The shape of the lesson
By the end of the lesson, learners can
- Spot the chiral centre: Pick a molecule with four different groups on one carbon in the simulator, look at its mirror image, then try to rotate the mirror image so it fits the original.
- More centres, more isomers: Large molecules often have several chiral centres.
- Chiral centre: C with four different groups.
- Enantiomers: non-superimposable mirror images.
- Racemic: 50 : 50, no net rotation.
- n centres: up to 2n stereoisomers.
One picture every idea returns to
A chiral centre is a carbon atom bonded to four different groups. A molecule with one chiral centre exists as two enantiomers: non-superimposable mirror images of each other. Enantiomers have identical physical properties and identical reactions with non-chiral reagents, but they rotate plane-polarised light by equal amounts in opposite directions. A racemic mixture contains equal amounts of the two and has no net effect on plane-polarised light. Living systems are chiral, so enantiomers can act very differently as drugs: one enantiomer of thalidomide is a sedative, the other caused birth defects. A molecule with n chiral centres has at most 2n stereoisomers.
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, Mirror images that do not match.. Your hands are mirror images, yet a left glove will not fit a right hand. 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: Spot the chiral centre | Pick a molecule with four different groups on one carbon in the simulator, look at its mirror image, then try to rotate the mirror image so it fits the original. Project tab 4 of the lab (Organic chemistry (A Level introduction) lab, chirality in 3D) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: More centres, more isomers | Large molecules often have several chiral centres. Project tab 4 of the lab (Organic chemistry (A Level introduction) lab, chirality in 3D) and let a student drive it while the class predicts. | Frayer Model |
| 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, 15 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 More centres, more 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.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
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 |
|---|---|
| A chiral centre needs four different groups, not four different atoms: in butan-2-ol the CH 3 and C 2 H 5 both join through carbon but are different groups. | A carbon in a C=C or C=O is never a chiral centre. Enantiomers have the same boiling point, density and reactions with non-chiral reagents; only the direction of rotation of plane-polarised light (and their behaviour with other chiral molecules) differs. 2 n is a maximum: symmetrical molecules can have fewer. |
Optical isomerism: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Define the terms (a) chiral centre, (b) enantiomers, (c) racemic mixture.
Draw three-dimensional diagrams of the two enantiomers of butan-2-ol.
Open-chain ribose is HOCH2CH(OH)CH(OH)CH(OH)CHO. Identify the chiral centres and state the maximum number of stereoisomers.
Explain how a polarimeter could distinguish a pure enantiomer from a racemic mixture of the same compound.
Thalidomide was sold in the late 1950s as a racemic mixture. Explain why this caused problems and why drug companies now aim to make single enantiomers.
Explain why propan-2-ol and aminoethanoic acid are not optically active but 2-aminopropanoic acid is.
Answer key and marking notes
- (a) A carbon atom bonded to four different atoms or groups.
- (b) A pair of stereoisomers that are non-superimposable mirror images.
- (c) An equimolar mixture of two enantiomers, which has no effect on plane-polarised light.
- Central C with H, OH, CH3 and C2H5: two bonds in the plane, one wedge, one dashed bond.
- The second diagram is the exact mirror image of the first, drawn either side of a mirror line.
- The CH2OH carbon has two H atoms and the CHO carbon has a C=O, so neither is chiral.
- The three CH(OH) carbons each carry four different groups: 3 chiral centres.
- Maximum stereoisomers = 23 = 8.
- The pure enantiomer rotates the plane of plane-polarised light by a measurable angle.
- In the racemic mixture equal and opposite rotations cancel, so the observed rotation is zero.
- One enantiomer had the desired sedative effect; the other caused birth defects.
- Receptors in the body are chiral, so the two enantiomers bind differently.
- A single enantiomer gives a lower dose and fewer side effects, though it needs chiral synthesis or separation.
- Propan-2-ol has two CH3 groups on C2, and aminoethanoic acid has two H atoms on its central carbon, so neither has four different groups.
- In 2-aminopropanoic acid, C2 carries H, NH2, CH3 and COOH: a chiral centre, so it has optically active enantiomers.
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 is a chiral centre?
- Aa carbon atom bonded to four different atoms or groups
- Ba carbon atom with a C=C double bond
- Ca carbon atom bonded to two identical groups
- Dany carbon atom with an OH group
Which compound shows optical isomerism?
- A2-chlorobutane, CH3CHClCH2CH3
- B1-chlorobutane, CH3CH2CH2CH2Cl
- C2-chloropropane, CH3CHClCH3
- D2-chloro-2-methylpropane, (CH3)3CCl
How do the two enantiomers of butan-2-ol differ?
