Optical isomerism (A-level only) — AQA A-Level Chemistry
Test yourself on Optical isomerism (A-level only) with AQA A-Level practice questions.
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Optical isomerism (A-level only) explained
Stereoisomers have the same structural formula, so the same atoms joined to the same neighbours, but a different arrangement of those atoms in space.
Read the full explanation
Optical isomerism is the kind that arises from chirality. A molecule is chiral when it contains a carbon bonded to four different groups, called an asymmetric or chiral centre; its mirror image cannot then be placed on top of the original however it is turned, in the same way that a left hand will not fit into a right glove. At this level you meet molecules with a single chiral centre, which therefore have exactly two optical isomers. Finding the chiral centre means checking all four groups on a carbon, including groups that differ only some distance away.
An asymmetric carbon atom is chiral and gives rise to optical isomers (enantiomers), which exist as non super- imposable mirror images and differ in their effect on plane polarised light.
The two forms are called enantiomers. Draw them by putting two bonds in the plane of the paper, one on a wedge coming towards you and one on a dashed line going away, then draw the mirror image on the other side of a mirror line. However you rotate one, it will not superimpose on the other. Physically they are identical in every property that does not itself involve chirality, so they have the same melting point, the same solubility and the same reactions with ordinary reagents. Two things do differ: they rotate the plane of plane polarised light by equal amounts in opposite directions, one clockwise and one anticlockwise, and they behave differently in a chiral environment, which is why an enzyme or a receptor accepts one enantiomer and rejects the other.
A mixture of equal amounts of enantiomers is called a racemic mixture (racemate).
A racemic mixture, or racemate, contains the two enantiomers of a compound in equal amounts, a fifty-fifty mixture by moles. It is optically inactive: each enantiomer rotates the plane of polarised light by the same angle, but in opposite directions, so the two rotations cancel exactly and the net rotation measured in a polarimeter is zero. The molecules themselves are still chiral, and the mixture still contains optical isomers; it is only the bulk sample that shows no rotation. Racemates arise whenever a new chiral centre is made from a flat, achiral species that a reagent can approach equally from either side, which is why so many laboratory syntheses of chiral molecules give no optical activity at all.
Students should be able to: draw the structural formulas and displayed formulas of enantiomers understand how racemic mixtures (racemates) are formed and why they are optically inactive.
For the drawing, a displayed formula shows every atom and every bond; for enantiomers you must also show the arrangement in space, so put two bonds in the plane, one on a wedge and one dashed, and reflect the whole thing across a mirror line. For the formation, look for a flat intermediate. In nucleophilic addition to a carbonyl the carbon under attack is trigonal planar, and in a substitution proceeding through a carbocation the positive carbon is planar too. A reagent can approach such a species from above or from below with equal probability, so the two enantiomers are produced in equal amounts. That equal production is exactly what makes the product racemic, and the equal and opposite rotations then cancel.
Your focus
- Define stereoisomerism and state how optical isomerism differs from E-Z isomerism.
- Identify the chiral centre in a given structure by listing the four groups attached to a carbon.
- Deduce how many optical isomers a molecule with one chiral centre has, and justify the number.
Show all 12 objectives
- Draw a pair of enantiomers in three dimensions using wedge and dashed bonds either side of a mirror line.
- Sort properties such as melting point, solubility, reaction with an achiral reagent, rotation of plane polarised light and the action of an enzyme into those identical for two enantiomers and those that differ.
- Explain why an enzyme can act on one enantiomer of a substrate but not on the other.
- Decide from the stated proportions of two enantiomers in a sample whether it is a racemate and whether it would rotate plane polarised light.
- Explain why a racemic mixture does not rotate plane polarised light.
- Distinguish between a chiral molecule and an optically active sample using a racemate as the example.
- Draw the structural and displayed formulae of both enantiomers of a compound with one chiral centre.
- Explain how a planar intermediate leads to equal amounts of the two enantiomers.
- Explain why the resulting racemate shows no rotation of plane polarised light.
Optical isomerism (A-level only) exam tips
Marking Points
- one mark for defining stereoisomers as having the same structural formula but a different arrangement of atoms in space
- one mark for identifying a chiral centre as a carbon atom bonded to four different groups
- one mark for stating that the two forms are non-superimposable mirror images
- one mark for correctly marking the chiral carbon in a given structure
- one mark for drawing the two enantiomers as three-dimensional mirror images either side of a mirror line
- one mark for using a wedge and a dashed bond to show the arrangement in space
- one mark for saying the two structures cannot be superimposed
- one mark for saying they rotate plane polarised light by equal amounts in opposite directions
- one mark for stating equal amounts, or equal numbers of moles, of the two enantiomers
- one mark for saying the mixture shows no rotation of plane polarised light
- one mark for the reason, that the two rotations are equal in size and opposite in direction so they cancel
- one mark for saying a racemate arises when the two enantiomers are produced in equal amounts
- one mark for a displayed formula showing every bond and atom around the chiral carbon
- one mark for a pair of structures drawn as mirror images using a wedge and a dashed bond
- one mark for identifying the planar intermediate, such as a carbocation or a carbonyl carbon
- one mark for saying attack is equally likely from above or below, giving equal amounts of each enantiomer
- one mark for saying the equal and opposite rotations cancel, so the racemate is optically inactive
Examiner Tips
- 💡Say "same structural formula" in the definition; "same molecular formula" alone does not score.
- 💡Check each candidate carbon by listing all four attached groups in full before deciding.
- 💡Mark the chiral centre with a clear asterisk when asked to identify it.
- 💡Draw the mirror line first, then build the second structure group by group as a reflection.
- 💡Check your pair by mentally rotating one; if it lands on the other, redraw it.
- 💡Say "equal but opposite" about the rotation, since both halves of the phrase carry credit.
- 💡Give both halves of the explanation: equal amounts, and equal but opposite rotations.
- 💡Distinguish carefully between a molecule being chiral and a sample being optically active.
- 💡Use the word racemate or racemic mixture in your answer, since the term itself is examined.
- 💡Say planar and then say attack from either side; the two statements are usually separate marks.
- 💡Displayed formulae must show all bonds, so do not abbreviate a methyl group to CH₃ when displayed is asked for.
- 💡Finish the argument at the cancellation of rotations, not just at the equal mixture.
Common Mistakes
- defining stereoisomers as having the same molecular formula only, which also describes structural isomers
- counting a carbon with two identical groups, such as two methyl groups, as chiral
- missing a chiral centre because two of its four groups differ only further along the chain
- confusing optical isomerism with E-Z isomerism, which comes from restricted rotation about a C=C
- drawing the second structure as the first one rotated, so the two are actually the same molecule
- drawing flat structures with no wedge or dashed bond and calling them enantiomers
- saying one enantiomer rotates the plane of polarised light and the other does not
- assuming an enzyme acts equally well on both enantiomers of a substrate, which AQA sets as an incorrect statement
- saying a racemate contains no optical isomers, rather than equal amounts of both
- claiming the individual molecules are achiral
- saying the mixture is inactive because the two enantiomers react with each other
- reading equal amounts as equal volumes rather than equal numbers of moles
- drawing the second structure as a rotation of the first, so both are the same enantiomer
- omitting the wedge and dashed bonds, leaving no three-dimensional information
- describing the intermediate as tetrahedral when the argument depends on it being planar
- saying the reagent attacks from the less hindered side, which would give unequal amounts