A sawhorse projection shows the three-dimensional arrangement of atoms or groups around a carbon-carbon single bond. In this representation, the molecule is viewed at an angle so that both carbon atoms and their attached groups can be seen clearly. This method is especially useful for visualizing different conformations based on the relative positions of these groups. ^[1–4]

How to Draw a Sawhorse Projection

To draw a sawhorse projection, choose the carbon-carbon bond to be represented and sketch it as a slanting line. In this representation, the carbon on the left is taken as the front carbon, and the carbon on the right as the rear carbon. Then draw three bonds from each carbon and add the attached atoms or groups. When the molecule is viewed along the carbon-carbon bond, the angle between the groups on the two carbon atoms is referred to as the dihedral angle. The arrangement of these groups gives rise to different conformations. ^[1,2]

Examples

1. Ethane

In ethane, each carbon atom is attached to three hydrogen atoms. Therefore, all the bonds around both carbons are occupied by hydrogen atoms. When the hydrogen atoms on the rear carbon lie directly behind those on the front carbon, the conformation is eclipsed, and the dihedral angle is 0°. When they lie in between, the conformation is staggered, and the dihedral angle is 60°. ^[1–3]

2. Propane

In propane, one carbon of the chosen bond is attached to one methyl group (CH₃) and two hydrogen atoms, while the other carbon is attached to three hydrogen atoms. After drawing the basic sawhorse framework, these groups are placed around the two carbons in their correct positions. If the groups on the rear carbon lie directly behind those on the front carbon, the molecule is in the eclipsed form. If they lie in between, it is in the staggered form. These correspond to dihedral angles of 0° and 60°, respectively.

3. Butane

In butane, the sawhorse projection is usually drawn along the bond between the two middle carbon atoms. Each of these carbons is attached to one CH₃ group and two hydrogen atoms. This gives rise to several conformations. When the two CH₃ groups are directly behind each other, the conformation is fully eclipsed, and the dihedral angle is 0°. When they are separated by 60°, the conformation is staggered and is referred to as gauche. When the dihedral angle is 120°, the conformation is partially eclipsed. When the two CH₃ groups are opposite each other, the conformation is staggered and is referred to as anti, with a dihedral angle of 180°.

Thus, the same basic method is used for ethane, propane, and butane, but the groups attached to the carbon atoms and their relative positions determine the conformation shown. This makes the sawhorse projection a useful way to understand the three-dimensional shapes of molecules and rotation about a single bond.