- Wedge-dash structures represent the 3D geometry of a molecule
- Wedges represent groups coming out of the plane of the page, and dashes represent groups going into the plane of the page
Line-bond structures are great for drawing organic molecules efficiently. While they can accurately reflect the geometry of bonds that are in the plane of the page, they don’t allow us to show the geometry of bonds that lie either above or below the plane of the page. For example, a carbon atom with four single bonds has a tetrahedral geometry. In a tetrahedral structure, a maximum of two bonds can be in the same plane. Try rotating the molecule below to convince yourself that in a tetrahedral structure, only two bonds can ever be in the same plane (note that you can zoom in if the model is too small).
When it’s important to specify the 3D arrangement of atoms, we can do so using wedge-dash structures. A wedge-dash structure uses a line-bond structure as a base. We add a wedge (
) to indicate bonds that come towards us out of the page, and a dash (
) to indicate bonds that go away from us into the page. Bonds in the plane of the page are represented by solid lines, just as in a line-bond structure. The table below shows how a wedge-dash structure relates to the 3D model of a molecule.
Chemists often combine line-bond and wedge-dash structures together when drawing organic molecules. This allows us to draw molecules efficiently by specifying the 3D structure only where it is necessary. For example, SAM, an important molecule in biosynthesis, is shown below using a combination of line-bond and wedge-dash representations.
Avoid these common errors when drawing molecules
- Double bonds should have a bond angle of 120°
- Triple bonds have a bond angle of 180°
- If a carbon atom is drawn in explicitly, then the hydrogen atoms must be drawn in, too
- A wedge and dash should be on the same side of a bend, to reflect the true geometry
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