Problem: Newman Projections and Alkane Conformations
Determine the most stable conformation of 2-methylbutane.
Analysis
As always, we must first translate this picture into a complete structure, taking into account the "shortcuts" involved in the bond-line structure shown. This involves several steps to remember:
The vertices at the end of all bond lines represent carbon unless the atomic symbol is explicitly shown.
All carbons will have a total of four bonds; therefore, any bonds not explicitly shown must be to hydrogens not shown.
Now, we have to decide which bond(s) will rotate to generate different conformers. The methyl groups will all rotate; however, other than staggered/eclipsed interations, there will be no distinctions since the hydrogens are all equivalent. However, the bond between C2-C3 has different sets of substituents, so let's look at the rotamers using Newman projections:
The eclipsing conformers are all higher in energy than the staggered ones. For the remaining 3, we look for "gauche interactions": alkyl groups that are within 60° of each other. The red arrows show these. Thus, the center and right hand structure are equivalent (they are mirror images of each other), and the one on the left is higher.
Look at the rotation in the 3-D structure.
