Need help with absolute configuration (R,S)

[JohnDoeDDS]

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Okay so I forgot how to do these and kaplan is confusing me. I remember I used an easier method I just can't remember it. So I would appreciate if someone could explain it to me please.

I understand assigning priority clock-wise =R; CC=S.

But I do not understand how to rotate the molecules in space. Meaning after I assign priority how to swith the lowest priority one to the right, and then properly position the others with the dotted and sloid triangle. I hope I am clear as to what I am asking. This is realy confusing me. Thanks.

 

[skyisblue]

R is clockwise. S is counterclockwise.

Assign priorities with 1 being largest and 4 being smallest, usually hydrogen. Position the substituents like on spokes of a steering wheel. you can place the smallest in back or front. if it's in back - start counting...if it's clockwise it's R. If it's in front - start counting...if it's clockwise, it now becomes S.

 

[JohnDoeDDS]

I get the clockwise part.

I am just not sure what it means to position something the back and how that effects the position of everything else. Thats what I am confused about. How to re-position the molecules.

 

[Streetwolf]

Dashed means in back, wedge is in front. If the lowest one isn't dashed then swap it with the substituent that is dashed. Then clockwise is S. If the lowest one IS dashed then clockwise is R. Provided, of course, that there IS a dashed substituent (whether or not it's the lowest ranking one).

You literally just go around clockwise/counterclockwise. You don't have to think about what it looks like in 3 dimensions. Just look at it on the paper and label 1, 2, 3, and 4. Look at where 4 is. If it's dashed, then look at 1, 2, and 3. If you can order them clockwise then you have R. If you can order them counterclockwise then you have S.

If 4 is not dashed, then swap it with the one that is dashed (in your head or on paper). Then look at the ordering. Clockwise is S and counterclockwise is R because when you swap two substituents you swap S and R.

 

[andyjl]

I think the easier way you are thinking of is just numbering the substituents with priorits 1 through 4. 4 being the lowest priority. Then just count 1,2,3,4. Now the trick here is that if the 4 is either on a vertical substituent, then the configuration is whatever 1 through 4 gives you, either clockwise or counterclockwise.

But if you count from 1 through 4 and determine the configuration and the substituent 4 is on the horizontal position, then just reverse the configuration. it will either be r --> s or s --> r.

There is no need to orient the molecules in a 3d manner.

 

[smec]

I think the easier way you are thinking of is just numbering the substituents with priorits 1 through 4. 4 being the lowest priority. Then just count 1,2,3,4. Now the trick here is that if the 4 is either on a vertical substituent, then the configuration is whatever 1 through 4 gives you, either clockwise or counterclockwise.

But if you count from 1 through 4 and determine the configuration and the substituent 4 is on the horizontal position, then just reverse the configuration. it will either be r --> s or s --> r.

There is no need to orient the molecules in a 3d manner.

Great approach.👍

 

[AbsoluteConfig]

The following method (web address to method) is for Beginning students, as well as others, who are still using the model-set (ball and stick) or those still having a little trouble trying to mentally rotate the stereogenic carbon (stereocenter) with the 4 groups/elements bonded to it.

The method I developed (4-6 years ago), is the most representative method, depicting the 3-dimensioinal spatial arrangement and the Tetrahedral shape. It is by far better than any of the methods published since the 1970's, the method itself accounts for the tetrahedral shape better than any method, therefore the student (viewer) can imagine the 3-dimensional structure better within one's brain.

Being able to mentally rotate is Important, in addition to assigning RS notations (configurations) to the stereogenic carbon. ALL the textbooks state use a model. As well, all the Chemistry teachers/professors state at the biginning of RS Nomenclature (absolute configuration), to use a model-set (ball and stick).

Therefore, it makes sense to have a method that also resembles the tetrahedral shape as much, as humanly possible, it should make it (the stereogenic carbon and 4 groups/elements bonded to the carbon) easier to mentally visualize.

It is the next best thing to using a model (model-set or ball and stick) and you can use it while taking a quiz, exam or final exam.

Hope it helps out 🙂

Absolute Config

www.AbsoluteConfiguration.com

 

[DQLEUCD]

I get the clockwise part.

I am just not sure what it means to position something the back and how that effects the position of everything else. Thats what I am confused about. How to re-position the molecules.

Ignore the lowest priority and just give it the stereochemistry and go with this. If the lowest priority is pointing into the page or away from you, the specific stereochemistry is correct, if it's pointing towards you then it's opposite of how you would name it. If it's in the plane of the page your going to have to look down the center of the carbon opposite of the lowest priority and assign it that way. Hope that helps. I think you can just name it normally without switching it. I'm trying it in my head and I believe you can name it normally but will have to try some other problems out.

 

[AbsoluteConfig]

I am just not sure what it means to position something the back and how that effects the position of everything else.

Using the example of the heart, the Vena Cava and the Aorta are major vessels of the heart. Those vessels are right and left, respectively, from the patients point of view. If a patient was getting a heart transplant, and the Aorta was positioned to the right instead of the left (in other words, turned around), it would certainly have an effect on where the other vessels are located, right?

There is a good reason to try to develop one's ability to visualize the 3-dimensional chemical structure, and once an individual has fine tuned and tweaked their ability to visualize 3-dimensional structures, an individual has provided himself/herself with a better set of tools to practice Dentistry or Medicine, than their classmates 🙂

AbsoluteConfig 🙂