Question

Why can't we add a methyl group to to the first carbon atom while making an isomer of butane?

Answer 1

Answer:

You can add a methyl group to the first carbon atom of a propane parent chain, but that would be equivalent to butane, or normal, unbranched butane.

Therefore, the only way to attach a methyl group to propane in order to make it be a structural isomer of butane is to do it at carbon 2, otherwise you'll end up with unbranched butane again.