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
(ii) More than One Chirality Center

Index


Molecules to be dealt with in this section have at least two sp3 carbon atoms which, having four different groups attached, are chirality centres. The methods to pick these out are what you might expect: examine the molecule for these atoms.

In the following examples, pick out the stereocentres.

Line drawing pdb model

In each of these cases there are two chirality centres:



    In the 3-bromobutan-2-ol, carbons 2 and 3 are chirality centres.
    In the 2,3-dibromobutane, carbons 2 and 3 are chirality centres.
    In D-erythrose, carbons 2 and 3 are chirality centres.

Number of stereoisomers of a given structure.

For one chirality sp3 centre we have seen that there are two possible stereoisomers which are mirror images of each other.

If a second chirality centre is present in a molecule, the number of possible stereoisomers doubles to four: each chirality centre may by either R or S, so the four possibilities are RR, SS, RS, and SR.

If a third chirality centre is present, the number of possible stereoisomers doubles again to eight (third centre R with the above four, and third centre S with the above four).

And for each subsequent chirality centre there is another doubling of the possibilities. It follows from this that four four chirality centres there will be 2 x 2 x 2 x 2 (ie 24) possible stereoisomers.
In general, for n chirality centres the possible number of stereoisomers is 2n.

Example:
Cholic acid has 11 chirality centres. How many possible stereoisomers are there?


cholic acid

With 11 chirality centres, cholic acid has a possible 211 = 2048 stereoisomers. The model shows the only one, out of all possible stereoisomers, which exists naturally.


Naming compounds with more than one stereocentre.

The naming of compounds with more than one chirality centre follows closely the naming of molecules with one chirality centre:

    At each chirality centre, decide whether the configuration is R or S.
    Name the compound with all R/S designations as prefixes, but with locants to identify which chirality centre is being refered to by the R or the S.

Example.

The four stereoisomers of 3-bromobutan-2-ol are named as follows:

Line drawing IUPAC name pdb model
(2R,3R)-3-bromobutan-2-ol
(2S,3S)-3-bromobutan-2-ol
(2S,3R)-3-bromobutan-2-ol
(2R,3S)-3-bromobutan-2-ol

Further worked examples.

Line drawing IUPAC name pdb model
(2S,5R)-5-aminohexan-2-ol
(3S,5S)-5-isopropyl-2,3-dimethyloctane
(2R,3S)-2-chloro-4-methylpentan-3-amine
(2R,4R)-2-chloro-4-hydroxyhexanoic acid
Try the following Self-study Questions.
Review R/S (i) One Chirality Centre.

Next Page: Chiral With No Chirality Center


Index

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