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enter image description here

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.

enter image description here

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    $\begingroup$ 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. $\endgroup$ – LDC3 Jun 4 '14 at 2:41
  • $\begingroup$ Ah, I think I need to buy a model, or convert this to a Fisher projection. $\endgroup$ – Dissenter Jun 4 '14 at 2:46
  • $\begingroup$ You can download ACD ChemSketch, and draw the molecules. Then you can rotate them in 3D. $\endgroup$ – LDC3 Jun 4 '14 at 2:48
  • $\begingroup$ 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. $\endgroup$ – ron Jun 4 '14 at 2:50
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    $\begingroup$ No matter whether someone is a beginner or advanced, models are extremely helpful. $\endgroup$ – ron Jun 4 '14 at 2:52
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enter image description here

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.! enter image description here

Note that both are (S)

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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)

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