Wedge and Dash Notation
Table of Contents
Wedge and dash notation is a way to show a molecule’s three-dimensional shape on a flat page or screen. It is especially useful in organic chemistry for showing the spatial arrangement of atoms or groups around a tetrahedral atom, such as an sp3–hybridized carbon. [1–4]
Because molecules are three-dimensional, ordinary flat drawings can sometimes be misleading. If all bonds around a tetrahedral carbon are drawn as simple straight lines, the molecule may appear flat even though its atoms are arranged in 3D space. Wedge and dash notation solves this problem by showing which bonds point toward the viewer, which point away, and which lie in the plane of the page.
How Wedge and Dash Notation Works
Different bond styles represent different directions in space: [1–7]
- A solid wedge shows a bond projecting out of the page toward the viewer.
- A hashed or dashed wedge shows a bond extending behind the page away from the viewer.
- A normal straight line shows a bond lying in the plane of the page.
In many introductory drawings of a tetrahedral carbon, two bonds are shown as normal lines, one bond is shown as a solid wedge, and one bond is shown as a dashed wedge. This is a common way to represent the tetrahedral shape clearly, although real wedge–dash drawings can be oriented in different valid ways.
One useful detail is that the wide end of a solid wedge is the part closest to the viewer. The narrow end is attached to the atom farther back in the drawing.
Example of a Wedge and Dash Structure
For example, imagine a carbon atom bonded to chlorine (Cl), hydrogen (H), a methyl group (CH3), and a hydroxyl group (OH). [6]
If the bond from carbon to CH3 is drawn as a solid wedge, the CH3 group is coming out of the plane of the page toward the viewer.
If the bond from carbon to H is drawn as a hashed or dashed wedge, the hydrogen atom is pointing behind the plane of the page, away from the viewer.
If the bonds from carbon to Cl and OH are drawn as regular lines, they lie in the plane of the page.
Together, these bond styles help show the actual three-dimensional arrangement of the groups around the carbon atom.
Wedge and Dash Notation in Stereochemistry
Wedge and dash notation is especially important in stereochemistry. Some compounds have the same molecular formula and the same atomic connectivity but differ in the three-dimensional arrangement of their atoms. These compounds are called stereoisomers, and they can have different chemical, physical, or biological properties. [3]
By showing the spatial arrangement of bonds, wedge and dash notation helps chemists distinguish between different stereoisomers. It is commonly used when studying chirality, enantiomers, diastereomers, and R/S configuration.
For example, changing which group is shown on a solid wedge and which group is shown on a dashed wedge can change the stereochemical configuration of a molecule. In some cases, this change can represent a different stereoisomer with different properties.
Wedge and Dash Notation vs. Other Representations
| Representation | Main purpose | Key idea |
|---|---|---|
| Wedge and dash notation | Shows the 3D arrangement around a tetrahedral center | Solid wedge = toward viewer; dashed wedge = away |
| Fischer projection | Shows stereochemistry in a simple 2D form | Horizontal bonds come forward; vertical bonds go backward |
| Newman projection | Shows the arrangement of atoms when looking directly along a carbon–carbon single bond | Useful for studying conformations and rotation around C–C bonds |
| Sawhorse projection | Shows the arrangement around a carbon–carbon single bond from an angled view | Draws the C–C bond diagonally to give a 3D-like view |
Wedge–dash and Fischer projections are commonly used to show stereochemistry, while Newman and sawhorse projections are especially useful for studying conformations and rotation around carbon–carbon single bonds. [3]
