The shape of the lesson
By the end of the lesson, learners can
- Cis and trans: Rotate the molecules in the 3D stereoisomerism simulator.
- Optical isomers: Look for carbon atoms with four different groups attached.
- Cis-trans: C=C with two different groups on each carbon.
- Chiral centre: C with four different groups.
- Enantiomers: non-superimposable mirror images.
- Racemic: 50 : 50, optically inactive.
One picture every idea returns to
Stereoisomers have the same structural formula but a different arrangement of atoms in space. Cis-trans isomerism arises because rotation about a C=C bond is restricted; it needs two different groups on each carbon of the double bond. Optical isomerism arises when a carbon atom carries four different groups (a chiral centre); the two forms, enantiomers, are non-superimposable mirror images that rotate plane-polarised light equally in opposite directions. A 50 : 50 mixture is a racemic mixture and has no overall effect on the light.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Same connections, different space.. Two molecules can have every atom joined in exactly the same order and still be different, because their groups point in different directions. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Cis and trans | Rotate the molecules in the 3D stereoisomerism simulator. Project tab 4 of the lab (Organic chemistry lab, stereoisomerism) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Optical isomers | Look for carbon atoms with four different groups attached. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 12 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Optical isomers, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Cis-trans isomerism needs two different groups on each carbon of the C=C, so but-1-ene does not show it. | A chiral centre needs four different groups; a carbon with two identical groups, such as the central carbon in propan-2-ol, is not chiral. |
Cis-trans and optical isomerism: worksheet
6 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced.
Define stereoisomers and state the two types.
Explain why 1,2-dichloroethene has cis and trans isomers but 1,1-dichloroethene does not.
Draw the cis and trans isomers of pent-2-ene.
Identify the chiral centre in 2-hydroxypropanoic acid and list the four groups attached to it.
State how a pair of enantiomers can be distinguished, and what is meant by a racemic mixture.
State which of these have optical isomers: (a) 2-bromobutane, (b) 2-bromopropane, (c) butan-2-ol.
Answer key and marking notes
- Compounds with the same structural formula but a different arrangement of atoms in space.
- Cis-trans (geometric) isomers and optical isomers.
- Rotation about C=C is restricted; in CHCl=CHCl each carbon has two different groups (H and Cl), so they can be on the same or opposite sides.
- In CH2=CCl2 each carbon has two identical groups, so swapping them gives the same molecule.
- Cis: CH3 and C2H5 on the same side of the C=C, both H on the other side.
- Trans: CH3 and C2H5 on opposite sides.
- The central carbon (C2).
- H, OH, CH3 and COOH.
- They rotate plane-polarised light by equal amounts in opposite directions.
- A racemic mixture contains equal amounts of the two enantiomers and has no net effect on plane-polarised light.
- (a) yes: C2 has H, Br, CH3, C2H5; (c) yes: H, OH, CH3, C2H5.
- (b) no: C2 carries two CH3 groups.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What are stereoisomers?
- Acompounds with the same structural formula but a different arrangement of atoms in space
- Bcompounds with the same molecular formula but a different structural formula
- Ccompounds with the same functional group in different positions on the chain
- Ddifferent conformations of one molecule produced by rotation about a single bond
The diagram shows two isomers of but-2-ene. Which statement is correct?
- AA is cis-but-2-ene and B is trans-but-2-ene; they cannot interconvert because rotation about C=C is restricted
- BA is trans-but-2-ene and B is cis-but-2-ene
- CA and B are the same molecule, because one can be turned into the other by rotating about the C=C bond
- DA and B are structural isomers, because their atoms are joined in a different order
Which compound shows cis-trans isomerism?
- A1,2-dichloroethene, CHCl=CHCl
- Bbut-1-ene, CH2=CHCH2CH3
- C1,1-dichloroethene, CH2=CCl2
- Dpropene, CH2=CHCH3
What is a chiral centre?
- Aa carbon atom bonded to four different atoms or groups
- Ba carbon atom bonded to two identical groups and two different groups
- Ca carbon atom that forms a C=C double bond
- Dany carbon atom bonded to four other atoms
Which statement about a pair of enantiomers is correct?
- Athey rotate plane-polarised light by the same angle in opposite directions, so a racemic (50 : 50) mixture shows no overall rotation
- Bthey rotate plane-polarised light in the same direction by different angles
- Cthey have different boiling points and different molecular formulae
- Da racemic mixture rotates plane-polarised light twice as much as one enantiomer
The diagram shows two forms of 2-hydroxypropanoic acid (lactic acid). Which statement is correct?
