Diastereomers, Meso Compounds and Fischer Projections · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

When mirror images are not enough

Science · Chemistry · ages 19-20
Name ______________________   Date ____________

  1. Two stereoisomers differ in melting point. What must they be?

    • Enantiomers
    • Identical samples
    • Diastereomers

  2. A molecule has 3 stereocentres. What is the maximum number of stereoisomers?

    • 6
    • 8
    • 3

  3. In a Fischer projection, vertical bonds go back and horizontal bonds come forward.

    Circle one:   True   False

  4. A compound has stereocentres but shows no optical rotation. What best explains it?

    • It must be a single enantiomer
    • It may be meso, with halves cancelling through a mirror plane
    • Stereocentres never affect light

  5. A student quotes 8 stereoisomers for a 3 centre molecule with a plane of symmetry. What is wrong?

    • The maximum is 6, never 8
    • Symmetry cuts the count, so the maximum overclaims it
    • Planes of symmetry always double the count

  6. While converting to a Fischer projection you swap two groups once. What happened?

    • Nothing changed at that centre
    • The whole molecule turned meso
    • That stereocentre inverted

  7. Two samples share boiling points and spectra but twist light oppositely. Can ordinary crystallisation separate a mix of them?

    • Yes, because opposite rotation means easy separation
    • No, they behave as enantiomers with identical scalar properties
    • Yes, boiling point alone separates them

  8. Four tartaric acid forms are drawn as Fischer crosses. How do you sort them?

    • Find the meso form by its mirror plane, then pair the true mirror images
    • All four must be identical
    • Melting points are useless for sorting them
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Answer key

For grown-ups. Fold this page away before handing over the rest.

When mirror images are not enough W1-mt_1r5vCBK9zr-s1

  1. Diastereomers · Only diastereomers differ in scalar properties.
  2. 8 · Raise 2 to the power n before checking for symmetry.
  3. True · That is how the cross encodes depth.
  4. It may be meso, with halves cancelling through a mirror plane · Internal mirroring cancels the rotation within one molecule.
  5. Symmetry cuts the count, so the maximum overclaims it · Check for duplicates before quoting the maximum.
  6. That stereocentre inverted · Track each group deliberately, since one swap flips the assignment.
  7. No, they behave as enantiomers with identical scalar properties · Only diastereomers differ enough for ordinary separation.
  8. Find the meso form by its mirror plane, then pair the true mirror images · Symmetry first, then mirror pairing, then separation by crystallisation.
Worksheet · LightMySky