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Enantiomers: R and S Nomenclature

All chiral molecules have nonsuperimposable mirror images. And as a general rule of thumb, chiral molecules must have one or more chiral centers -- that is, carbons that have four non-identical substituents around it. (There are, of course, exceptions to this rule). A classic case of a simple chiral molecule is the following halogenated methane derivative:

This carbon atom has four non-identical substituents around it, making this carbon a chiral center, and as proof of its chirality the molecule has a non-superimposable mirror image. A fancy term used in textbooks and in the literature to describe molecules that are mirror images of each other is enantiomers, as in "the enantiomer of the left molecule above is the molecule on the right, its mirror image."

To distinguish between enantiomers, chemists use the R and S classification system. Stereocenters, (sometimes called chiral centers, or stereogenic centers) are carbons that have four non-identical substituents on them, and are designated as either of R stereochemistry or S stereochemistry. If a molecule has one stereocenter of R configuration, then in the mirror image of that molecule, the stereocenter would be of S configuration, and vice-versa.

Determination of R or S configuration can be applied in three steps:

1. Order the substituents coming off the stereogenic carbon atom using the Cahn-Ingold-Prelog rules.
2. Rotate the molecule until the lowest priority (number 4) substituent is in the back
3. Draw a curve from number 1 to number 2 to number 3 substituent. If the curve is clockwise, the stereocenter is of R configuration. If the curve is counterclockwise, the stereocenter is of S configuration.

1. Order the substituents: Order the substituents coming off the carbon stereocenter from 1 to 4, with 1 being the highest priority substituent and 4 being the lowest priority substituent. To assign priority using the Cahn-Ingold-Prelog rules, compare the first atoms of the substituents.

  • Give those substituents with higher molecular weight atoms a higher priority number. In our example below, iodine would be 1, bromine 2, chlorine 3, and fluorine 4, because iodine has the highest molecular weight (and is therefore highest priority) and fluorine has the lowest molecular weight (and is therefore the lowest priority).

  • If the first atom of two substituents happen to be identical identical in molecular weight, go to the next atom and make the molecular weight comparison (eg. an ethyl group would have higher priority over a methyl group).

  • Assigning priorities for double bonds becomes a bit more challenging (this applies in the same fashion to carbonyls, C=O, and imines, C=N). A carbon with a double bond to another carbon is treated as a carbon singly bonded to two carbons, as shown below. This means that, for example, an ethylene substituent, R-CH=CH2, will have a higher priority then an ethyl substituent (R-CH2CH3)

    Shown here is an ethylene substituent (often called an allyl substituent). By the Cahn-Ingold-Prelog rules for assigning R and S nomenclature, this allyl group can be redrawn with each double bond carbon singly bonded to an additional carbon with three "phantom ligands," that are ignored.

Our example molecule can now be numbered as follows:


2. Rotate the molecule. There are a couple of different ways to go from the priority numbering to determining R and S confiuguration. One of the best methods taught by a lot of undergraduate textbooks is to rotate the molecule until the number 4 priority subsituent is in the back, as shown below. (Remember that dashed lines mean a bond going into the computer screen, and a solid wedged line indicates a bond coming out of the screen). This takes some practice, especially if you are like most people and have difficulty visualizing molecules in three dimensions.

The molecule is rotated until the 4 priority substituent is in the back.


3. Draw the curve. A curve is then drawn from the 1 to 2 to 3 priority substituents, ignoring the 4th priority subsituent (as shown below). If that curve goes clockwise then that stereocenter is of the R configuration. If the curve goes in a counterclockwise direction, then that stereocenter is of S configuration. In our example below, the curve goes counterclockwise so the stereocenter is of S configuration.

The name of the compound above, then, would be (S)-bromo, chloro, fluoro, iodomethane, and the name of its enantiomer, or its mirror image, would be (R)-bromo, chloro, fluoro, iodomethane.

Click here for a short quiz on assigning R or S configuration.

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