2-bromobutene can exist as cis and trans isomers. Describe the structural features required for cis-trans isomerism to occur in an alkene, and explain why these features prevent rotation that would interconvert the two isomers.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cis-trans isomers are a form of stereoisomerism found in certain alkenes. The two isomers of 2-bromobutene have identical molecular formulae and the same bonds, yet they are distinct compounds with different properties.
Model answer (5 marks)
A double bond between two carbon atoms is required – the C=C bond consists of a σ‑bond and a π‑bond, the latter preventing rotation.
Each of the two sp² carbons must bear two different substituents (here Br and H, and two different alkyl groups).
The π‑bond is formed by sideways overlap of p orbitals above and below the plane of the molecule; this overlap is lost if the molecule rotates.
Because rotation about the C=C bond is blocked, the relative positions of the substituents are fixed.
Thus the two possible arrangements – both Br on the same side (cis) or on opposite sides (trans) – are distinct, non‑superimposable stereoisomers.
Each of the two sp² carbons must bear two different substituents (here Br and H, and two different alkyl groups).
The π‑bond is formed by sideways overlap of p orbitals above and below the plane of the molecule; this overlap is lost if the molecule rotates.
Because rotation about the C=C bond is blocked, the relative positions of the substituents are fixed.
Thus the two possible arrangements – both Br on the same side (cis) or on opposite sides (trans) – are distinct, non‑superimposable stereoisomers.
Examiner tips
- State the double bond and the need for two different groups on each carbon.
- Explain that the π‑bond blocks rotation and why that locks the geometry.
- Show that cis/trans arise from the fixed relative positions of the groups.
- Use the exact terms ‘cis’ and ‘trans’ and mention the double bond’s σ and π bonds.
Common mistakes
- Claiming rotation is possible because the σ‑bond allows it.
- Forgetting that both carbons must have two different groups.
- Using vague language like ‘fixed’ without linking it to the π‑bond.”
Mark scheme (5 marks)
- The molecule must contain a C=C double bond (which restricts rotation)
- Each carbon of the double bond must have two different groups/atoms attached to it
- The C=C double bond consists of a sigma bond and a pi bond; it is the pi bond that prevents rotation
- The pi bond is formed by sideways overlap of p orbitals above and below the plane; rotating would break this overlap
- Because rotation cannot occur, the groups on either side of the double bond are locked in position, giving rise to two distinct (non-superimposable) arrangements — the cis and trans isomers
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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