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### Course: Organic chemistry>Unit 4

Lesson 2: Enantiomers

# 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 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.
• 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)
• In the 3 carbon (the second chiral center) how do we know that the h has a wedge instead of a dash or a stick. How can we determine that?
• Every carbon in this representation has 2 bonds in the plane (normal lines) of the paper (or screen in this case...), 1 bond coming out of the plane (wedged line) and 1 bond going behind the plane (dashed line)

So since we see two normal lines and a dashed line the implied H must have a wedged line.
• 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.
• Why is it called 2R? At
• The highest priority group on the second carbon (the first chiral carbon in the chain) is a hydroxyl group and it’s in the R configuration.
• How do you know whether the hydrogen is going away or pointing at us?
• See if it’s wedged or dashed....
In these drawings there will always be 2 bonds with normal lines, one wedged bond and one dashed bond
• At , Jay drew a H coming away from us from the chiral center containing the OH group. How can we assume that that H is facing away from us? And why doesn't he assume these orientations on any of the other hydrogens he draws?