How can I convert 3-methylpentane #C_2-C_3# bond from Newman projection to bond line notation?
We start with a Newman projection of 3-methylpentane.
Note that there is a carbon atom at the centre of the "Y" in the middle of the circle, and there is also a "hidden" carbon behind the circle.
We hunt for the longest continuous chain of carbon atoms.
It goes from the methyl group in the front, to C-2 in the middle of the Y, to the hidden C-3, and then to the two carbons of the ethyl group.
That makes a 5-carbon chain.
Now we look for substituents.
There are none on C-1 or C-2, but there is a methyl group on the hidden C-3, and the ethyl group is part of the main chain.
So we have 3-methylpentane.
The bond-line structure consists of a zig-zag line of five carbon atoms with a methyl group attached to C-3.