Some Hydrocarbons don’t follow the general formulae.
Hydrocarbons are categorised according to the longest chain available in the compound.
This Butane that has 5 Carbons, and 12 Hydrogens.
It is still considered a butane, as its longest chain holds four Carbons.
To include the new added Carbon, it is called a Methyl-Butane.
We refer to it as 2 – MethylButane to indicate that the added Carbon branches off the 2nd carbon in the longest chain.