- Athey rotate plane-polarised light by equal angles in opposite directions
- Bthey have different boiling points
- Cthey have different structural formulae
- Donly one of them reacts with sodium
What is a racemic mixture?
- Aan equimolar mixture of two enantiomers, with no net effect on plane-polarised light
- Ba mixture of cis and trans isomers
- Ca single enantiomer that rotates light strongly
- Da mixture of structural isomers with the same boiling point
Thalidomide was given to pregnant women in the late 1950s as a racemic mixture. What does this case show about chiral drugs?
- Athe two enantiomers of a drug can have very different effects in the body
- Benantiomers always have identical biological effects
- Cracemic mixtures are optically active
- Donly achiral molecules can be used as drugs
The diagram shows the amino acid isoleucine. How many chiral centres does it have, and what is the maximum number of stereoisomers?
- A2 chiral centres, 4 stereoisomers
- B1 chiral centre, 2 stereoisomers
- C3 chiral centres, 8 stereoisomers
- D2 chiral centres, 2 stereoisomers
Optically active 3-chloro-3-methylhexane is hydrolysed by an SN1 mechanism. The product alcohol shows no optical activity. Why?
- Athe planar carbocation intermediate is attacked equally from both faces, giving a racemic mixture
- Bthe product alcohol has no chiral centre
- CSN1 inverts every molecule to the other enantiomer
- DOH− attacks only from the side opposite the leaving group
Which molecule has exactly two chiral centres?
- A3-chlorobutan-2-ol, CH3CHClCH(OH)CH3
- Bbutan-2-ol, CH3CH(OH)CH2CH3
- Cpentan-3-ol, CH3CH2CH(OH)CH2CH3
- D2,3-dimethylbutane, (CH3)2CHCH(CH3)2
Quiz key
A carbon with four different groups has a mirror image that cannot be superimposed on it, so the molecule exists as two enantiomers. A C=C carbon has only three groups and is planar, which gives cis/trans isomerism, not optical isomerism. Two identical groups make the molecule superimposable on its mirror image. An OH group alone is not enough: propan-2-ol has an OH but its C2 carries two CH3 groups.
C2 of 2-chlorobutane carries H, Cl, CH3 and C2H5: four different groups, so it is chiral. In 1-chlorobutane every carbon has at least two H atoms. In 2-chloropropane C2 has two CH3 groups. In 2-chloro-2-methylpropane the central carbon has three CH3 groups.
Enantiomers are mirror images, so their physical properties are identical except for the direction in which they rotate plane-polarised light. Boiling points are the same because the intermolecular forces are identical. They share one structural formula, CH3CH(OH)CH2CH3. Both have an OH group and react identically with non-chiral reagents such as sodium.
A racemic mixture contains 50% of each enantiomer; their equal and opposite rotations cancel, so the mixture is optically inactive. Cis and trans isomers are geometric (E/Z) isomers, not enantiomers. A single enantiomer is optically pure, the opposite of racemic. Structural isomers differ in their bonding and are not involved.
One enantiomer of thalidomide was an effective sedative, while the other caused serious birth defects, because chiral receptors interact differently with each mirror image. Enantiomers only behave identically towards non-chiral species, and the body is chiral. A racemic mixture is optically inactive because the rotations cancel. Many safe drugs are chiral; the lesson is to make and test single enantiomers, not to avoid chirality.
C2 carries H, NH2, COOH and the rest of the chain; C3 carries H, CH3, C2H5 and CH(NH2)COOH. Both are chiral, so n = 2 and there are up to 22 = 4 stereoisomers. One centre misses C3, whose methyl and ethyl groups are different. C1 has a C=O, and C4 and C5 carry two or three H atoms, so there is no third centre. Two centres give 2 × 2 = 4 combinations, not 2.
In SN1 the C–Cl bond breaks first to give a trigonal planar carbocation; water or OH− can attack either face with equal probability, so equal amounts of both enantiomers form: a racemic mixture. The product, 3-methylhexan-3-ol, still has a chiral C3 (OH, CH3, C2H5, C3H7). Complete inversion is the result of SN2, and the product would then be optically active. Attack only from the opposite side also describes SN2, not SN1.
In 3-chlorobutan-2-ol, C2 carries H, OH, CH3 and CHClCH3, and C3 carries H, Cl, CH3 and CH(OH)CH3: two chiral centres. Butan-2-ol has only one (C2). In pentan-3-ol, C3 carries two identical ethyl groups, so it has none. In 2,3-dimethylbutane each CH carries two CH3 groups, so it has none.
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.