Heptane constitutional isomers

Constitutional isomerism becomes more complex as the size of the hydrocarbon molecule is increased. For example, there are three constitutional isomers of pentane, C5H12. The number of constimtional isomers increases quite rapidly with an increasing number of carbon atoms. Thus, there are five constimtional isomers of hexane, CeH, nine isomers of heptane, C7H16, 75 isomers of decane, C10H22, and 366,319 isomers of eicosane, C20H42. You can begin to understand why it is possible to make so many different molecules based on carbon. 

There are 18 constitutional isomers with molecular formula CsHis- Without drawing all 18 isomers, determine how many of the isomers will have a parent name of heptane. 

We are looking for constitutional isomers of CgHig that have a parent chain of seven carbon atoms (heptane). The extra CH3 group can be placed in any of three positions (C2, C3, or C4) giving the following three constitutional isomers ... 

Similarly, we cannot connect the CH3 group to position C6 as that would generate the same structure as placing the CH3 group at the C2 position. In summary, there are only three constitutional isomers of CgHig that have a parent name of heptane (shown above). 

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