Basic Organic Nomenclature
7. Stereochemistry (iii)
|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.
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.
Illustrate the four stereoisomers of 3-bromobutan-2-ol using
Again, make sure that you see the different isomer represented by each diagram.
DiastereoisomerismThis 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:
Diastereoisomers are stereoisomers which are not enantiomers.
ExercisesTry the following exercises.
Continue illustration conventions with one of the following:
Review Enantiomerism: the phenomenon.
Next page: R and S nomenclature.
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