Basic Organic Nomenclature
7. Stereochemistry (iii)
These two methods of representing molecules in 2-D find most use in the representation of 3-D structures in chemical reactions where the relative positions of two groups on adjacent atoms in the molecule are important, and in conformational analysis.
Sawhorse diagrams.In this method, the spacial orientation at a particular bond in the molecule is shown using a solid wedge line for the bond chosen and simple lines for all other bonds. By this convention, simple lines from the front atom (thick end of the wedge) are representing bonds coming out of the plane of the paper, whilst lines from the rear atom (thin end of the wedge!) are representing bonds going back into the plane of the paper
Illustrate the two enantiomers of 2-bromopropanoic acid using sawhorse diagrams, use the C2-C3 bond with C3 forward:
Compare the two diagrams to the 3-dimensional models, and note how
the two diagrams do indeed represent the two possible enantiomers in
that they represent mirror images of each other.
Again, make sure that you see the different isomer represented by each diagram. And note again that exchanging any two groups/atoms on the chirality centre (C2) converts one enantiomer into the other.
ExercisesTry the following sawhorse diagram exercises.
Newman projections.Like sawhorse projections, Newman projections focus on one bond in the molecule. In this case the molecule is viewed as if looking straight down the bond, so that the two bonded atoms lie one behind the other.
The front atom of the two is a point, the rear atom of the two is a circle.
Bonds to the front atom are shown by lines issuing from the point which represents the front atom.
Bonds to the rear atom are shown by lines starting from the circle which represents the rear atom.
To be clear, the projections are usually shown with the front and back bonds staggered, as in the following illustrations.
Note how the two enantiomers differ from each other: exchanging any two of the groups/atoms on the chirality centre (C2) gives the enantiomer.
Note how the two enantiomers differ: exchanging any two groups/atoms on the chirality centre (C2) changes the projection for one enantiomer into the projection for the other.
|Try the following Newman
Continue illustration conventions with one of the following:
Review More than One Chirality centre.
Next page: R and S nomenclature.
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