How to determine RnS for multiple chiral structures

[duvaldentist]

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Im a little confused for distinguishing RnS for structures with multiple chiral centers. I know if you only have one chiral center you take the lowest atom and place it down, then rank the rest and see if its left for "S" or right turn for "R" but on chads videos he said that dosent work for multiple chiral centers so how do you solve for those? Ex. o-chem #118 from 2011 DAT destroyer.

 

[Impulse155]

To determine R and S
Im going mainly by memory so il check in a little bit to make sure nothing is reversed.
1) find your chiral carbon (carbon with 4 different attachements)
2) Look at the elements directly attached to the carbon (usually there will be 1 CH3 or 1 hydrogen)
-Hydrogen/CH3 are always going to be priority number 4 (however hydrogen beats CH3 when comparing both of them)
3) Your labeling the elements directly attached based on atomic number. Highest atomic number gets the highest priority (O is more important than C, N is more important than C, O is more important than N, etc etc)
-When your have 2 of the same atoms attached to the chiral carbon look at what is attacahed to the element. If your looking at 2 carbons, and one has 2x Hydrogens and another carbon where the other has the carbon, and an oxygen, the carbon bonded to the oxygen gets priotiy.
4) After you labeled out your 1-4 determine if it is turning clockwise or counterclockwise.
-If your going clockwise its R
-If your going counterclockwise its S
5) Next you need to find out if its true or false
- True means that your lowest priority (number 4) will be on the dashed or across from the dash.
- False means that your lowest priority (number 4) will be on the wedge, or across from the wedge. In which case you have to switch (if you thought it was R, its S for example.

If you have multiple, you just repeat.

Hope this helps.

 

[duvaldentist]

To determine R and S
Im going mainly by memory so il check in a little bit to make sure nothing is reversed.
1) find your chiral carbon (carbon with 4 different attachements)
2) Look at the elements directly attached to the carbon (usually there will be 1 CH3 or 1 hydrogen)
-Hydrogen/CH3 are always going to be priority number 4 (however hydrogen beats CH3 when comparing both of them)
3) Your labeling the elements directly attached based on atomic number. Highest atomic number gets the highest priority (O is more important than C, N is more important than C, O is more important than N, etc etc)
-When your have 2 of the same atoms attached to the chiral carbon look at what is attacahed to the element. If your looking at 2 carbons, and one has 2x Hydrogens and another carbon where the other has the carbon, and an oxygen, the carbon bonded to the oxygen gets priotiy.
4) After you labeled out your 1-4 determine if it is turning clockwise or counterclockwise.
-If your going clockwise its R
-If your going counterclockwise its S
5) Next you need to find out if its true or false
- True means that your lowest priority (number 4) will be on the dashed or across from the dash.
- False means that your lowest priority (number 4) will be on the wedge, or across from the wedge. In which case you have to switch (if you thought it was R, its S for example.

If you have multiple, you just repeat.

Hope this helps.

Wow thank you so much, you really broke it down good for me. Can you just verify that this is right before I memorize the wrong method because I know u said u were just going off memory. Also does this method still work for multiple chiral centers?

 

[chippy]

Impulse115, I hope that when you said "hydrogen beats CH3 when comparing them" you meant that hydrogen beats it as in, hydrogen will be 4th priority.

For multiple chiral centers, you can pretty much follow Impulse's guide above for each chiral center. So lets say, you have a molecule with 2 chiral centers, you could have RR, SS, RS, or SR. BUT if your two chiral centers are the same (as in they both have the exact same 4 substituents) then it turns out that your RS and SR are exactly the same. SO instead of saying that we have RR, SS, RS, and SR, we actually have RR, SS, and meso, which is the term for that RS-SR isomer.

 

[duvaldentist]

Impulse115, I hope that when you said "hydrogen beats CH3 when comparing them" you meant that hydrogen beats it as in, hydrogen will be 4th priority.

