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I'm having trouble assigning priorities for this molecule:

I tried assigning priorities with the rules for doubly bonded atoms in mind. I know that the lowest priority group should be going into the page (not on a wedge) but I read that I can simply assign a priority with the fourth priority group on a wedge and then take the opposite of whatever I assigned as the true configuration of the molecule. Problem is, I get R. Answer key says S.

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Your drawing is correct and yes, "I read that I can simply assign a priority with the fourth priority group on a wedge and then take the opposite of whatever I assigned" this is true too. In your drawing 1->2->3 is a clockwise rotation (therefore "R" but), but the lowest priority item (4) is coming out towards you. Therefor, the R/S assignment needs to be reversed (from "R") to "S".

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  • $\begingroup$ Thank you! I think I need to not only purchase a molecular model kit but also an analog clock. $\endgroup$
    – Dissenter
    Commented Jun 2, 2014 at 23:01
  • $\begingroup$ I once had a friend who had an analog wall clock very near to the corner of a room. On the adjoining wall, very near to the corner was a mirror...don't do that. $\endgroup$
    – ron
    Commented Jun 3, 2014 at 1:12
  • $\begingroup$ The clock method should be ignored as this example demonstrates. Point your thumb in the direction of the lowest priority group (4). Then your fingers will point in descending order (1 -> 2 -> 3). If not, you are using the wrong hand. Right hand = R; left (sinister) hand = S. $\endgroup$
    – user55119
    Commented Jul 25, 2019 at 1:20

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