2.1 Newman Projection
2.1 Newman Projection
It was understood that the rotation about C-C single bond of ethane and related molecules to cause the change of molecular shape. It is necessary, however, to prepare a set of rules beforehand in order to express the change of three-dimensional structure in a two-dimensional way.
Schematic Drawings
The first step to express molecular structure in a two-dimensional way is use of schematic drawings; i.e., to draw molecular structure as it looks. In order to stress the three-dimensional nature, several conventions are used. dash and wedge drawings are some examples.
(a) perspective (b) dash and wedge drawing
Fig. 2.1 Schematic drawings of ethane.
It is impossible, however, for these drawings to reproduce the exact shape of the molecules. As we have learned before, the dihedral angle defined by four atoms, H-C-C-H, is a very good parameter of molecular structure. It is not practical, however, to draw such a figure Fig. 1.8 as for a large compound. In order to solve the problem a method of drawing molecular structure was proposed by the American chemist M. S. Newman in the middle of 20th century. The characteristic point of this method is that the dihedral angle is clearly demonstrated. The way of drawing a Newman projection is exemplified with ethane. The molecule (or rather, molecular model) of ethane is projected against the wall behind the molecule along the direction of C-C bond. On the wall, the shadow of six hydrogens and two carbons should appear. However, since the projection is made along the C-C bond, the two carbon atoms overlapped and only one carbon in front can be seen and the other is behind the front one. To avoid this inconvenience, the front carbon is now designated as a dot, while a large circle represents the carbon behind. The dihedral angle is defined by the angle made by two bond, CA-HA and CB-HB on the Newman projection. The angle is positive if the overlapping of CA-HA bond with CB-HB bond is achieved by moving the former clockwise, while the angle is negative if the same process requires an anticlock movement of CA-HA bond. In Fig. 2.2, two conformations of ethane 1 and 2 given in Fig. 1.8 are shown.
Fig. 2.2 Newman projection of ethane