For multiple chiral centers, you can pretty much follow Impulse's guide above for each chiral center. So lets say, you have a molecule with 2 chiral centers, you could have RR, SS, RS, or SR. BUT if your two chiral centers are the same (as in they both have the exact same 4 substituents) then it turns out that your RS and SR are exactly the same. SO instead of saying that we have RR, SS, RS, and SR, we actually have RR, SS, and meso, which is the term for that RS-SR isomer.

Ya I interperated the H beats CH3 as beats it for the 4th priority. And thanks for the clarification. Sorry for the dumb questions but as I go threw destroyer I am realizing I don't have a strong grasp on chiral centers. If your analyzing a C to see if it's a chiral center if it's connected to two other C's does it matter what's on those carbons or does the fact that it's 2C's mean it's not a chiral center. Sorry if that dosnt make much sense

 

[Impulse155]

Ya I interperated the H beats CH3 as beats it for the 4th priority. And thanks for the clarification. Sorry for the dumb questions but as I go threw destroyer I am realizing I don't have a strong grasp on chiral centers. If your analyzing a C to see if it's a chiral center if it's connected to two other C's does it matter what's on those carbons or does the fact that it's 2C's mean it's not a chiral center. Sorry if that dosnt make much sense

First off, I apologize for using the term "beats" very losely lol.
It definately matters.
For example if you have something like
*ANY PERIODS YOU SEE, DISREGARD THEM* (there just there b/c otherwise the pic kept ****ing up)

.......N
.......|
C-C-C-C-C-C-C
.......|
.......OH

Lets not worry about stereochem.
The carbon were looking at is 3rd from the left. We can see that even though it has 2 carbons on it, both of them are not the same. To the left you have ethane, to the right you have butane.
When your in a situation like this, your going to follow the pattern until you reach a differential point. If you cant find one, itsnot a chiral carbon.
Therefore you have to see whats attached to it.
Carbon to the left:
It has 1 Carbon attached, and that carbon has 3 hydrogens.
Carbon to the right:
Has 1 carbon attached, however its followed by another Carbon that has another carbon attached to it.
Carbon takes priority over hydrogen. Therefore, the group on the right has a higher priority than the group on the left (lower number).

So in this one,
Oxygen would be highest priority:1
Nitrogen follows
Then the group on the right
Then the group on the left.

If you have any questions feel free to let me know or repost it. Im pretty solid with ochem (prolly better at it than bio)

Edit:
One sec, theres an error with the drawing, wont let me draw it out.

 

[duvaldentist]

First off, I apologize for using the term "beats" very losely lol.
It definately matters.
For example if you have something like
*ANY PERIODS YOU SEE, DISREGARD THEM* (there just there b/c otherwise the pic kept ****ing up)

.......N
.......|
C-C-C-C-C-C-C
.......|
.......OH

Lets not worry about stereochem.
The carbon were looking at is 3rd from the left. We can see that even though it has 2 carbons on it, both of them are not the same. To the left you have ethane, to the right you have butane.
When your in a situation like this, your going to follow the pattern until you reach a differential point. If you cant find one, itsnot a chiral carbon.
Therefore you have to see whats attached to it.
Carbon to the left:
It has 1 Carbon attached, and that carbon has 3 hydrogens.
Carbon to the right:
Has 1 carbon attached, however its followed by another Carbon that has another carbon attached to it.
Carbon takes priority over hydrogen. Therefore, the group on the right has a higher priority than the group on the left (lower number).

So in this one,
Oxygen would be highest priority:1
Nitrogen follows
Then the group on the right
Then the group on the left.

If you have any questions feel free to let me know or repost it. Im pretty solid with ochem (prolly better at it than bio)

Edit:
One sec, theres an error with the drawing, wont let me draw it out.

Wow thank you so much, and props for your figure lol... But just to verify so in your example it's "S" right because going N,OH,right C it's counter clockwise which is S right?

 

[Impulse155]

Wow thank you so much, and props for your figure lol... But just to verify so in your example it's "S" right because going N,OH,right C it's counter clockwise which is S right?

What gets priority, O or N 🙂