Question

What is the most stable conformer for 3-methylpentane, viewed along the `C_2-C_3` bond using Newman projections?

Answer 1

The most stable conformer for 3-methylpentane viewed along the `C_2 - C_3` bond will have the ethyl group anti to a methyl group, with a methyl - methyl gauche interaction.

So, start with the bond line notation for 3-methylpentane

Notice that you have two hydrogens and a methyl group attached to `C_2`, and one hydrogen, one methyl, and one ethyl group attached to `C_3`.

Start by placing these groups on a Newman projection template. The `C_2` carbon will be in the front and the `C_3` carbon will be in the back, which will result in

Projection 1

All the groups are accounted for: two hydrogen atoms and a methyl group connected to `C_2` and a methyl group, a hydrogen atom and an ethyl group connected to `C_3`.

In order to determine which staggered projection will describe the most stable conformer, you need to draw the other two as well - you do this by rotating the front carbon by `"120"^@`.

Projection 2 and Projection 3

The most stable conformer has the fewest gauche interactions. The first projection will have two methyl groups in anti positon and a gauche interaction between two methyl groups.

Projection 2 will have two methyl groups in anti postition as well, but this time you have gauche interaction between a methyl and an ethyl group, which is less stable because of the greater repulsion between these two groups.

Projection 3 will be the least stable of the group, since you have gauche interactions between two methyl groups and an ethyl group.

As a conclusion, projection 1 represents the most stable conformer for 3-methylpentane viewed along the `C_2-C_3` bond.