Determining absolute configuration (R or S)

msbbc833

Full Member
10+ Year Member
I know that when you have a chiral center, you rank the 4 substituents and view the compound with the lowest priority group in the back, and assign it an R (right) or S (left) orientation. Lets assume we have a compound where the #4 substituent is H and it has a dashed line so it is already in the back, making it easy to determine R or S. If the H were to have a wedge instead, then you do the same thing but flip the orientation so S becomes R and R becomes S. Now what happens when the H bond is in the plane (straight line)?

From what I remember, you redraw the compound leaving all the bond connectivities (lines, dashed, wedge) exactly how they are, except you would switch the H with the compound that has the dashed line, and then determine R or S. Is this correct?

G1SG2

Full Member
10+ Year Member
5+ Year Member
I know that when you have a chiral center, you rank the 4 substituents and view the compound with the lowest priority group in the back, and assign it an R (right) or S (left) orientation. Lets assume we have a compound where the #4 substituent is H and it has a dashed line so it is already in the back, making it easy to determine R or S. If the H were to have a wedge instead, then you do the same thing but flip the orientation so S becomes R and R becomes S. Now what happens when the H bond is in the plane (straight line)?

From what I remember, you redraw the compound leaving all the bond connectivities (lines, dashed, wedge) exactly how they are, except you would switch the H with the compound that has the dashed line, and then determine R or S. Is this correct?

Yes, that's what I always do.

Seraph 84

Full Member
I always number them and rotate the molecule to get the lowest priority pointing away and use the "steering wheel" technique.

0Complications

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10+ Year Member
I always number them and rotate the molecule to get the lowest priority pointing away and use the "steering wheel" technique.

I just number them, from highest to lowest and draw then arrow. If the lowest priority group is facing out of the page you reverse it, so say you had an S, you make it an R.

This way is idiot-proof, which is what I need. I used to miss these q's like crazy, now I don't miss a single one.

Just remember the lowest priority group has to face into the page, so if it doesn't, reverse the configuration.

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