# How to rotate a chiral carbon

I know that to correctly assign R,S priorities I have to rotate the above figure until the H is projecting into the page (away from me). However, I don't know how to rotate the H and the rest of the molecule when the H is in the plane of the page.

When I imagine tilting the molecule back I get a see-saw shaped molecule ... obviously incorrect.

• Envision that you are rotating the molecule on the C-Ethyl bond with the hydrogen going into the page. Remember, it won't lie flat.
– LDC3
Jun 4, 2014 at 2:41
• Ah, I think I need to buy a model, or convert this to a Fisher projection. Jun 4, 2014 at 2:46
• You can download ACD ChemSketch, and draw the molecules. Then you can rotate them in 3D.
– LDC3
Jun 4, 2014 at 2:48
• The "1" and "3" should be solid lines in the plane of the paper; the "4" going back as a dashed wedge as you've shown it, and the "2" being a solid wedge projecting towards the observer.
– ron
Jun 4, 2014 at 2:50
• No matter whether someone is a beginner or advanced, models are extremely helpful.
– ron
Jun 4, 2014 at 2:52

Imagine viewing it from right side:

• H is upwards and CH3CH2 is downwards.
• OH is leftwards and CH3 rightwards.
• H and CH3CH2 are backwards(from right) so they must be on vertical line of Fischer projection and similarly for OH and CH3.

Alternatively:

• Put the dotted wedge CH3 in any vertical position let it be bottom.
• Then imagine all other groups to be planar, we see in clockwise order:
• CH3CH2 , H , OH
• Draw them similarly on the Fischer projection in clockwise sense.!

Note that both are (S)

Swap number 4 for the number of the bond/molecule/atom that is pointing away from you. Decipher whether it is R/S by drawing from 1 to 3. The answer is opposite to this.

https://www.youtube.com/watch?v=x28IcxUhu3A (From 9 minutes 25 seconds on)