Basic Organic Nomenclature

An Introduction

Dave Woodcock,
Associate Professor Emeritus UBC (Okanagan)
©1996,2000, 2008

7. Stereochemistry (iii)
Chirality

IV. Naming Enantiomers
R/S System
(i) One Chirality Center

Index


So there are only ever two enantiomers, therefore requiring only two names to distinguish them. The two designations are R and S, used in parenthesis in front of the compounds name in the same fashion that E and Z are used for alkenes.

To determine whether a particular isomer is the R or the S isomer, the Cahn-Ingold-Prelog priority rules are used in the same way that they are used to determine E or Z for alkenes. For a review of these rules, see the page on alkene nomenclature. In the case of a chirality carbon atom, there are four groups attached and so priorities 1 (lowest) to 4 (highest) must be determined.

As a review make sure that you see the priority order in the following compounds:


1. Butan-2-ol:

Priority order is:

Priority atom(group)
4 O (OH)
3 C (CH2CH3)
2 C (CH3)
1 H


2. 3-Methylpent-1-en-4-yne:

Priority order is:

Priority atom(group)
4 C (yne)
3 C (ene)
2 C (CH3)
1 H

Identifying the R or S isomer.

Once the four groups/atoms around the chirality centre have been prioritized, the R or S prefix can be determined as follows:

  1. Rotate the molecule such that the group of lowest priority is pointing directly away from you.

    In effect, you are looking at the molecule as a Newland projection along the bond to the atom of lowest priority, with the chirality center in front.

  2. The other three groups are ordered from highest to lowest priority and the direction of circular motion from highest to lowest is obtained.
    • If the direction is clockwise, the label is R (for rectus).
    • If the direction is anticlockwise, the label is S (for sinister).

  3. The designation R or S is prefixed to the name of the compound.

Worked examples

Follow through the examples.

1. Butan-2-ol.

Prioritize the groups on the chirality centre:OH (4), C2H5 (3), CH3 (2), H (1).

The lowest priority group in this example is the H atom. Rotate the model so that the hydrogen is pointing away from you:

Now trace the motion as you go from the highest priority group (the hydroxyl) to the second highest (the ethyl) and finally to the third highest (the methyl). In this case the motion is clockwise (to the right) and so the enantiomer is designated R.

The full name for the isomer shown is (R)-butan-2-ol.


2. 3-Methylpentan-1-en-4-yne.

Prioritize the groups: yne (4), ene (3), CH3 (2), H (1).

The lowest priority group in this example is the H atom. Rotate this model so that the hydrogen is pointing away from you:

Now trace the motion as you go from highest priority to lowest in the three remaining groups. That is from yne through ene to methyl. The resulting motion is anticlockwise or to the left: the isomer is the S enantiomer.
The full name for the isomer shown is (S)-3-methylpentan-1-en-4-yne.


Examples for you to work on.

The following structures are named. Make sure that you obtain the same name by following through the process as given above.
To repeat it is:
  1. Prioritize the four attached groups or atoms according to the Cahn-Ingold-Prelog rules.

  2. Rotate the model so that the lowest priority group of the four is pointing away from you.

  3. Trace the path between the remaining three groups starting with the one of highest priority and working down the priority list.

  4. If the traced path is clockwise (to the right) the enantiomer is designated R.

    If the traced path is anticlockwise (to the left) the enantiomer is designated S.


Example 3:
(R)-Bromochloroiodomethane.


Example 4:
(S)-2-Iodobutane.


Example 5:
(R)-2-Bromopropanoic acid.


Example 6:
(S)-2,2-dimethylcyclopentan-1-ol.

Exercises

Try the following exercises.
Review More the One Chirality Centre.

Next page: Naming: More than One Chirality Centre.


Index

Main index page of MolecularModels.ca.

page upkeep by Dave: (email) who is solely responsible for the contents.

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