R and S configurations

[hpets]

For compounds that contain a chiral center, i am having trouble labeling them R and S. I understand how to do them, but when the lowest priority group is not in the back, i can never get them right. When i make two switches/interconversions of groups, the molecule sometimes still seems to be the incorrect configuration. For example (94 on topscore 1, choice B) top group Br, right an aldehyde, left Fl, and bottom H. Isn't that an R configuration since H is on the vertical axis (and thus pointing away from me)? If anyone can enlighten me, that'd be great!

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[Hopin']

For compounds that contain a chiral center, i am having trouble labeling them R and S. I understand how to do them, but when the lowest priority group is not in the back, i can never get them right. When i make two switches/interconversions of groups, the molecule sometimes still seems to be the incorrect configuration. For example (94 on topscore 1, choice B) top group Br, right an aldehyde, left Fl, and bottom H. Isn't that an R configuration since H is on the vertical axis (and thus pointing away from me)? If anyone can enlighten me, that'd be great!

I may be little off, but here's what I remember:

If the lowest priority group isn't in the back, just put it in the back and then lable other groups in lowest to highest priority. If you get clockwise configuration, then it is R, BUT THE TRICK IS THAT SINCE YOU SWITCHED THE POSITION OF THE LOWEST PRIORITY GROUP, YOU'D NEED TO SWITCH THE CONFIGURATION AS WELL. SO THIS BECOMES S instead of R.
I think this is how we learned it in Orgo class. This was a dirty little trick and is not described in any textbook (i think).
Might wanna check it out if it works for ya.
Good luck

 

[Chga]

yeah just flip the configs when ever the lowest priority group is in the front

 

[hpets]

So, shouldn't the molecule i talked about be R since the H is already in the back? Topscore says its S.

I know that trick too, thanks for the reminder!

 

[Notoriousjae]

Is the compound represented as a fischer projection or perspective formula because there are different methods to finding out the answer to the different ways it is represented.
Also keep in mind, if you switch two groups to make H have the lowest priority, you would be determining the configuration of the enantiomer of the original molecule. So if the arrow points clockwise, the enantiomer (with the switched groups) has the R configuration, which means the original molecule has the S configuration and vice versa if it points counter clockwise.
hope this helps

 

[Zabispurs]

It took me forever to figure out R and S configurations but I have a strategy that works 99% of the time. Order from 1-4 from highest to lowest priority. If the lowest priority, usually an Hydrogen, screws up the order then switch from R to S or S to R. It is kind of hard to explain in words, I wish I could write it out on paper for you guys.. Basically if the Hydrogen(the least priority group) screws up the order of the configuration then switch from R to S or S to R

 

[hpets]

screws up the method? do you think you could try to explain a little more? i personally know chemistry is screwing up my sanity right now!

so is the H in the molecule i gave "screwing" it up? thanks!

 

[billyjones]

This is how I do it.
1. Number substituents 1-4 based on priority. Remember that the priortiy lies in the atomic number, NOT electronegativity.
2. Based on this priliminary information determine if the configuration is R or S
3. Determine if the Hydrogen is pointing toward you or away from you.
4a. If the hydrogen is pointing away from you keep the configuration as you 1st determined it.
4b. If the hydrogen is pointing toward you, simply choose the OTHER configuration.