Basic Organic Nomenclature

An Introduction

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

7. Stereochemistry (iii)
Chirality

III. Drawing 3-D Structures in 2-D
(iii) Fischer Projections

Index


Fischer projections are "coded" representations of wedge diagrams, and are extremely useful for illustrating structures which contain more than one chirality centre. All the bonds in these projections are simple lines, but by convention:
horizontal lines are bonds projecting forward, out of the plane of the paper; vertical lines are bonds projecting backwards, into the plane of the paper.

When more than one chirality centre is shown, they are usually connected with vertical lines and these vertical lines are best considered to be in the same plane as the paper).

Examples.

In the following examples, the equivalent wedge diagram is also shown so that the orientation of the bonds in the Fischer projections can be assimilated.

Example 1.
Illustrate the two enantiomers of 2-bromopropanoic acid using Fischer projections at the chirality centre (C2):

Fischer projection pdb model

Compare the two diagrams to the 3-dimensional models, and note how the two diagrams do indeed represent the two possible enantiomers and that they represent mirror images of each other.
In particular, note that exchanging any two groups/atoms at the chirality centre produces the mirror image of that centre, and in this case (only the one chirality centre) the enantiomer.

Example 2.
Fischer projections are most often used when more than one chirality centre is present in the molecule. This second example illustrates this use of these projections.

Illustrate the four stereoisomers of 3-bromobutan-2-ol using Fischer projections:
(note: 4 isomers = 2n, with n = 2)

Fischer projection pdb model

Again, make sure that you see the different isomer represented by each diagram.


Diastereoisomerism

This last example has produced an example of stereoisomers which are not enantiomers. Remember that enantiomers are mirror images of each other, and so there are only ever two. Obviously then, all four of the stereoisomers of 3-bromobutan-2-ol above cannot be enantiomers. Recall that stereoisomers which are not mirror images (enantiomers) are termed diastereoisomers.

For the first example (2-bromopropanoic acid) we noted that to change a Fischer projection from one enantiomer to the other required only the interchange of two of the groups/atoms on the chirality centre. If there are two or more chirality centres, then to obtain the enantiomer of a molecule, two groups must be interchanged at each chirality centre as reflection in a mirror will invert everything in the molecule. (Do not under any circumstance exchange groups/atoms between different chirality centres!!)

Check back to the second example and see that projections 1 and 2 represent one pair of mirror images (enantiomers), and that projections 3 and 4 represent another pair of mirror images (enantiomers).

Thus diagrams 1 and 3, 1 and 4, 2 and 3, and 2 and 4 represent the diastereoisomer pairs possible for this molecule.


To repeat the definitions:

    Enantiomers are mirror images of each other.

    Diastereoisomers are stereoisomers which are not enantiomers.

Recall also that (E)-but-2-ene and (Z)-but-2-ene are stereoisomers which are not enantiomers, and so are diastereoisomers.

Exercises

Try the following exercises.
Continue illustration conventions with one of the following:

Sawhorse diagrams and Newman projections

Wedge diagrams


Review Enantiomerism: the phenomenon.

Next page: R and S nomenclature.


Index

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