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# Absolute Configuration: R-S Sequence Rules

To name the enantiomers of a compound unambiguously, their names must include the "handedness" of the molecule. The method that has been developed to do this is formally known as R,S nomenclature.

### Introduction

The method of unambiguously assgning the handiness of molecules was originated by three chemists: R.S. Cahn, C. Ingold, and V. Prelog and, as such, is also often called the Cahn-Ingold-Prelog rules. In addition to the Cahn ingold system, there are two ways of experimentally determining an enantiomer's absolute configuration:

1. X-ray diffraction analysis. Note that there is no correlation between the sign of rotation and the structure of a particular enantiomer.
2. Chemical correlation with a molecule whose structure has already been determined via X-ray diffraction.

However, for non-lab purposes, it is beneficial to focus on the R-S system. The sign of optical rotation, although different for the two enantiomers of a chiral molecule,at the same temperature, cannot be used to establish the absolute configuration of an enantiomer. This is because the sign of optical rotation for a particular enantiomer may change when the temperature changes.

### Stereocenters are labeled R or S

The "right hand" and "left hand" nomenclature is used to name the enantiomers of a chiral compound. The stereocenters are labeled as R or S.

Look at the first picture, then draw a curved arrow from the highest priority (1) substituent to the lowest priority (4) substituent. If the arrow goes in a counterclockwise direction (left when leaving the 12'o clock position), the configuration at stereocenter is considered S ("Sinister" → Latin= "left").  If, however, the arrow turns clockwise,(Right when leaving the 12'o clock position) then the stereocenter is labeled R ("Rectus" → Latin= "right").

The R or S is then added as a prefix ,in parenthesis, to the name of the specific enantiomer we are concerned about.

Example 1

(R)-2-Bromobutane

(S)-2,3- Dihydroxypropanal

### Sequence rules to assign priorities to substituents

Before you can apply the R and S nomenclature to a stereocenter, you have to prioritize your substituents. Follow these rules to prioritize:

#### Rule 1

First look at the atoms that are directly attached to the stereocenter of the compound. A substituent that has a higher atomic number takes precedence over a substituent that has a lower atomic number. Hydrogen is the lowest possible priority substituent, because it has the lowest atomic number.

1. When you are dealing with isotopes, the atom with the higher atomic mass receives higher priority.
2. When visualizing the molecule, the lowest priority substituents should always go away from the viewer (dashed line indicates going away from the viewer). To understand how this works or looks, imagine that you have a clock and a pole.
3. Attach the pole to the back of the clock, so that when you look at the face of the clock the pole points away from you. That is the same way the lowest priority substituent should point away from you.
4. Then, draw an arrow from the highest priority atom to the 2nd highest priority atom to the 3rd highest priority atom. Since you have placed the 4th highest priority atom in the back, you arrow should seem like it is going across the face of a clock. If it is going clockwise, then it is an R-enantiomer; If it is going counterclockwise, it is an S-enantiomer.

When looking at a problem with wedges and dashes: if the lowest priority atom is not on the dashed line going away from you, you must rotate the molecules so that the lowest priority bond is facing away from you.
Remember that

• Wedges mean coming towards the viewer.
• Dashes mean going away from you.

#### Rule 2

When you have two substituents with equal rank, you must proceed along the two substituent chains until you find a point of difference. First, you determine which of the chains has the first connection to an atom with the highest priority-the highest atomic number. That chain will have the higher priority.

If the chains are similar keep going down the chain, until you can find a point of difference.

For example: an ethyl substituent will take priority over a methyl substituent. At the connectivity of the stereocenter, both have a Carbon, which are equal in rank. Going down the chains, a methyl has only has Hydrogen atoms attached to it while the ethyl has another Carbon attached to it. The Carbon on the ethyl is the first point of difference and has a bigger atomic number than Hydrogen:therefore the ethyl takes priority over the methyl.

#### Rule 3

If a chain is connected to the same kind of atom twice or three times. Check to see if the atom it is connected to has a greater atomic number than any of the atoms that the competing chain is connected to.

• If none of the atoms connected to the competing chain(s),at the same point, has a greater atomic number: the chain bonded to the same atom multiple times has the greater priority
• If however, one of the atoms connected to the competing chain has a bigger atomic number: then that chain will have the higher priority.

Example 2
A 1-Methylethyl substituent will take precedence over an Ethyl substituent.  Connected to the first carbon, ethyl only has one other Carbon whereas the 1-Methylethyl has two Carbons attached to the first Carbon; this is the first point of difference. Therefore, 1-Methylethyl will rank higher in priority than Ethyl, as shown below:

However:

Remember that being double or triple bonded to an atom means that the atom is connected to the same atom twice. And in such a case you would follow the same method as above.

Caution!!
Keep in mind that priority is determined by the first point of difference along the two similar substituent chains. After you have reached the first point of difference, the rest of the chain is irrelevant.

When you are looking for the first point of difference on similar substituent chains, you may encounter branching. If there is branching, we choose the branch that is higher in priority. When the two substituents have similar branches, you rank the elements within the branches until you reach a point of difference.

After all your substituents have been prioritized in the correct manner, you can now name/label the molecule  R or S.

1. Put the lowest priority substituent in the back (dashed line).
2. Go from 1 to 2 to 3. (it is helpful to draw or imagine an arcing arrow that goes from 1--> 2-->3)
3. Determine if the direction from 1 to 2 to 3 clockwise or counterclockwise

i) If it is clockwise it is R
ii) if it is counterclockwise it is S.

USE YOUR MODELING KIT: Making models will help you visualize the structure. When you make a model, make sure the lowest priority is pointing away from you. Then determine what direction you have to go from the highest priority substituent to the lowest: Clockwise (R) or Counterclockwise(S).

IF YOU DO NOT HAVE A MODELING KIT: remember that the dashes mean the bond is going into the screen and the wedges means that bond is coming out of the screen. If your lowest priority bond is not pointing to the back, mentally rotate it so that it is. However, it would help you greatly when learning organic chemistry to get one.

If you have a modeling kit use it to help you solve the following practice problems.

### Problems

Are the following R or S?

### Solutions

1. S:  I > Br > F > H. The lowest priority substituent, H, is already going towards the back. It turns left going from I to Br to F, so it's a S.
2. R: Br > Cl > CH3 > H.  You have to switch the H and Br in order to place the H, the lowest priority, in the back. Then, going from Br to Cl, CH3 is turning to the right, giving you a R.
3. Neither R or S: This molecule is achiral. Only chiral molecules can be named R or S.
4. R: OH > CN > CH2NH2 > H. The H, the lowest priority, has to be switched to the back. Then, going from OH to CN to CH2NH2, you are turning right, giving you a R.
5. (5) S: $$\ce{-COOH}$$ > $$\ce{-CH_2OH}$$ > $$\ce{C#CH}$$ > $$\ce{H}$$. Then, going from $$\ce{-COOH}$$ to $$\ce{-CH_2OH}$$ to $$\ce{-C#CH}$$ you are turning left, giving you a S configuration.

### References

1. Schore and Vollhardt. Organic Chemistry Structure and Function. New York:W.H. Freeman and Company, 2007.
2. McMurry, John and Simanek, Eric. Fundamentals of Organic Chemistry. 6th Ed. Brooks Cole, 2006.

### Contributors

• Ekta Patel (UCD), Ifemayowa Aworanti (University Of Maryland Baltimore County)

15:26, 31 Oct 2014

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