to pck24:
stereochemistry is assumed by fisher projections that substituents drawn to the top and bottom project back into the plane of the screen or paper and those at east and west positions project out from the plane.
to 3dObject: read the message to pck24 first. Next, when H is projecting into the plane of the paper and follwing the substituents in order of priority lead in a clockwise or rightward motion then it is R. rightward = R
If, with the H projecting into the plane of paper, the priority goes counterclockwise you have S.
Now, if the H is projecting outwards from the paper and priority leads you rightward or clockwise you have S. So depending on which way the hydrogen is projecting, clockwise can mean different absolute configuration.
Hope this helps,
bw800402
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