- Athe starred carbon is bonded to H, OH, CH3 and COOH, so the two forms are non-superimposable mirror images (enantiomers)
- Bthe two forms are mirror images that can be superimposed, so they are the same molecule
- Cthe two forms are cis-trans isomers, because the OH group is on opposite sides
- Dthe two forms are structural isomers, because the OH is joined to a different atom
Which compound has a chiral centre?
- A2-bromobutane, CH3CHBrCH2CH3
- Bpropan-2-ol, CH3CH(OH)CH3
- C1-bromobutane, CH2BrCH2CH2CH3
- D2-bromopropane, CH3CHBrCH3
How many chiral centres are there in 3-methylpentan-2-ol, CH3CH(OH)CH(CH3)CH2CH3?
- A2
- B1
- C3
- D0
Quiz key
Stereoisomers have the same atoms joined in the same order (same structural formula), but the atoms are arranged differently in space. The two kinds are cis-trans (geometric) and optical isomers. Same molecular formula with a different structural formula is the definition of structural isomers. A functional group in a different position is positional isomerism, a type of structural isomerism. Rotation about a C-C single bond only gives different conformations of the same molecule, which interconvert freely; stereoisomers cannot interconvert without breaking bonds.
In A both CH3 groups are on the same side of the C=C, so A is cis; in B they are on opposite sides, so B is trans. Turning one carbon would break the π bond, so rotation about C=C is restricted and the two cannot interconvert at room temperature: they are distinct cis-trans isomers. Swapping the labels reverses the definition (trans means across). Free rotation applies to single bonds, not C=C. In both isomers each carbon is bonded to the same atoms in the same order, so the structural formula is the same; the difference is only in space, making them stereoisomers, not structural isomers.
In CHCl=CHCl each alkene carbon has an H and a Cl, so the two Cl atoms can be on the same side (cis) or opposite sides (trans). But-1-ene is the classic trap: it has a C=C, but C1 carries two H atoms, so swapping them gives the same molecule. 1,1-dichloroethene has two Cl on one carbon and two H on the other, so neither carbon has two different groups. Propene also has a CH2= end with two identical H atoms, so it has no cis-trans isomers.
A chiral centre is a carbon with four different groups attached. Its mirror image cannot be superimposed on it, giving a pair of optical isomers (enantiomers). If two groups are identical, the molecule has a plane of symmetry and its mirror image can be superimposed, so it is not chiral; this is the classic error. A C=C carbon has only three groups and is planar, so it cannot be a chiral centre; C=C is linked to cis-trans isomerism instead. Four bonds alone are not enough: in methane or the CH3 of ethane the groups are the same.
Enantiomers are non-superimposable mirror images with identical physical properties, except that they rotate the plane of plane-polarised light by equal amounts in opposite directions. In a racemic mixture (equal amounts of each) the rotations cancel, so there is no net rotation. Rotation in the same direction would not distinguish mirror images. Enantiomers have the same molecular and structural formula and the same boiling point. The racemic mixture shows zero rotation, not double, because the two effects cancel rather than add.
The starred carbon carries four different groups, H, OH, CH3 and COOH, so it is a chiral centre. The two forms are mirror images (see the mirror line) and no rotation will place every group of one onto the other, so they are enantiomers, a type of optical isomer. They could be superimposed only if two groups on the carbon were identical. Cis-trans isomerism requires restricted rotation about a C=C; here the carbon has only single bonds. In both forms the OH is bonded to the same carbon, so the structural formula is unchanged; only the 3D arrangement differs.
In 2-bromobutane, C2 carries H, Br, CH3 and C2H5: four different groups, so it is chiral and exists as two enantiomers (butan-2-ol is chiral for the same reason). In propan-2-ol, C2 carries two identical CH3 groups, so it is not chiral; this is the classic trap. In 1-bromobutane, C1 carries two H atoms, and no other carbon has four different groups. 2-bromopropane, like propan-2-ol, has two CH3 groups on the central carbon.
C2 is bonded to H, OH, CH3 and CH(CH3)CH2CH3: four different groups, chiral. C3 is bonded to H, CH3, C2H5 and CH(OH)CH3: also four different groups, chiral. Every other carbon is a CH3 or CH2 with at least two identical H atoms. So there are 2 chiral centres. 1 spots only the carbon carrying the OH and misses C3. 3 wrongly counts the CH2 carbon, which has two identical H atoms. 0 overlooks that a carbon with a branch can still carry four different groups.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.