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I am struggling with determining R and S configuration for the following compound enter image description here

I know how to give R and S configuration for a cyclic compound like the compound above if the lowest priority group is on a wedge or dash bond but on the above compound the lowest priority group is on the plane of the paper so how do I determine the R and S configuration for the chiral carbon on this compound?

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  • $\begingroup$ Build a model and turn it around. $\endgroup$
    – orthocresol
    Commented Oct 5, 2018 at 9:45
  • $\begingroup$ @orthocresol yeah I could simply do that but on the exam we are not allowed to use a model so how could I determine the Rand S conformation without a model? $\endgroup$
    – AussieKenDoll
    Commented Oct 5, 2018 at 9:46
  • $\begingroup$ Just determine the priorities on the nearest atoms (not super easy in this case), then dissect the cycle (possibly redraw it with numbers or letters instead of chemical groups), and swap any two substituents twice in order to get a comfortable structure orientation, if you have not 3D imagination to rotate the molecule or your point of view in your head. (And note that you're determining configuration, not conformation) $\endgroup$
    – mykhal
    Commented Oct 5, 2018 at 10:03
  • $\begingroup$ This similar question may help? chemistry.stackexchange.com/questions/55676/… $\endgroup$
    – owjburnham
    Commented Oct 5, 2018 at 10:55
  • $\begingroup$ Try this method after assigning priorities: chemistry.stackexchange.com/questions/99721/… $\endgroup$
    – user55119
    Commented Oct 5, 2018 at 20:03

1 Answer 1

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The only two ways to do this are either:

  1. Move yourself (or imagine your eye) to the right place so that the lowest priority substituent is away from you, or
  2. Rotate the molecule so that the lowest priority substituent is away from you as you look at the page from above.

Either approach takes a bit of mental gymnastics and gets practice to perfect. For (1), I fine it helps to hold the paper up or move my head so I am close to the page, which makes it easier for me to imagine looking along that bond. For (2), it's easier if you abstract away all the detail by replacing the four groups with just the priority numbers. Even if you can't use a model in the exam, I still think models help to get to grips with the mental manipulations.

Here's a quick sketch - hope it helps: Getting to R or S - change frame of reference or rotate molecule. Good luck!

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  • $\begingroup$ Thanks! Is it possible to use the swap method for this cyclic compound? If yes, Do I only need to consider the molecules which I gave the priority (1,2,3,4) when doing molecule swap method to determine the R and S configuration of the above compound(4th priority on the plane of the paper)? $\endgroup$
    – AussieKenDoll
    Commented Oct 5, 2018 at 13:56
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    $\begingroup$ Yes, I shouldn't have been so hasty in saying there's only two ways to do this! Swapping two substituents always inverts the configuration (eg. S -> R), so doing one swap, followed by a second swap gets back to the original configuration (eg. S -> R -> S). So, rather than rotating like I did in (2), you could draw out the chiral centre with 1, 2, 3, 4, then do two sequential swaps that end up putting the lowest priority group going away from you (ie. with the hashed bond). $\endgroup$
    – Organic Chemistry Explained
    Commented Oct 5, 2018 at 14:15

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