AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Students should be able to: draw the structural formulas and displayed…

    Full specification wording

    Optical isomerism (A-level only): 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…

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 4 of 4
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    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.

    What earns the marks

    • A displayed formula showing every bond and atom around the chiral carbon.
    • A pair of structures drawn as mirror images using a wedge and a dashed bond.
    • Identifying the planar intermediate, such as a carbocation or a carbonyl carbon.
    • Saying attack is equally likely from above or below, giving equal amounts of each enantiomer.
    • Saying the equal and opposite rotations cancel, so the racemate is optically inactive.

    Common mistakes and how to put them right

    • 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.

    Exam tips

    • 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.

    Try the lesson

    The start of the Optical isomerism (A-level only): 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. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which statement describes the relationship between the two enantiomers of butan-2-ol, CH₃CH(OH)CH₂CH₃?

    Listen

    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. A strong answer earns marks for these points. a displayed formula showing every bond and atom around the chiral carbon. a pair of structures drawn as mirror images using a wedge and a dashed bond. identifying the planar intermediate, such as a carbocation or a carbonyl carbon. saying attack is equally likely from above or below, giving equal amounts of each enantiomer. Exam tip: Say planar and then say attack from either side; the two statements are usually separate marks.

    See it

    A top-down flowchart. At the top is a node labelled 'Flat intermediate: trigonal planar carbonyl carbon or planar carbocation'. Two arrows lead from it: one to 'Reagent can approach from above' and one to 'Reagent can approach from below'. The above node leads to 'One enantiomer formed' and the below node leads to 'Other enantiomer formed'. Both of these lead into a single node, 'Equal amounts of each enantiomer', which leads to 'Racemic mixture (racemate)', then to 'Equal and opposite rotations cancel', and finally to 'Optically inactive'.
    It shows how a planar intermediate gives equal amounts of two enantiomers, whose equal and opposite rotations cancel to make the racemate optically inactive.

    Key words

    Say what each word means before you turn the card.

    displayed formula

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    planar intermediate

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    attack from either side

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    why racemates are inactive

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    See every definition
    displayed formula
    A formula that shows every atom and every bond, and for enantiomers also shows the arrangement in space with two bonds in the plane, one on a wedge and one dashed.
    planar intermediate
    A flat species such as a trigonal planar carbonyl carbon under attack or a planar positive carbon in a carbocation.
    attack from either side
    A reagent can approach a planar species from above or from below with equal probability, so the two enantiomers are produced in equal amounts.
    why racemates are inactive
    The two enantiomers are produced in equal amounts, and their equal and opposite rotations then cancel.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. At the top is a node labelled 'Flat intermediate: trigonal planar carbonyl carbon or planar carbocation'. Two arrows lead from it: one to 'Reagent can approach from above' and one to 'Reagent can approach from below'. The above node leads to 'One enantiomer formed' and the below node leads to 'Other enantiomer formed'. Both of these lead into a single node, 'Equal amounts of each enantiomer', which leads to 'Racemic mixture (racemate)', then to 'Equal and opposite rotations cancel', and finally to 'Optically inactive'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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    Test your understanding of Optical isomerism (A-level only): 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.

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