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Stereoisomerism: Flashcards

Revise structural isomers and stereoisomers, cis–trans and E/Z isomerism, chiral centres and enantiomers.

14 cardsIntermediate

14 cards

  1. Question
    What is the difference between structural isomers and stereoisomers?
    Answer

    Structural: atoms joined in a different order. Stereoisomers: same order, different arrangement in space.

  2. Question
    Why can C=C double bonds give cis–trans isomers?
    Answer

    The π bond prevents rotation about the double bond.

  3. Question
    What condition is needed for cis–trans isomerism in an alkene?
    Answer

    Each carbon of the C=C must carry two different groups.

  4. Question
    Does but-1-ene show cis–trans isomerism?
    Answer

    No: one carbon of the double bond has two H atoms.

  5. Question
    What do Z and E mean?
    Answer

    Z: higher-priority groups on the same side. E: on opposite sides. Priority = higher atomic number.

  6. Question
    What is a chiral centre?
    Answer

    A carbon atom bonded to four different atoms or groups.

  7. Question
    What are enantiomers?
    Answer

    A pair of non-superimposable mirror-image molecules.

  8. Question
    How do enantiomers differ in their properties?
    Answer

    They rotate plane-polarized light in opposite directions and can interact differently with other chiral molecules (e.g. enzymes).

  9. Question
    What is a racemic mixture?
    Answer

    A 50 : 50 mixture of two enantiomers; it does not rotate plane-polarized light.

  10. Question
    Which carbon in butan-2-ol is chiral?
    Answer

    Carbon 2, bonded to H, OH, CH₃ and CH₂CH₃.

  11. Question
    State the three CIP priority rules.
    Answer

    1. Higher atomic number of the attached atom wins. 2. If tied, move outwards and compare at the first difference. 3. Multiple bonds count as duplicated atoms.

  12. Question
    How do you assign R or S to a chiral centre?
    Answer

    Rank groups 1–4; point group 4 away; trace 1 → 2 → 3. Clockwise = R, anticlockwise = S. If 4 points towards you, reverse the answer.

  13. Question
    Rank –COOH, –CHO, –CH₂OH and –CH₃.
    Answer

    –COOH (O,O,O) > –CHO (O,O,H) > –CH₂OH (O,H,H) > –CH₃ (H,H,H).

  14. Question
    Does R mean the compound rotates light clockwise (+)?
    Answer

    No. R/S comes from the 3D arrangement; + / − is measured with a polarimeter. They are not linked.