3.5 : Proiezioni di Newman

A common way of representing a three-dimensional structure of a molecule on a two-dimensional surface is using the dash-wedge notation.

In this notation, solid lines represent bonds lying in the same plane; solid wedges indicate bonds coming out of the plane, and dashed wedges represent bonds situated behind the plane.

In an organic molecule like 1,2-dibromo-1,2-dichloroethane,  single bonds — due to the sigma nature of their orbital overlap — can rotate freely. Therefore, the molecule can adopt different shapes without breaking the bonds.

The three-dimensional temporary shapes generated by such rotations are called conformations, and each structure is referred to as a conformer.

Now consider the molecule’s dash-wedge notation.

A 45° rotation about the molecule’s vertical axis generates the sawhorse projection. The molecule is viewed from an oblique angle, with all atoms and bonds being visible.

In comparison, a 90° rotation of the molecule gives rise to a planar projection called the Newman projection. 

In this, the molecule is viewed end-on, along the C-C bond — called the projected bond. The circle represents the carbon atom farther from the observer, and the bonds connected to this carbon are drawn from the circle's periphery.

The center of the circle represents the carbon nearer to the observer, and the bonds attached to this carbon are drawn as lines extending from the center.

The planar bonds and groups are placed at the center, on the top, and the bottom of the circle. The solid wedge bonds are placed on the right, and all dashed bonds are placed on the left.

Alternatively, when the molecule is observed from the right, the solid and dashed wedges exchange their positions.

The angle between a bond at the nearer carbon and a bond at the farther carbon is the dihedral angle.