Ranking the Stability of Newman Projections

We have seen that Newman projections are a powerful way to show the different conformations of a molecule. We mentioned earlier that there are staggered conformations and eclipsed conformations. In fact, there are three staggered and three eclipsed conformations. Let s draw all three staggered conformations of butane. The best way to do this is to keep the back carbon motionless (so the fan in the back is not spinning), and let s slowly turn the groups in the front (only the front fan is spinning)  

If we were to go back to our comparison between molecules and people, we would say that the anti conformation would be like lying down in a bed, and the gauche conformations are both like sitting in a chair. All of these are comfortable positions, but lying down is the most comfortable. The anti conformation is the most stable. 

Now let s look at the three eclipsed conformations of butane. Again, let s keep the back where it is, and let s just rotate the front carbon with its three groups  

To summarize, the most stable conformation will be the staggered conformation where the large groups are as far apart as possible (anti), and the least stable conformation will be the eclipsed conformation where the large groups are eclipsing each other. 

EXERCISE 6.8 Draw the most stable and the least stable conformations of the following compound, using Newman projections looking down the following bond  

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Draw all possible staggered and eclipsed conformations of l-bromo-2-chloroethane (see the followingball-and-stick model), using Newman projections. Underneath each structure, draw the corresponding dash-wedge and sawhorse structures. Rank the conformations in order of decreasing stability. 

Draw Newman projections of the staggered and eclipsed conformations of propane. Rank them in stability. 

Draw Newman projections of four different conformations of butane. Rank them in stability. Hint Two of the conformations represent the most stable forms and the other two the least stable forms.)... 

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