Question

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

Answer 1

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.

Here's why that would be so. Below are the two isomers of butane, butane and 2-methylpropane

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If you start with the bond line notation for propane, or `C_3H_8`, you'll get something like this

Now, a methyl group is represented as a simple line. If you look closely at propane's structure, you'' notice that placing the methyl group on either carbon 1 or carbon 3 will produce one of these two structures (the methyl group is drawn in blue)

These structures are identical with the first isomer of butane, which looks like this

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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.