Help on visualization of Fischer Diagrams (Example)

[Chem_Mod]

Question: If I draw the (2R, 3R)1,3-dichloro-2-butanol molecule horizontally so that the H's were on vertical lines, I get (2R, 3S) because for the third Carbon. I drew the arrow from Cl left and down to the other carbon bonded to H, OH and ClCH2. Why drawing horizontally, instead of vertically, gives 3S? Shouldn't the answers be the same even if I drew the same molecule horizontally?

[Chem_Mod]

Answer: When drawing Fischer projections, remember that vertical bonds point down and horizontal bonds point up. If you draw the (2R,3R)-1,3-dichloro-2-butanol as a horizontal fischer projection, with all the carbons in a straight line from left to right, then the OH group should point up and the Cl group should point down. It is difficult to describe fischer projections, so practice drawing the molecule several different ways. Also, don't forget to use model kits to study.