AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Optical isomerism is a form of stereoisomerism and occurs as a result of…

    Full specification wording

    Optical isomerism (A-level only): Optical isomerism is a form of stereoisomerism and occurs as a result of chirality in molecules, limited to molecules with a single chiral centre.

    Stereoisomers have the same structural formula, so the same atoms joined to the same neighbours, but a different arrangement of those atoms in space.

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

    • Point 1 of 4
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Stereoisomers have the same structural formula, so the same atoms joined to the same neighbours, but a different arrangement of those atoms in space. 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.

    What earns the marks

    • Defining stereoisomers as having the same structural formula but a different arrangement of atoms in space.
    • Identifying a chiral centre as a carbon atom bonded to four different groups.
    • Stating that the two forms are non-superimposable mirror images.
    • Correctly marking the chiral carbon in a given structure.

    Common mistakes and how to put them right

    • 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.

    Exam 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.

    Try the lesson

    The start of the Optical isomerism (A-level only): Optical isomerism is a form of stereoisomerism and occurs as a result of chirality in molecules, limited to molecules with a single chiral centre. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which feature of a molecule is responsible for optical isomerism?

    Listen

    Stereoisomers have the same structural formula, so the same atoms joined to the same neighbours, but a different arrangement of those atoms in space. 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. A strong answer earns marks for these points. defining stereoisomers as having the same structural formula but a different arrangement of atoms in space. identifying a chiral centre as a carbon atom bonded to four different groups. stating that the two forms are non-superimposable mirror images. correctly marking the chiral carbon in a given structure. Exam tip: Say "same structural formula" in the definition; "same molecular formula" alone does not score.

    See it

    A top-down flowchart. The first box states that stereoisomers have the same structural formula but a different arrangement of atoms in space. An arrow leads to a box saying optical isomerism arises from chirality. An arrow leads to a box defining a chiral centre as a carbon atom bonded to four different groups. An arrow leads to a box saying to check all four groups on a carbon, including groups that differ only further along the chain. An arrow leads to a box stating there are two optical isomers. A final arrow leads to a box stating they are non-superimposable mirror images.
    It shows how stereoisomerism leads to optical isomerism through a single chiral centre, giving two non-superimposable mirror images.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Chiral centre: carbon bonded to four different groups
    2. 2. Optical isomerism arises from chirality
    3. 3. Stereoisomers: same structural formula, different arrangement in space
    4. 4. Non-superimposable mirror images
    5. 5. Two optical isomers
    6. 6. Check all four groups on a carbon, including groups differing only further along the chain

    Key words

    Say what each word means before you turn the card.

    stereoisomerism

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    chirality

    Try answering aloud or in your head before turning the card.

    four different groups

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    non-superimposable mirror images

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    See every definition
    stereoisomerism
    A form in which molecules have the same structural formula, so the same atoms joined to the same neighbours, but a different arrangement of those atoms in space.
    chirality
    The property of a molecule whose mirror image cannot be placed on top of the original however it is turned.
    four different groups
    What a carbon must be bonded to for a molecule to be chiral, making that carbon an asymmetric or chiral centre.
    non-superimposable mirror images
    The relationship between the two forms of a molecule with a single chiral centre, in the same way that a left hand will not fit into a right glove.

    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. The first box states that stereoisomers have the same structural formula but a different arrangement of atoms in space. An arrow leads to a box saying optical isomerism arises from chirality. An arrow leads to a box defining a chiral centre as a carbon atom bonded to four different groups. An arrow leads to a box saying to check all four groups on a carbon, including groups that differ only further along the chain. An arrow leads to a box stating there are two optical isomers. A final arrow leads to a box stating they are non-superimposable mirror images.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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