Question

5. Which are constitutional isomers? A. Hexane and 3-methylpentane B. Hexane and 2,3-dimethylpentane C. Hexane and 2,3-dimethylbutane D. Hexane and cyclohexane

Answer 1

The five isomers of hexane are: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane. They are constitutional isomers because they each contain exactly the same number and type of atoms, in this case, six carbons and 14 hydrogens and no other atoms. For conventional, stable, organic molecules it is necessary that each carbon atom forms four bonds to other atoms and each hydrogen atom forms only one bond. With these "rules" it is fairly straitforward to see that _all_ molecules containing just six carbons and 14 hydrogens will have the following things in common:

1. A contiguous carbon skeleton and..
2. Hydrogens will only be bonded to carbons.

With this in mind, constructing all of the isomers of hexane can be accomplished by first ignoring the hydrogens and determining all the unique carbon-carbon bonding arrangements possible with six carbon