Drawing Newman Projections
So far, we have seen different ways of drawing molecules such as straight chain, fully written, and condensed formats. There is yet another way to draw and visualize a molecule, called a Newman projection. This section will focus on understanding and drawing Newman projections
A Newman projection is a way to take a snapshot of what a molecule looks like at a particular moment in time from a different angle than we’re used to. Newman projections focus on any two carbons and the groups coming off them in a molecule by shifting the view from which the molecule is visualized.
When drawing Newman projections, look at the molecule from a different perspective by looking down two of the carbon atoms, so you can only see the front carbon atom and not the back carbon (as it is blocked by the front carbon). If you look at the groups coming off the carbons, they will make a Y shape (often, but not always, a right-side-up Y or an upside-down Y).
When drawing Newman projections, the front carbon is indicated by the central point of from “Y-like” shape, and the back carbon is not explicitly shown, although it is assumed to be right behind the front carbon.
In the figure above, we have rotated the back carbon in 60˚ increments to emphasize the free rotation around single bonds, but you don’t always have to rotate by 60 degrees. Furthermore, we can rotate both the front and the back carbons as we wish.
As each carbon rotates, there is some overlap as the atoms move. This is because of steric hindrance, a repulsive force exerted by substituents in a molecule. Essentially, large groups want to be as far apart as possible, but in a Newman projection, the atoms are forced to be fairly close to one another, so there is a repulsive force present. The overlap, and the energy difference associated with this overlap, leads to two energetic subgroups of Newman projections: eclipsed and staggered.
Eclipsed conformations result in more steric hindrance between two atoms than staggered conformations because of how close the atoms can get to one another. Eclipsed conformations are therefore less stable than staggered conformations.
Eclipsed conformations are higher in energy and less stable than staggered conformations.
Staggered conformations are a fairly stable conformation as the atoms are spread apart to minimize steric hindrance. An example of a staggered conformation looks like this:
There are 2 other terms used to describe staggered conformations:
- Anti conformation
- Gauche conformation
The most stable form of the Newman projection is the anti conformation. In this form, the largest substituent coming off the front carbon is exactly 180o degrees away from the largest substituent on the back carbon; therefore, the two largest substituents on each carbon of the Newman projection are as far apart from one another as possible, leading to the least possible steric hindrance. In the example used above, the anti conformation looks like:
Gauche conformations are staggered molecules that have some steric hindrance. Whereas the most stable anti conformation has the two largest substituents 180° away from each other, gauche conformations have the two largest molecules 60° apart from one another. These conformations are more stable than eclipsed conformations (see next section), but less stable than anti conformations as there is some steric hindrance interaction present. Let’s look at the gauche conformations in this same molecule:
The larger the groups, the greater the gauche effect as more steric hindrance would be present.
In the eclipsed conformation, groups coming off the two carbons of focus in the Newman projection interact with and repel each other, creating steric hindrance as they directly overlap with one another. Larger substituents – such as alkyl groups, halogens, and oxygen-containing groups, for example – create more hindrance. The larger the substituents, the more hindrance present.
For example, think about the 3 possible eclipsed conformations the following molecule could have:
Of the possible eclipsed conformations, one form is less stable than the others, as shown by the diagram below. This is created by the overlap of the two largest substituents on the two carbons of focus for the Newman projection. In the diagram below, the two largest substituents on each carbon are labeled in pink. Fluorine is also a large atom but not as large as the cyclic substituent (which is labeled as C6H8).
The eclipsed conformation is so unstable that it only exists as a transition state between staggered conformations. It only exists for a short time as the molecule rotates from one staggered state to another.
The following diagram is a summary of the energy of all the possible conformations: