Question

Question #35a3b

Answer 1

Consider looking at 2-methylpentane from the perspective below,

This would be the exact Newman projection,

It really helps if you have a molecular model kit to visualize this happening. Just imagine looking right at the molecule in front, and imagine what you would see in the back.

There would be conformations at `60°` increments of varying energies. The one I drew is probably the lowest energy you can feasibly get this molecule.

Keeping the front carbon's angle constant, we rotate the back carbon, and the substituents "in the back" would rotate close to substituents in the front or in different orientations "between" them (and incur various steric hindrances, Gauche interactions, and torsional strains as a result).

A general rule is that large substituents don't like to be close together. For instance, if the propyl group in the back was eclipsing one of the methyl groups up front, lots of steric hindrance and torsional strain would occur. This isn't stable.