitals above and below the axis. Rotating one end of the bond would break that overlap, so rotation is restricted and the attached groups stay where they are. The two carbon atoms and the four atoms attached to them all lie in one plane. That fixes two distinct arrangements whenever each doubly bonded carbon carries two different groups: but-2-ene exists as Z-but-2-ene, with the methyl groups on the same side, and E-but-2-ene, with them on opposite sides. A mark scheme for exactly this question rewards naming both isomers and then stating that there is no free rotation about the C=C. A strong answer earns marks for these points. stating that there is restricted or no free rotation about the C=C double bond. naming both isomers, such as Z-but-2-ene and E-but-2-ene. stating that each carbon of the double bond must carry two different groups. referring to the π bond or the planar arrangement as the reason rotation is restricted. Exam tip: Write the phrase restricted rotation about the C=C, because that is the marking point.
Key words
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restricted rotation
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sideways p orbital overlap
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planar double bond
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E and Z isomers
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See every definition
- restricted rotation
- Rotation about the planar carbon–carbon double bond is restricted because rotating one end would break the sideways overlap of p orbitals.
- sideways p orbital overlap
- The π bond is formed by sideways overlap of p orbitals above and below the axis.
- planar double bond
- The two carbon atoms and the four atoms attached to them all lie in one plane.
- E and Z isomers
- Two distinct arrangements occur whenever each doubly bonded carbon carries two different groups, such as Z-but-2-ene and E-but-2-ene.
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