Question

Draw the five structural isomers of hexane $\left({\mathrm{C}}_6{\mathrm{H}}_{14}\right)$.

Short answer

The five structural isomers of hexane ${\mathrm{C}}_6{\mathrm{H}}_{14}$ are: 1. n-hexane: $${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_3$$ 2. 2-methylpentane: $${\mathrm{CH}}_3-\mathrm{CH}(-{\mathrm{CH}}_3)-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_3$$ 3. 3-methylpentane: $${\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{CH}(-{\mathrm{CH}}_3)-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_3$$ 4. 2,2-dimethylbutane: $${\mathrm{CH}}_3-\mathrm{CH}(-{\mathrm{CH}}_3)-\mathrm{CH}(-{\mathrm{CH}}_3)-{\mathrm{CH}}_2-{\mathrm{CH}}_3$$ 5. 2,3-dimethylbutane: $${\mathrm{CH}}_3-\mathrm{CH}(-{\mathrm{CH}}_3)-\mathrm{CH}(-{\mathrm{CH}}_3)-{\mathrm{CH}}_3$$

Step-by-step solution

Draw the straight chain isomer

Let's start by drawing the straight chain isomer, which has all six carbon atoms connected linearly. To complete this isomer, connect each carbon atom to the appropriate number of hydrogen atoms, creating four bonds with each carbon atom. The structure will look like this: $${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_3$$ This is called n-hexane.

Draw the branched isomers

Now, let's consider the branched isomers, which have one or more carbon atoms branching off the main chain. We'll start with a structure containing a single carbon branching off at the second carbon. The first branched isomer has the structure: $${\mathrm{CH}}_3-\mathrm{CH}(-{\mathrm{CH}}_3)-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_3$$ This is called 2-methylpentane.

Create another branched isomer

The next branched isomer has a single carbon branching off at the third carbon. The structure is as follows: $${\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{CH}(-{\mathrm{CH}}_3)-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_3$$ This is known as 3-methylpentane.

Make a more branched isomer

Another branched isomer can be created with a carbon branching off at the second carbon and an additional carbon branching off the central carbon. The structure will be: $${\mathrm{CH}}_3-\mathrm{CH}(-{\mathrm{CH}}_3)-\mathrm{CH}(-{\mathrm{CH}}_3)-{\mathrm{CH}}_2-{\mathrm{CH}}_3$$ This is called 2,2-dimethylbutane.

The last branched isomer

The final branched isomer has one carbon branching off at the second carbon and another at the third. The structure will be: $${\mathrm{CH}}_3-\mathrm{CH}(-{\mathrm{CH}}_3)-\mathrm{CH}(-{\mathrm{CH}}_3)-{\mathrm{CH}}_3$$ This is known as 2,3-dimethylbutane. We have now drawn all five structural isomers of hexane: n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane.