I know that if the smallest atomic number group is not in the back, we can twist the molecule so that it is in the back and then determine if it is R/S (Left image). But if the chiral center is attached to a ring, can we still simply twist the molecule (right image)? If so, how? Or is there another way to make the smallest atomic number group go to the back for molecules with rings?
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1$\begingroup$ If you mean "rotate" (with the center in the desired chiral atom), then you're right. You can rotate freely. $\endgroup$– mykhalCommented Jul 22, 2018 at 7:41
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$\begingroup$ Wishing it would not have been closed. I had another idea and now can't render it. $\endgroup$– Oscar LanziCommented Jun 7 at 12:28
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1$\begingroup$ @OscarLanzi I can reopen if you are interested in adding an answer. $\endgroup$– Tyberius ♦Commented Jun 17 at 19:52
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1 Answer
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Rotating structures is a recipe for disaster because errors in transcription will invariable occur. Use the best chirality tools you have---your hands. Leave the structure where it is! Use either hand to make a "thumbs up" gesture. Point your thumb in the direction of the lowest priority group. The remaining fingers should point in the direction of the highest to next to lowest priority group (OH>C2H5 >CH3). If not, you are using the wrong hand. Left hand = S. Right hand = R. Try to rotate structures 1-3 and see how it compares with the "hand method". Structures 1-3 are of the S-configuration. Cyclopentene 4 is Jon's structure with substituents. It is of the R-configuration (=CH>CH2>CH3), a conclusion reached without redrawing the structure.
answered Jul 22, 2018 at 19:29