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SolutionsStereochemistry of complex molecules

  1. Completing projections

    Complete the following formulas:

    Solution: the same projections completed

  2. Reduction of a diketone

    The ketone groups of 5-nitrononane-2,8-dione (figure) can be reduced to the alcohol (OH) function one at a time, giving a set of isomers of 8-hydroxy-5-nitrononan-2-one on the one hand and of 5-nitrononane-2,8-diol on the other. Write them all down and say how they are related to each other (enantiomers, diastereomers or meso forms).

    Solution: possible isomers of 8-hydroxy-5-nitrononan-2-one and of 5-nitrononane-2,8-diol

    Compounds 1 (8-hydroxy-5-nitrononan-2-one)

    There cannot be meso forms, because no plane of symmetry divides the molecule into two equal halves.

    • 1a: (5R, 8S)
    • 1b: (5R, 8R)
    • 1c: (5S, 8S)
    • 1d: (5S, 8R)
    • 1e: (5S, 8R)
    • 1f: (5S, 8S)
    • 1g: (5R, 8R)
    • 1h: (5R, 8S)

    Therefore 1a and 1h are identical, as are 1b and 1g, 1c and 1f, and 1d and 1e. There are only 4 isomers, as expected from the 2n rule with n = 2 (stereocentres at positions 5 and 8).

    • 1a = 1h is the enantiomer of 1d = 1e
    • 1b = 1g is the enantiomer of 1c = 1f
    • All other relationships are diastereomeric.

    Compounds 2 (5-nitrononane-2,8-diol)

    Here there can be meso forms, because in some cases a plane of symmetry divides the molecule into two equal halves.

    • 2a = 2d = meso (superimposable object and image)
    • 2b = 2c = meso (superimposable object and image)
    • 2e = 2g (the mirror image of 2e is 2f, non-superimposable)
    • 2f = 2h (the mirror image of 2f is 2e, non-superimposable)

    Although there are three “apparent” stereocentres at positions 2, 5 and 8, only 4 stereoisomers of this structure exist. 2e = 2g is the enantiomer of 2f = 2h; all other relationships are diastereomeric.