Determine R and S configurations

[3dObject]

Based on what I know, I would have thought that molecule B has an R configuration also, but the solution states that only molecule A has an R configuration. Is this correct or a possible type? Thanks.

[Neecze]

I just hope you'll understand my english...

Only molecule A has R configuration. Molecule B has S configuration since priority of substituents is Br>F>CHO. So, if you want to go from Br to CHO substituent, you don't move clockwise. And since hydrogen atom is under plane of Fisher's projection - molecule B has S configuration.

[pck24]

How can the stereochemistry be assigned when the molecules are drawn without stereochemistry? Any of these molecules could be R or S! Or am I being silly...

[bw800402]

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