enter image description here enter image description here

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?

  • 1
    $\begingroup$ If you mean "rotate" (with the center in the desired chiral atom), then you're right. You can rotate freely. $\endgroup$
    – mykhal
    Jul 22 '18 at 7:41

Rotating structures is a recipe for disaster because errors in transcription will invariable occur. Use the best chirality tool 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.

enter image description here


Not the answer you're looking for? Browse other questions tagged or ask your own question.