There are four structural isomers with the molecular formula C₄H₉Br.
You start by writhing the linear carbon chain and the you start moving the bromide atom until you find all the compounds. In our case you have bromide atom in the position 1 and 2 on the carbon chain.
Then you branch the carbon chain and you have isobutane. Here the possible positions for bromide is 1 and 2 on the primary carbon chain.
