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# More R,S practice

How to assign configurations to a molecule with two chirality centers.

## Want to join the conversation?

• I have a hard time figuring out how to rotate so that the molecule/atom with the least priority is going "in to the paper". With the second chiral center in this example could one have just kept the pentanol group still and rotated the other three groups around the carbon so the hydrogen ends up in the back?
• Yes, you could, and you would still get the same answer.
But there's an even easier way.

If the H is in front, don't bother to rotate. Just determine the priorities in the usual way.
If the 123 direction is clockwise (R), just change it to (S).
It works every time.
• Why is it always necessary that the lowest priority group into the space??
• You mean away from the chiral centre, right?
I guess it is so that we could determine the whether it is R or S. All the high priority groups being in the same plane helps in determine whether it is clockwise or anticlockwise rotation. As the structure is 3-D it is kind of like neglecting the lowest priority group by placing it in the different plane from other groups and there by making it easier to see the rotation.
I hope this helps.
• I am unable to understand the orientation of the lowest priority group away from our vision. pls help
• The lowest priority group (the group with the lowest atomic number) should be projecting away from you ("into the page"). So, in our example, Hydrogen is the lowest priority so it needs to be pointing away from you in order to assign a R or S chirality. Then you can use the other three groups (generally 1, 2, 3) to determine which direction your circle goes in: clockwise-R, counterclockwise-S.
(1 vote)
• Is there a trick when your 4th priority is at the sheet plane? Just like in the previous video.
• Unfortunately, no. Because your wedges and dashes will be in arbitrary locations.
• I still don't know how to visualize rotations of the molecules (like when the lowest priority group is in the linear position, and I have to make it go away from me). Help me, please!
• Yes, that is not easy. Firstly, like Sal did in the last video, it helps to simplify the diagram and just draw #1, #2, #3, #4 instead of drawing bulky groups. Also, use your fingers, and make a tripod! assign groups to each of your fingers, and try! Have a nice day );
• What does R and S stand for?
(1 vote)
• actually its vice versa R for rectus and S for sinister
• It's really difficult for me to rotate the molecule whenever we use skeletal structures. If the 4th group is on the back, then it's easy but in problems like these, I just can't do it without the model kit. I've noticed, though, that if you get R configuration whenever the 4th group is NOT in the back, you can change it to S and that will be the right answer without having to mentally rotate the molecule. Would this work with all molecules? I'm not sure if I explained myself correctly.
• I can understand where the "S" comes from but where does the "3" and "2R" come from?
• The 2R and 3 are basically the position of the chiral carbon on the Carbon chain.
(1 vote)
• I'm having trouble understand how Jay arrived at an S configuration for the second chiral center in the pentanol example. If you rotate the molecule holding the carbon bonded to the alcohol in place, the hydrogen pointed towards us would end up facing away, the methyl group facing away would end up in the plane of the molecule, and the C2H5 that was in the plane of the molecule would end up pointing towards us. That would make it go 1, 2, 3, left, top, right. Which is R configuration.
(1 vote)
• Remember it is not “top” it’s towards us in space. It will be an anticlockwise rotation (left->towards->right) so it is S.

Honestly though rather than worrying about rotating the molecule, there’s a really neat trick to working these that will save time. If you swap any two groups around you change a chiral centre from R to S or S to R.
Using this idea, if you swap the hydrogen and methyl so the hydrogen is going away while the methyl is coming towards us, you should be able to see that is R (left->right->towards=clockwise), so if that’s R then the stereocentre of this chiral centre will be S.

This video introduced the trick to me: https://youtu.be/Z10oC7BF4ig