Answer:
Structure 1:
The longest chain is made up of 4 carbons (butane). There is a 1-carbon branch (methyl). The lowest possible position of the branch is on the 2nd carbon. This makes the alkane: 2-methylbutane.
Structure 2:
The longest chain is made up of 7 carbons (heptane). There is a 2-carbon branch (ethyl). The lowest possible position of the branch is on the 4th carbon. This makes the alkane: 4-ethylheptane.
Structure 3:
The longest chain is made up of 4 carbons (butane). There are 2, 1-carbon branches (dimethyl). The lowest possible position of both branches is on the 2nd carbon. This makes the alkane: 2,2-dimethylbutane.
Structure 4:
The longest chain is made up of 3 carbons (propane). There are 2, 1-carbon branches (dimethyl). The lowest possible position of both branches is on the 2nd carbon. This makes the alkane: 2,2-dimethylpropane.
Structure 5:
The longest chain is made up of 6 carbons (hexane). There is a 1-carbon branch (methyl). The lowest possible position of the branch is on the 2nd carbon. This makes the alkane: 2-methylhexane.
Structure 6:
The longest chain is made up of 3 carbons (propane). There is a 1-carbon branch (methyl). The lowest possible position of the branch is on the 2nd carbon. This makes the alkane: 2-methylpropane.
The isomers are structures 1 and 4 because they contain the same amount of carbons (5) and hydrogens (12) while assuming different structures.