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These projections are drawn by looking directly along a particular bond in the system (here a C-C bond) and arranging the substituents symmetrically around the atoms at each end of that bond. The protocol requires that the atoms within the central bond are defined as shown below:
In order to draw a Newman projection from a wedge-dash diagram, it is useful to imagine putting your "eye" in line with the central bond in order to look along it.
Let's work through an example, consider drawing a Newman projection by looking at the following wedge-dash diagram of propane from the left hand side.
- First draw the dot and circle to represent the front and back C respectively
- Since the front carbon atom has an H atom in the plane of the page pointing up we can add that first
- The back carbon atom has an H atom in the plane of the page pointing down
- Now add the other bonds to each C so that it is symmetrical
- The groups / bonds that were forward of the plane of the page in the original wedge-dash diagram are now to our right
- Those behind the plane are now to our left
- Now you try the same thing, but looking from the right to generate the other Newman projection.
Newman Projections are used mainly for determining conformational relationships. Recall that conformers are molecules that can be converted into one another by a rotation around a single bond. Newman Projections are also useful when studying a reaction involving prochiral molecules that have a double bond, in which the addition of a new group creates a new stereocenter.
In this notation, you are actually viewing a molecule by looking down a particular carbon-carbon bond. The front carbon of this bond is represented by a dot, and the back carbon is represented by a large circle. The three remaining bonds are drawn as sticks coming off the dot (or circle), separated by one another by 120 degrees. A Newman Projection can be drawn such that the groups on the front carbon are staggered (60 degrees apart) or eclipsed (directly overlapping) with the groups on the back carbon. Below are two Newman Projections of ethane, C2H6. The structure on the left is staggered, and the structure on the right is eclipsed. These are the simplest Newman Projections because they have only two carbons, and all of the groups on both carbons are identical.
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