How many isomers does #C_4H_9Br# have?
First we should consider the formula as
Then, how can we arrange the carbon atoms?
The first three, only can be arranged in a linear mode:
The fourth one can be allocated in the ends:
or in the middle carbon:
Now, where can we put the Br atoms?
In butane we can put the Br in the first carbon (equivalent to the last)
or in the second which is equivalent to the third:
In the case of isobutane, the external carbons are equivalent, giving:
but we can also substitute in the carbon of the middle:
The remaining positions we add hydrogen atoms, making each carbon atom four bonds.
So there are four isomers of